fundamentals and applications of organic...

CONTENTSCoverTitle PageCopyrightAbout the AuthorsPrefaceIntroduction

ReferencesChapter 1: Fundamental Principles ofOrganic Electrochemistry: FundamentalAspects of Electrochemistry Dealing withOrganic Molecules

1.1 Formation of Electrical Double Layer1.2 Electrode Potentials (RedoxPotentials)1.3 Activation Energy and Overpotential1.4 Currents Controlled by ElectronTransfer and Mass TransportReferences

Chapter 2: Method for Study of OrganicElectrochemistry: ElectrochemicalMeasurements of Organic Molecules

2.1 Working Electrodes2.2 Reference Electrodes2.3 Auxiliary Electrodes2.4 Solvents and Supporting Electrolytes

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2.5 Cells and Power Sources2.6 Steady-State and Non-Steady-StatesPolarization Curves2.7 Potentials in ElectrochemicalMeasurements2.8 Utilization of Voltammetry for theStudy of Organic ElectrosynthesisReferences

Chapter 3: Methods for OrganicElectrosynthesis

3.1 Selection of Electrolytic Cells3.2 Constant Current Electrolysis andConstant Potential Electrolysis3.3 Direct Electrolysis and IndirectElectrolysis3.4 Electrode Materials and ReferenceElectrodes3.5 Electrolytic Solvents and SupportingElectrolytes3.6 Stirring3.7 Tracking of Reactant and Product3.8 Work-Up, Isolation andDetermination of Products3.9 Current Efficiency and Effect of thePower UnitReferences

Chapter 4: Organic Electrode Reactions

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4.1 General Characteristics of ElectrodeReactions4.2 Mechanism of Organic ElectrodeReactions4.3 Characteristics of OrganicElectrolytic Reactions4.4 Molecular Orbitals and ElectronsRelated to Electron Transfer4.5 Electroauxiliaries4.6 Reaction Pattern of OrganicElectrode Reactions4.7 Electrochemically GeneratedReactive SpeciesReferences

Chapter 5: Organic Electrosynthesis5.1 Electrocatalysis5.2 Electrogenerated Acids and Bases5.3 Electrochemical AsymmetricSynthesis5.4 Modified Electrodes5.5 Paired Electrosynthesis5.6 Reactive Electrodes5.7 Electrochemical Fluorination5.8 Electrochemical PolymerizationReferences

Chapter 6: New Methodology of OrganicElectrochemical Synthesis

6.1 SPE Electrolysis and Its Applications

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6.2 Electrolytic Systems Using SolidBases and Acids6.3 Solid-Supported Mediators6.4 Biphasic Electrolytic Systems6.5 Cation Pool Method6.6 Template-Directed Methods6.7 Electrolysis in Supercritical Fluids6.8 Electrolysis in Ionic Liquids6.9 Thin-Layer Electrolytic Cells6.10 Electrochemical Microflow Systems6.11 Electrolysis Under Ultrasonication6.12 Electrosynthesis Using SpecificElectrode Materials6.13 Photoelectrolysis and Photocatalysis6.14 Electrochemical Polymer ReactionsReferences

Chapter 7: Related Fields of OrganicElectrochemistry

7.1 Application in Organic ElectronicDevices7.2 Electrochemical Conversion ofBiomass to Valuable Materials7.3 Application to C1 Chemistry7.4 Environmental CleanupReferences

Chapter 8: Examples of CommercializedOrganic Electrode Processes

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8.1 Avenue to Industrialization8.2 ExamplesReferences

Appendix A: Examples of OrganicElectrosynthesis

A.1 Electrochemical FluorinationA.2 Electrosynthesis Using aHydrophobic ElectrodeA.3 Natural Product Synthesis UsingAnodic OxidationA.4 Kolbe ElectrolysisA.5 Indirect Electrosynthesis Using aMediatorA.6 Electrosynthesis of ConductingPolymersReferences

Appendix B: Tables of Physical DataIndexEnd User License Agreement

List of TablesTable B.1

Table B.2

Table B.3

Table B.4

Table 5.1

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Table 5.2

Table 5.3

Table 5.4

Table 5.5

Table 6.1

Table 6.2

Table 6.3

Table 6.4

Table 6.5

Table 6.6

List of IllustrationsFigure A.1

Figure A.2

Figure A.3

Figure B.1

Figure B.2

Figure 1.1

Figure 1.2

Figure 1.3

Figure 1.4

Figure 1.5

Figure 1.6

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Figure 1.7

Figure 2.1

Figure 2.2

Figure 2.3

Figure 2.4

Figure 2.5

Figure 2.6

Figure 2.7

Figure 2.8

Figure 2.9

Figure 2.10

Figure 2.11

Figure 3.1

Figure 3.2

Figure 3.3

Figure 3.4

Figure 3.5

Figure 3.6

Figure 3.7

Figure 3.8

Figure 4.1

Figure 4.2

Figure 4.3

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Figure 4.4

Figure 4.5

Figure 4.6

Figure 4.7

Figure 4.8

Figure 4.9

Figure 4.10

Figure 4.11

Figure 4.12

Figure 4.13

Figure 4.14

Figure 4.15

Figure 4.16

Figure 5.1

Figure 5.2

Figure 5.3

Figure 5.4

Figure 5.5

Figure 5.6

Figure 5.7

Figure 5.8

Figure 5.9

Figure 5.10

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Figure 5.11

Figure 5.12

Figure 5.13

Figure 5.14

Figure 5.15

Figure 5.16

Figure 5.17

Figure 5.18

Figure 5.19

Figure 5.20

Figure 6.1

Figure 6.2

Figure 6.3

Figure 6.4

Figure 6.5

Figure 6.6

Figure 6.7

Figure 6.8

Figure 6.9

Figure 6.10

Figure 6.11

Figure 6.12

Figure 6.13

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Figure 6.14

Figure 6.15

Figure 6.16

Figure 6.17

Figure 6.18

Figure 6.19

Figure 6.20

Figure 6.21

Figure 6.22

Figure 6.23

Figure 6.24

Figure 6.25

Figure 6.26

Figure 6.27

Figure 6.28

Figure 6.29

Figure 6.30

Figure 6.31

Figure 6.32

Figure 6.33

Figure 6.34

Figure 6.35

Figure 6.36

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Figure 7.1

Figure 7.2

Figure 7.3

Figure 7.4

Figure 7.5

Figure 7.6

Figure 7.7

Figure 7.8

Figure 7.9

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Fundamentals andApplications ofOrganicElectrochemistry

Synthesis, Materials, DevicesToshio Fuchigami and Shinsuke Inagi

Department of Electronic Chemistry, TokyoInstitute of Technology, Japan

Mahito Atobe

Department of Environment and SystemSciences, Yokohama National University,Japan

15

This edition first published 2015

© 2015 John Wiley & Sons, Ltd

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© Toshio Fuchigami, Mahito Atobe andShinsuke Inagi, 2012

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Library of Congress Cataloging-in-PublicationData

Fuchigami, Toshio.

19

Fundamentals and applications of organicelectrochemistry : synthesis, materials, devices/ Toshio

Fuchigami, Mahito Atobe, Shinsuke Inagi.

pages cm

Includes bibliographical references and index.

ISBN 978-1-118-65317-3 (cloth)

1. Organic electrochemistry. 2.Electrochemistry. 3. Chemistry, Organic. I.Atobe, Mahito, 1969- II.

Inagi, Shinsuke. III. Title.

QD273.F83 2014

547′.137–dc23

2014017644

A catalogue record for this book is availablefrom the British Library.

ISBN: 9781118653173

20

About the AuthorsDr Toshio Fuchigami is an instituteprofessor at the Tokyo Institute of Technology,having received his PhD from the same institutein 1974. He has authored more than 420publications, including review articles and bookchapters. His current research interests arecentred on the new hybrid fields oforganofluorine electrochemistry and newelectrolytic systems for green sustainablechemistry. He has received many awards inelectrochemistry, including the ElectrochemicalSociety of Japan Award and theElectrochemical Society (ECS) Manuel M.Baizer Award. He is also an ECS fellow.

Dr Mahito Atobe was appointed to aprofessorship at the Graduate School ofEnvironment and Information Sciences,Yokohama National University in July 2010. Hereceived his PhD from the Tokyo Institute ofTechnology in 1998. His current researchfocuses on organic electrosynthetic processesand electrochemical polymerisation underultrasonication, electrosynthetic processes in aflow microreactor, and organic electrochemicalprocesses in supercritical fluids (140publications).

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Dr Shinsuke Inagi is an associate professorat the Tokyo Institute of Technology. Hereceived his PhD from Kyoto University in2007. After postdoctoral research (ResearchFellowship for Young Scientists from the JapanSociety for the Promotion of Scientists) atKyoto University, he joined ProfessorFuchigami's research group as an assistantprofessor at the Tokyo Institute of Technologyin 2007. He was promoted to associateprofessor in 2011. His current research interestsinclude electrochemical synthesis of polymericmaterials.

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PrefaceOrganic electrochemistry is electrochemistryfocused on organic molecules, while inorganicelectrochemistry deals with inorganicmolecules, which is a major part offundamentals and applications ofelectrochemistry. In fact, most industrializedelectrode processes are inorganic.Electrochemistry is mainly based on physicalchemistry such as thermodynamics andkinetics. Many mathematical equations areused in electrochemistry textbooks and organicchemists therefore think that electrochemistryis difficult. Similarly, organic chemistry dealswith organic molecules and complicatedreactions, therefore physical chemists oftendislike organic chemistry.

Organic electrochemistry, particularly organicelectrosynthesis, has developed byincorporating new organic reactions andorganic synthesis. The 21st century issometimes called the ecological century, andorganic electrosynthesis is a typical greensustainable chemistry since it does not requireany hazardous reagents and produces less wastethan other chemical synthesis. Furthermore,organic electrochemistry has also recentlydeveloped as integrated field including not onlyorganic electrosynthesis but also materials

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chemistry, catalysis chemistry, biochemistry,medicinal chemistry and environmentalchemistry. In our daily lives organic andpolymer materials play important roles intechnologies such as biosensors, conductingpolymers, liquid crystals, electroluminescencematerials, dye-sensitized solar cells and so on.To understand these technologies we muststudy the basics of both organic chemistry andelectrochemistry. In this century, the area ofinterest is diversity, therefore students,particularly graduate students, can no longer beengaged in developments in cutting-edgetechnology unless they understand thefundamental principles of various sciences suchas organic chemistry, inorganic chemistry andphysical chemistry, regardless of their ownspecialized scientific background.

In addition, organic electrochemistry alsoinvolves organic electron transfer chemistryusing electrical energy. In this way organicelectrochemistry is quite similar tophotoelectron transfer, which is an importantfield of organic photochemistry using lightenergy. Although a number of fundamentalbooks dealing with organic photochemistryhave been published, there has been notextbook dealing with the basic aspects oforganic electrode electron transfer and itsapplications together with new fields.

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In this book, the authors have conciselyproduced their organic electrochemistry lecturenotes for graduate students. The text isarranged for graduate students, researchers andengineers to easily understand the basicprinciples of electrochemistry, electrochemicalmeasurements and organic electrosynthesis,including its new methodologies. Someexperimental examples of organicelectrosynthesis are also described in detail.

Online supplementary material for the book canbe found at http://booksupport.wiley.com

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IntroductionToshio Fuchigami

The concept of organic electrochemistry isrelatively new, even though it has a longhistory. In 1800, an Italian physicist namedVolta invented the well-known Voltaic pile.Three years later, Petrov in Russia published apaper on the electrolysis of alcohols andaliphatic oils. A year after that, Grotthuss inLithuania, who proposed the ionic conductingmechanism, found that a diluted solution ofindigo white could be readily electrochemicallyoxidized to indigo blue. In 1833, Faradaydiscovered Faraday's law, and one year later hefound that hydrocarbons could be formed bythe electrolysis of an aqueous solution of theacetic acid salt. Unfortunately, he could notidentify the products. In 1849, Wöhler'sdisciple, Kolbe, discovered the electrochemicaloxidation of a carboxylic acid (RCOOH) to thedimeric alkane (R–R) and CO2, known as Kolbeelectrolysis [1]. Consequently, Faraday andKolbe are pioneers in the investigation oforganic electrochemical processes. From theend of the 19th century to the early 20thcentury, electrochemical oxidation andreduction processes of various compounds wereintensively investigated. Thus, the applicationof electrolysis for preparing organic compounds

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continued in the first half of the 20th century. Atypical example is the electrochemical reductionprocess of nitrobenzene to aniline. Importantly,organic electrochemistry was also developedalong with the discovery of newelectroanalytical techniques such aspolarography, which was developed byHeyrovský and Tachi in the early 1920s [2].However, organic electrosynthesis research hadto be completely halted during the SecondWorld War.

In 1964, Baizer developed the electrochemicalhydrodimerization of acrylonitrile, which is ahighly useful industrial process for themanufacture of adiponitrile. This inventionrestimulated organic electrosynthesis researchby many electrochemists and organic chemists.Since then, the development of organicelectrochemistry, particularly organicelectrosynthesis, has been marked byincorporating new types of organic reactionsand modern organic synthesis. Furthermore,various aprotic polar organic solvents havebeen developed, and these enable us to detectelectrogenerated unstable intermediates. Inaddition, cyclic voltammetry and relatedelectroanalytical techniques have assisted in theunderstanding of kinetics and mechanisms oforganic electrode processes.

Organic electrochemistry has recentlydeveloped as an integrated field including not

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only organic electrosynthesis but also materialschemistry, catalysis chemistry, biochemistry,medicinal chemistry and environmentalchemistry.

The 21st century is known as the ecologicalcentury. Organic electrosynthesis is expected tobe a typical green chemistry process since itdoes not require any hazardous reagents andproduces less waste than conventional chemicalsynthesis. Recently, a novel pairedelectrosynthesis of phthalide and p-t-butylbenzaldehyde has been developed andindustrialized by BASF in Germany, and theyconsider electrosynthesis to be the mostpromising green synthetic process. These factshave prompted organic electrochemists as wellas organic chemists to make great efforts todevelop new systems of organic electrosynthesisin order to achieve green and sustainablechemistry. In fact, a number of successful newgreen organic electrolytic systems have beendeveloped to date, as illustrated in this book.We believe that cutting-edge developments inorganic electrochemistry will be achievedthrough hybridization with other scientificfields, as mentioned above.

References1. Vijih, A.K. and Conway, B.E. (1967) Chem.Rev., 67, 623–664.

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2. Zuman, P. (2012) Chem. Rec., 12, 46–62.

32

Mahito Atobe

Chemists often encounter situations in which areaction does not proceed at a convenient rateunder the initially selected set of conditions. Inchemistry, activation energy is defined as theminimum energy required to start a chemicalreaction, and hence the activation energy mustbe put into a chemical system in order for achemical reaction to occur. Catalysts are oftenused to reduce the activation energy but a hightemperature is still required for the reaction toproceed at an appreciable rate. Electrochemicalreactions, however, can generally be carried outunder mild conditions (room temperature andambient pressure).

In electrochemical reactions there is anadditional experimental parameter, theelectrode potential, involved in themanipulation of electrochemical reaction rates.Electron transfer rates can easily be varied overmany orders of magnitude at a singletemperature by proper control of the electrodepotential. Indeed, electrode potential is sopowerful a parameter for controlling the ratesof electrochemical reactions that most reactionscan be carried out at or near room temperature.

An understanding of the nature of thedependence of electron transfer rates onpotential is important for understanding

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electrode processes and constitutes the centraltheme of this chapter. Because electron transferat an electrode surface is necessarily aheterogeneous process, it will be necessary toexamine briefly the structure of theelectrode–solution interface and its effects onthe course of an electrochemical reaction. It isnot enough, however, to simply derive therelationship between electron transfer rate andelectrode potential. This is because as a result ofthe dramatic changes in these rates withpotential, it is generally found that at certainpotentials electron transfer is so fast that theoverall process is actually limited by the rate ofmass transport of the substrate from the bulksolution to the electrode surface. There aredifferent modes of mass transport, and theydiffer in efficiency, therefore it will be necessaryto examine each of these influences.

1.1 Formation of Electrical DoubleLayerWhen electrodes are polarized in an electrolytesolution, the charge held at the electrodes isimportant. In order to neutralize a chargeimbalance across the electrode–solutioninterface, the rearrangement of charged specieslike ions in the solution near the electrodesurface will occur within a few hundredths of asecond, and finally result in strong interactionsoccurring between the ions in solution and the

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electrode surface. This gives rise to theelectrical double layer, whose thickness isusually between 1 and 10 nm (Figure 1.1) [1].There exists a potential gradient over theelectrical double layer and the gradient is nolonger confirmed to the bulk electrolytesolution. The potential difference between theelectrode surface and the bulk solutionillustrated in Figure 1.1 may amount to a volt ormore, over the rather short distance of thethickness of the double layer, and hence this isan extremely steep gradient, in the order of 106

V cm−1 or greater, which is an electrical field ofconsiderable intensity. This is the driving forcefor the electrochemical reaction at electrodeinterfaces, therefore when the polarizationbetween anode and cathode is increasedgradually, the potential gradient in the vicinityof the anode and cathode is also increased andconsequently the most oxidizable and reduciblespecies in the system are subject to anelectron-transfer reaction at the anode andcathode, respectively. Because a chargeimbalance in the vicinity of an electrode takesplace after the electron-transfer reaction, ionsare transferred to the electrode interface toneutralize the imbalance, and consequently thecontinued Faradic current is observed. Thus,the electrolyte in a solution plays a role in theformation of the electrical double layer and theneutralization of a charge imbalance afterelectrolysis.

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Figure 1.1 Electrical double-layer model andpotential distribution in the double layer

1.2 Electrode Potentials (RedoxPotentials)In all electrochemical experiments the reactionsof interest occur at the surface of the workingelectrode therefore we are interested incontrolling the potential drop across theinterface between the surface of the workingelectrode and the solution. However, it isimpossible to control or measure this interfacialpotential without placing another electrode inthe solution. Thus, two interfacial potentialsmust be considered, neither of which can bemeasured independently. Hence, onerequirement for the counter electrode is that itsinterfacial potential remains constant so that

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any changes in the cell voltage produceidentical changes in the working electrodeinterfacial potential. An electrode whosepotential does not vary with the current isreferred to as an ideal non-polarizableelectrode, but there is no electrode that behavesin this way. Consequently, the interfacialpotential of the counter electrode in thetwo-electrode system discussed above varies asthe current is passed through the cell. Thisproblem is overcome by using a three-electrodesystem in which the functions of the counterelectrode are divided between the reference andauxiliary electrodes (Figure 1.2) [2]. Thisensures that the potential between the workingand reference electrodes is controlled and thecurrent passes between the working andauxiliary electrodes. The current passingthrough the reference electrode is furtherdiminished by using a high-input impedanceoperational amplifier for the reference electrodeinput.

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Figure 1.2 Experimental setup for thethree-electrode system

By employing the three-electrode system wecan control or measure the working electrodepotential. We then consider the essentialmeaning of the potential control using thefollowing simple redox couple (Eq. 1.1):

(1.1)

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where Red and Ox represent the reduced andoxidized forms of a given species.

It is helpful to focus on the energy of electronsin the working metal electrode and in the Redspecies in the electrolyte solution, as depicted inFigure 1.3. The behaviour of electrons in ametal electrode can be partly understood byconsidering the Fermi-level (EF) [3]. Metals arecomposed of closely packed atoms that havestrong overlap between one another. A piece ofmetal therefore does not possess the individualwell-defined electron energy levels that wouldbe found in a single atom of the same material.Instead a continuum of levels exists, with theavailable electrons filling the states from thebottom upwards. The Fermi-level correspondsto the energy of the highest occupied orbitals(HOMO). This level is not fixed and can bemoved by supplying electrical energy (seeFigure 1.3). We are therefore able to alter theenergy of the Fermi-level by applying apotential to an electrode (when a negativepotential is applied, the Fermi-level moves tohigher energy; when a positive potential isapplied, it moves to a lower energy.).Depending on the position of the Fermi-level itmay be thermodynamically feasible to reduce/oxidize species in solution. Figure 1.3 shows theFermi-level within a metal along with theorbital energies (HOMO and LUMO) of amolecule (Red) in solution.

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Figure 1.3 Fermi-level within a metal alongwith the orbital energies (HOMO and LUMO)of a molecule (Red) in solution

As shown in Figure 1.3a, the Fermi-level has ahigher value than the HOMO of Red. It istherefore thermodynamically unfavourable foran electron to jump from the HOMO to theelectrode. However, as shown in Figure 1.3c,when the Fermi-level is below the HOMO ofRed it is thermodynamically favourable for theelectron transfer to occur and we can observecurrent for the oxidation of Red. The criticalpotential at which this electron-transfer processoccurs identifies the standard potential, Eo, ofthe redox couple Red/Ox (see Figure 1.3b).

1.3 Activation Energy andOverpotentialAs mentioned in section 1.2, depending on therelative position of the Fermi-level to the orbitalenergies (HOMO and LUMO) of a substrate

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molecule in solution, it may bethermodynamically feasible to reduce/oxidizethe molecule. However, in generalelectrochemical reactions possess energybarriers that must be overcome by the reactingspecies. This energy barrier is called theactivation energy (see Figure 1.4). Hence, thepotential difference above the equilibrium value(the standard potential, Eo) is usually requiredto produce a current. This potential differencebetween the standard potential and thepotential at which the redox event isexperimentally observed is called theoverpotential [4].

Figure 1.4 The activation energy in theelectron transfer process at an electrode

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1.4 Currents Controlled byElectron Transfer and MassTransportAlthough electrode potential is an extremelyimportant experimental parameter formanipulation of electrochemical reaction rates,other parameters such as mass transport canalso affect reaction rates. We now consider thesimplest electrochemical model, which iscomposed of the electron transfer process andthe mass transfer process, as shown in Figure1.5. In this case, depending on the electrodepotential, the rate-determining step might beeither the electron transfer rate or the rate ofmass transport of the substrate to the electrodesurface. To examine the quantitative andsemiquantitative interrelationships betweenpotential, electrochemical reaction rates andmass transport, a wide variety of voltammetricexperiments are commonly used [5].

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Figure 1.5 Electrochemical model showing theelectron transfer process and the mass transferprocess

We will now consider the factors affecting therelative heights and shapes of voltammetricwaves such as those in Figure 1.6. As theelectrode potential is scanned during avoltammogram (to more negative potentials fora reduction or more positive potentials for anoxidation), the electron transfer rates aredramatically increased and a voltammetriccurve passes through a mixed region in whichthe rates of mass transport and electrontransfer both limit the electrolysis current.Finally, the electron transfer rate eventuallyreaches a high enough rate that the currents arepurely mass transfer limited, and themass-transport-limited voltammetric peak andplateau currents are observed in quiet andstirred solution, respectively. For this reasonthe equations dealing with the electrolysiscurrent are different for each rate-determiningstep.

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Figure 1.6 Voltammetric waves in quiet andstirred solution

When the electrochemical reaction is controlledby the electron transfer step, the net electrolysiscurrent density (i) is represented by theButler–Volmer equation (Eq. 1.2) [4]. Thisequation describes how the current density (i)on an electrode depends on the overpotential(η), considering that both a cathodic and ananodic reaction occur on the same electrode:

(1.2)

where ia and ic are the individual anodic andcathodic current densities, respectively, i0 is theexchange current density, α is the chargetransfer coefficient (its value lies between 0 and1, frequently being about 0.5 at lower

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overpotentials), n is the number of electronsinvolved in the electrode reaction, F is theFaraday constant and η is the overpotential. Asindicated by the Butler–Volmer equation, thenet current density is the difference betweenthe cathodic and anodic current densities. Inaddition, the exchange current density is thatcurrent in the absence of net electrolysis and atzero overpotential.

When the electrochemical reaction is controlledby the mass transfer step, the electrolysiscurrent density relates to the magnitude of thegradient of the substrate moleculeconcentration at the electrode surface,represented by Eq. 1.3 [6]:

(1.3)

where n is the number of electrons involved inthe electrode reaction, F is the Faradayconstant, D is the diffusion coefficient (the rateconstant for motion of the substance throughthe given medium by diffusion), c is thesubstrate molecule concentration, x is thedistance from the electrode surface and hence(dc/dx)x=0 represents the gradient of thesubstrate molecule concentration at theelectrode surface. The concentration profiles forthe substrate and product in quiet solution areshown in Figure 1.7. Since the thickness of thediffusion layer and hence the concentrationgradient change as electrolysis proceeds in

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quiet solution, the decrease in electrolysiscurrent is observed in its voltammogram, asillustrated in Figure 1.6a.

Figure 1.7 Change in the diffusion layerthickness with electrolysis time

In the presence of convection, for examplestirring, the variation of the diffusion layer withtime is inhibited and hence the concentrationgradient is constant. In this case, a limitingcurrent density (id) related to the diffusion layerthickness (δ) would be observed [5]:

(1.4)

Equation 1.4 describes the current density atthe plateau of a voltammogram measured instirred solution (Figure 1.6b), provided that thepotential is scanned rapidly, so theconcentration of the substrate in the bulk of thesolution is not significantly depleted during thetime needed to measure the voltammogram. Inaddition, since δ is reduced by more efficientstirring, the limiting current densities in stirredsolution increase with the stirring rate. They areof course much larger than the current densitiesin quiet solution because convection is so much

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more efficient than diffusion at transporting thesubstrate molecule to the electrode surface.

References1. Bard, A.J. and Faulkner, L.R. (2001)Electrochemical Methods, Fundamentals andApplications, 2nd edn, John Wiley & Sons, Inc.,New York, Chapter 13.

2. Bard, A.J. and Faulkner, L.R. (2001)Electrochemical Methods, Fundamentals andApplications, 2nd edn, John Wiley & Sons, Inc.,New York, Chapter 2.

3. Compton, R.G. and Sanders, G.H.W. (1996)Electrode Potentials, Oxford University Press,Oxford, Chapter 1.

4. Izutsu, K. (2009) Electrochemistry inNonaqueous Solutions, Wiley-VCH VerlagGmbH, Weinheim, Chapter 5.

5. Fry, A.J. (1989) Synthetic OrganicElectrochemistry, John Wiley & Sons, NewYork, Chapter 2.

6. Rifi, M.R. and Covitz, F.H. (1974)Introduction to Organic Electrochemistry,Marcel Dekker, New York, Chapter 2.

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1Fundamental Principles of OrganicElectrochemistry: Fundamental Aspects ofElectrochemistry Dealing with OrganicMolecules

48

Mahito Atobe

One of the main aims of this book is tounderstand the mechanism of organicelectrochemical reactions in order to use themmost effectively or in new ways. Nowadays anumber of specific electrochemicalmeasurements are used to obtain mechanisticinformation about organic electrochemicalreactions. Among these measurements,voltammetry is one of the most frequently usedtechniques since it provides enough evidenceconcerning the mechanism of an electrodeprocess to allow us to use this processintelligently in synthesis and to develop newelectrochemical reactions.

In this chapter we will introduce voltammetrictechniques to obtain mechanistic informationfor organic electrochemical reactions. Becausethe success or failure of voltammetricmeasurements depends to a great degree on theproper selection of experimental componentssuch as electrochemical cells, electrodes,solvents and electrolytes, we will discuss someconcerns about experimental components forvoltammetry. In addition, this chapter will alsoshow how voltammetry can be used to obtaininformation about the mechanism of a neworganic electrode reaction.

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2.1 Working ElectrodesVoltammetry is the group of electrochemicaltechniques where current is studied as aresponse to the potential of the workingelectrode. Experiments are usually carried outusing a three-electrode system in which thepotential between the working and referenceelectrodes is controlled and the current passesbetween the working and auxiliary electrodes[1]. Since the measured electrochemicalreaction occurs at the working electrode, theselection of the working electrode material iscritical to the experimental success ofvoltammetry.

The electrolytes can be used withoutappreciable degradation only in limit ranges ofelectrical potential. This potential windowshould be as wide as possible to allow for thegreatest degree of studied samplecharacterization [2]. The upper and lowerpotential limits are determined not only by theelectrolytic solution but also by the electrodematerial. In aqueous electrolytes, oxygen andhydrogen evolution reactions limit the potentialwindow, and hence the window is usuallynarrower than that in non-aqueous electrolytes.To overcome this problem, cathode materialswith high hydrogen overpotential and anodematerials with high oxygen overpotential areusually used as working electrode materials forvoltammetry in aqueous electrolytes. Platinum

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and gold are good anode materials because oftheir higher oxygen overpotential, whilemercury, zinc and lead are good candidates forcathode materials because of their higherhydrogen overpotential. However, the toxicityof mercury has lead to a limited use. On theother hand, the non-aqueous electrolytes arestable and their potential windows are generallylarger than those of aqueous electrolytes,therefore there are few limitations for thechoice of working electrode materials. The mostcommonly used working electrode materials forvoltammetry in non-aqueous electrolytes areplatinum, gold and carbon. Although almost allnoble metals can be used as both anode andcathode materials, base metals are unsuitableas anode materials because of their dissolutionunder anodic polarization.

Various shapes (e.g. disks, wires, plates) andsizes (e.g. a few square centimetres for plateelectrodes, a few micrometres to a fewcentimetres in disk diameter) of solid electrodesare used for voltammetric measurements(Figure 2.1). Solid electrodes for voltammetricmeasurements are most often fabricated byencapsulating the electrode material in anon-conducting sheath of glass or inertpolymeric material like Teflon, Kel-F(poly-chlorotrifluoroethylene) or PEEK(poly-etheretherketone). Most commonly, theexposed electrode material is in the form of adisk (Figures 2.1a and b). Common

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commercially available disk diameters rangefrom 1 μm to 1 cm. Metal plate electrodes areusually connected to a lead wire by spotwelding, and the wire part is encapsulated in anon-conducting sheath of glass or inertpolymeric material (Figure 2.1c).

Figure 2.1 Examples of handmade workingelectrodes

Ideally, a working electrode should behave inthe same way each time it is used. The factorsthat affect the electrochemical behaviour of asurface are its cleanliness, the kind and extentof chemical functionalities (including oxides)that are present, and the microstructure of theelectrode material itself. Generally, apre-treatment step or steps will be carried outprior to each experiment to ensure that theelectrode surface condition can be reproducedfrom experiment to experiment. These stepsmay be as simple as mechanical polishing, andmay include pre-scanning across a certainpotential range or exposure to a solvent or

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chemical species to activate the electrode.Specific procedures for different electrodematerials can be found in references [1] and [2],and those contained therein.

2.2 Reference ElectrodesThe potential of the reference electrode must bestable and reproducible. In aqueous solutions,the method for measuring electrode potentialshas been well established [3]. The standardhydrogen electrode (SHE) is the primaryreference electrode and its potential is definedas zero at all temperatures. Practicalmeasurements employ reference electrodes thatare easy to use, the most popular ones being asaturated calomel electrode (SCE) (Figure 2.2a)and a silver–silver chloride (Ag/AgCl) electrode(Figure 2.2b). In contrast, in non-aqueoussolutions the method for measuring electrodepotential has not been established. The mostserious problem is the reference electrode, thatis, there is no primary reference electrode suchas the SHE for non-aqueous electrolytes and noreference electrode as reliable as the aqueousAg/AgCl electrode. However, efforts are beingmade to improve this situation.

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Figure 2.2 Examples of handmade referenceelectrodes

The reference electrodes used in non-aqueoussystems can be classified into two types. Onetype is an aqueous reference electrode, usuallyan aqueous Ag/AgCl electrode or SCE.However, the aqueous reference electrodeshould not be dipped directly into thenon-aqueous solution under study because thesolution is contaminated with water and theelectrolyte (usually KCl). To prevent this, thereference electrode should be in a separatecompartment and a salt bridge used for theion-conducting connection between theworking electrode and reference electrodecompartments (see Figure 2.3c). The tip of thesalt bridge, which is filled with the non-aqueouselectrolytes under study, is dipped into thenon-aqueous solution. When such aqueousreference electrodes are used, the liquidjunction potential between the aqueous andnon-aqueous solutions must be taken into

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account. To improve this situation, the IUPACCommission on Electrochemistry proposed thatthe Fc+/Fc couple should be measured in thesame system and the electrode potentialreported as values referred to the apparentstandard potential of the system. As for theother method, the same solvent as that of thesolution under study is used for an internalsolvent in the reference electrode. The Ag/Ag+

electrode is the most popular referenceelectrode used in non-aqueous solutions, and itcan be used in a variety of solvents (Figure2.2c).

Figure 2.3 Examples of voltammetric cells

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Although the reference electrodes introduced inthis section are commercially available, theycan also be made in the laboratory.

2.3 Auxiliary ElectrodesThe auxiliary electrode, often also called thecounter electrode, is an electrode used in athree-electrode electrochemical cell forvoltammetry in which the electrical currentpasses between the working and auxiliaryelectrodes. The auxiliary electrode thereforefunctions as a cathode whenever the workingelectrode is operating as an anode and viceversa. The auxiliary electrode often has asurface area much larger than that of theworking electrode to ensure that thehalf-reaction occurring at the auxiliaryelectrode can occur fast enough so as not tolimit the process at the working electrode [4].Platinum (e.g. Pt wire, Pt plate) is a goodmaterial for the auxiliary electrode due to itshigh stability.

2.4 Solvents and SupportingElectrolytesThe important factors for solvents involtammetry are the potential window, thesolubility of the substrate molecule under studyand physical-chemical properties such as donoror solvating properties [5]. Although water is

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often used as a solvent for voltammetry, manyorganic substrates are water-insoluble or onlysparingly soluble. In addition, the oxygen andhydrogen evolution reactions limit the potentialwindow when water is used as a solvent forvoltammetry. The voltammetric experimentsfor organic substrates are therefore usuallycarried out in polar organic solvents in whichthe supporting electrolyte can dissociate intoions. Acetonitrile (dielectric constant = 38) isone of the most frequently used solvents sinceits high upper and lower potential limits allow itto be used as a solvent for both electrochemicaloxidation and reduction reactions. Otherfrequently used solvents for electrochemicaloxidation are dichloromethane ( = 9),nitromethane ( = 37), propylene carbonate (= 64) and 1,2-dimethoxyethane ( = 3), whileother frequently used solvents forelectrochemical reduction areN,N-dimethylformamide(DMF, = 37),dimethylsulfoxide (DMSO, = 47),tetrahydrofuran (THF, = 7) and benzonitrile (

= 26). Although hexamethylphosphoramide(HMPA, = 30) is also a frequently usedsolvent for electrochemical reduction, extremecare must be used in handling it because of itstoxicity.

A supporting electrolyte for use in voltammetryshould fulfil the following conditions: (i) itshould be soluble in the solvent under studyand should dissociate into ions to give enough

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conductivity to the solution, (ii) it should beresistant to oxidation and reduction, and shouldgive a wide potential window, and (iii) it shouldnot have an unfavourable effect on the electrodereaction to be measured. In addition, theinteraction between dissociated ions andintermediates formed by electrode reactionsmust be taken into account. For organicsolvents, the commonly used electrolytes aretetraalkylammonium salts. In general,tetraethylammonim ion (Et4N+) and

tetrabutylammonium ion (Bu4N+) arefrequently used as the cation part oftetraalkylammonium salts, while perchlorateion (ClO4

−), tosylate ion (TsO−),

tetrafluoroborate ion (BF4−) and

hexafluorophosphate ion (PF6−) are frequently

used as the anion part. Because halide ion (X−)may be oxidized to form halonium ion (X+), it isnecessary to be careful when using it as theanion part of the supporting electrolytes. On theother hand, for aqueous systems, inorganicsalts such as NaCl and KCl, although nottetraalkylammonium salts, can be employed assupporting electrolytes. In practice, theconcentration of supporting electrolytes shouldpreferably be above 0.1 M.

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2.5 Cells and Power SourcesThe most important decision to be made inplanning voltammetry is whether to use anundivided cell, in which the working, auxiliaryand reference electrodes are immersed in asingle chamber (Figure 2.3a), or a divided cell,in which the working and auxiliary electrodesare in separate compartments (Figure 2.3b) [6].In making this decision, we have to considerinterference by electrolysis products at theauxiliary electrode in voltammetricexperiments. If this happens a divided cell mustbe used. When the aqueous reference electrode,such as an Ag/AgCl electrode or SCE, is used innon-aqueous systems the reference electrode isalso in a separate compartment in order toprevent contamination with water (Figure2.3c), as mentioned in section 2.2. In this case,a salt bridge is used for the ion-conductingconnection between the working electrode andreference electrode compartments, and the tipof the salt bridge is dipped into thenon-aqueous solution. In addition, the tipshould be as close as possible to the workingelectrode to minimize the iR drop between thetip and the electrode.

A potentiostat is used as a power supply and isfundamental to voltammetry usingthree-electrode systems [6]. In addition, a wavegenerator is necessary for the non-steady statemeasurements like cyclic voltammetry (CV).

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Instrumentation for CV, consisting of both thewave generator and potentiostat in a singlepackage, is available from a number ofmanufactures. The voltammograms obtainedare output to an X–Y recorder or PC.

2.6 Steady-State andNon-Steady-States PolarizationCurvesA plot of current density against electrodepotential under a set of constant operatingconditions, known as a polarization curve, is thestandard electrochemical technique forcharacterizing the electrode reactions [7]. Asteady-state polarization curve describes therelationship between the electrode potentialand the current density, which is recorded byholding the electrode potential and recordingthe stable current response. Under vigorousstirring, the stable current response can be alsoobtained by potential scanning measurementsand the diffusion limiting current is dependenton the potential scan rate. The voltammogramobtained in this case corresponds to thesteady-state polarization curve. Anon-steady-state polarization curve can beobtained at a rapid potential scan rate in theabsence of any convections. However, evenunder still conditions a steady-statepolarization curve may be obtained by using aslow potential scan since the influence of

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thermal convection cannot be ignored in thiscase.

To evaluate the limiting current density, asteady-state polarization curve is usuallymeasured and the diffusion coefficient (D),diffusion layer thickness (δ) or the number ofelectrons involved in the electrode reaction (n)can be derived from the measured currentdensity and Eq. 1.4.

In order to obtain mechanistic informationabout organic electrochemical reactions,non-steady-state polarization curves such ascyclic and linear sweep voltammograms areusually measured. CV is the most frequentlyused technique in organic electrochemistry, andhence we will discuss it in more detail below.

In CV, the potential of a stationary electrode ischanged linearly towards increasingly negativeor positive directions until the electrode processof interest, either a reduction or oxidation,respectively, takes place, and then the directionof the potential scan is reversed. Thisexperiment is capable of providing a great dealof useful information about the redox potentialof the studied organic molecules and therelative rates of electron transfer, masstransport and any chemical reactions takingplace at the electrode surface.

Let us now consider the very simple CV for theredox reaction of ferrocene (Fc). It is well

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known that the redox reaction of ferrocene isrepresented by Eq. 2.1.

(2.1)

Because the Fc is the reduction state in thisredox reaction, the potential should be sweptfirstly towards the positive direction (anodicscan). In this case, the potential scan shouldbegin from the initial potential where nocurrent flows (rest potential). In addition, thescan rate is generally set in the range between50 and 200 mV s−1. As mentioned in section2.2, by changing the potential to an electrodethe energy of the Fermi-level is also alteredintentionally. When the potential is swepttowards the positive direction from the restpotential (process a–b of Figure 2.4; theFermi-level moves to a lower energy), the rateof electron transfer from the HOMO offerrocene to the Fermi-level of the electrode isdramatically increased and the potential passesthrough a mixed region in which the rates ofmass transport and electron transfer both limit

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the current. When the electron transfer rate ishigh enough the currents are purely masstransfer limited, and a mass-transport-limitedvoltammetric peak is observed (Figure 2.4b).When the final potential is reached (Figure2.4c), the direction of the potential scan isreversed (process c–d of Figure 2.4). Since theFermi-level moves to a higher energy by doingthis reverse scan, electron transfer from theFermi-level to the HOMO of the ferroceniumion (Fc+) should occur (the current for Fc+

reduction is observed in this case.). When mostof the Fc+ in the vicinity of the electrode surfacehas been converted to Fc, the reduction currentis reduced (process d–a of Figure 2.4).

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Figure 2.4 Cyclic voltammogram of ferrocene(Fc) and Fermi-level (EF) within a metalelectrode along with the HOMO level of Fc insolution

The form of cyclic voltammogram for ferroceneredox represents a typical reversible process.The oxidation peak current ( ) in a reversibleprocess like a ferrocene redox reaction is:

(2.2)

where n is the number of electrons involved inthe electrode reaction (1 in the case of theferrocene redox reaction), cFc and DFc are thebulk concentration and the diffusion coefficientof ferrocene, respectively, and v is the potentialscan rate. Thus for a reversible CV wave, thepeak current is proportional to the bulkconcentration of a substrate or the square rootof the potential scan rate.

However, the reversible redox reaction isuncommon in organic electrochemistry sincemany organic electrochemical reactions involvea fast chemical reaction subsequent to electrontransfer (an EC process: the electrode step is Eand the chemical step is C). In this case, there-oxidation or re-reduction peak may totallydisappear in the reversal potential scan.

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2.7 Potentials in ElectrochemicalMeasurementsIn the articles and books dealing with organicelectrode reactions, there are many kinds ofpotentials [8,9]. The following six kinds ofpotentials are frequently used and hence we willdiscuss them:

standard electrode potential

formal potential

peak potential

half-peak potential

half-wave potential

decomposition potential (onset potential).

If we know the standard state free energychange, ΔGo, for a chemical process, we cancalculate the standard electrode potential, Eo,for an electrochemical reaction based on thatprocess using the relationship between ΔGo andEo. Thus the Eo is not the experimental valuebut the calculated value. Therefore, even if theelectrode potential is measured using the SHEunder an equivalent state, the value measuredwill deviate from the real Eo value to a smalldegree. The value measured using the SHEunder an equivalent state is termed the formal

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potential (Eo′). The relationship between Eo

and Eo′ is represented by Eq. 2.3:

(2.3)

where γO and γR are the activity coefficients forthe oxidized and reduced forms of a studiedspecies. However, the deviation between Eo andEo′ is very small in a dilute solution and isusually less than 300 mV even in aconcentrated solution.

The potential at the peak of CV curves is termedthe peak potential. For the reversible redoxreaction, the anodic peak potential (Epa) andcathodic peak potential (Epc) are independentof scan rate and concentration, therefore thepeak potentials supply information about theidentity of the substrate species and thethermodynamic index for the oxidation/reduction of the studied species. In addition,the average value of the anodic and cathodicpeak potentials corresponds to the formalreduction potential (Eo′) for a reversible couple(Figure 2.5).

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Figure 2.5 Cyclic voltammogram for areversible process

On the other hand, when a relatively fastchemical reaction subsequent to electrontransfer is involved in the electrode process, asindicated in Eqs. 2.4 and 2.5 (EC process), thepeak potential shifts depending on themagnitude of its rate constant (k) and the sizeof the back peak in the reverse scan should bedecreased. In this case the peak potentialscannot be used as the thermodynamic index forthe oxidation/reduction of the studied species.

(2.4)

(2.5)

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The above discussion concerns voltammogramsrecorded at a fixed potential scan rate.However, if the scan rate is altered we are likelyto observe a variation in the voltammogramsrecorded. For example, when cyclicvoltammograms are recorded at a significantlyfaster scan rate (more than 1 V s−1), a reversibleresponse in CV may be observed due to the timetaken to record the voltammogram. If thepotential scan rate is sufficiently fast it ispossible that no Ox formed according to Eq. 2.4has had time to chemically react while thevoltammogram is recorded. In this case, we canestimate a thermodynamic potential such as theformal potential from the obtained reversiblevoltammogram.

Although the half-peak potential (Ep/2) is oftenconfused with the half-wave potential (E1/2), itis defined as the potential where the current ishalf of the peak current in CV. In addition, therelationship between Ep and Ep/2 isrepresented by Eq. 2.6:

(2.6)

where n is the number of electrons involved inthe redox electrode reaction. However, Ep/2 isnot an important thermodynamic parameter,and hence it is not used very much.

The half-wave potential (E1/2) is the potential atwhich the wave current in steady-state

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voltammetry is equal to half of the diffusionlimiting current (Figure 2.6a). E1/2 is equal tothe formal potential of the studied electrodereaction when the reaction is a redox reversibleprocess and the diffusion coefficients of theoxidized and reduced forms of a studied speciesare equal to each other. However, if the reactioninvolves a chemical reaction subsequent toelectron transfer, E1/2 is no longer equal to theformal potential (Figure 2.6b).

Figure 2.6 Steady-state voltammograms of asimple redox system (a) without and (b) withfollowing chemical reaction

The decomposition potential (Edec) is apotential where the Faradic current begins to beobserved on the voltammogram and is alsocalled the onset potential (Eonset). AlthoughEdec is often used as the thermodynamic indexfor the oxidation/reduction of the studiedspecies, it is not exactly a thermodynamicparameter.

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2.8 Utilization of Voltammetry forthe Study of OrganicElectrosynthesis

2.8.1 Voltammetric Analysis forSelective ElectrosynthesisMany organic electrode processes are inprinciple multi-pathway, and therefore manyreaction products are usually obtained underconstant-current electrolysis. For example,constant-current electrolysis for the reductionof aromatic aldehydes or ketones under acidicconditions gives two products, such as alcoholsand pinacols, simultaneously (Eq. 2.7) [10,11].Consider linear sweep voltammetry (LSV) forthis reduction under mechanical stirring(Figure 2.7). It is known that the first wavecorresponds to one-electron reduction of theprotonated substrate to form the correspondingradical intermediate, while the second wavecorresponds to a second one-electron reductionof the radical intermediate to form the alcohol,as shown in Eq. 2.7. Therefore, when thepotential of the working electrode is maintainedat potential (known as theconstant-potential electrolysis), one-electronreduction of the carbonyl compound willproceed clearly to the radical intermediate andconsequently the pinacol product can beobtained selectively. If, on the other hand, thecathode is maintained at potential , a

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two-electron reduction as well as theone-electron reduction will occur, and amixture of two products is obtained.

(2.7)

Thus LSV in this process provides informationabout the reaction mechanism that allows us touse the process selectively in synthesis.

Figure 2.7 Linear sweep voltammogram forthe reduction of an aromatic carbonylcompound under mechanical stirring

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2.8.2 Clarification of the ReactionMechanismIn general, organic electrode reactions includeseveral varieties of coupled chemical reactions,and hence their mechanism is usually complexcompared to that for inorganic electrodereactions. When we undertake a new organicelectrode reaction it is therefore desirable toconduct CV measurements in advance in orderto understand the mechanism.

A CV experiment conducted by Michielli andElving is a good example for clarifying thecomplex reaction mechanism [12]. Theyemployed wave clipping and addition of aproton donor (phenol) to obtain informationabout intermediates in the electrochemicalreduction of benzophenone (1). As shown inFigure 2.8a, CV for the reduction gives twosuccessive cathodic peaks Ic and IIc at −1.8 and−2.0 V (relative to the Ag/AgNO3 referenceelectrode), and a single anodic peak Ia, as thepotential is scanned from 0 to −2.3 V and backto −1.0 V. Wave clipping, that is, reversal of thescan direction at a potential (−1.9 V) betweenpeaks Ic and IIc, causes peak Ia to remain, asshown in Figure 2.8b. This proves very nicelythat peak Ia is associated with oxidation of thespecies formed in the first reduction step (Ic)and reduction at peak Ic therefore forms aspecies that is long lived on the CV time scale.On the other hand, peak IIc must produce a

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very reactive species because it is totallyirreversible. Addition of increasing amounts ofphenol to the solution increased the height ofpeak Ic and diminished peaks IIc and Ia untilfinally peak Ic had doubled in size and peaks IIcand Ia were no longer evident.

Figure 2.8 Cyclic voltammograms for thereduction of benzophenone. (a) Potential isscanned from 0 to −2.3 V and back to −1.0 V.(b) Potential is scanned from 0 to −1.9 V andback to −1.0 V

These results are consistent with the followingmechanism:

(2.8)

(2.9)

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(2.10)

(2.11)

(2.12)

(2.13)

The couple Ic–Ia is associated with reversibleformation of benzophenone ketyl 2 (Eq. 2.8).Peak IIc should be associated with reduction of2 to the reactive dianion 3 (Eq. 2.9). In thepresence of phenol, 2 is protonated (Eq. 2.10),and the resulting radical 4 is then furtherreduced to form the monoanion 5 (Eq. 2.12).Finally 5 is protonated to form the alcohol 6(Eq. 2.13).

As we have seen in the above example, CVprovides preliminary information about themechanism of an unknown electrode reaction.

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To further clarify the mechanism, we can carryout the electrochemical reaction on a largeenough scale to permit the products andintermediates to be isolated and identified. CVmeasurements of the products andintermediates isolated by preparative scaleelectrolysis may also help us to understand themechanism of the studied electrochemicalreaction. Thus, both CV measurement andpreparative scale electrolysis are absolutelyessential to clarify the complex mechanism ofan unknown organic electrode reaction.

2.8.3 Voltammetry for Selection ofMediatorMost organic electrode reactions are carried outby direct electrolysis, that is, by electrontransfer between the organic substrate and theelectrode. On the other hand, by adding a redoxmediator to the medium it is possible to carryout electrochemical reactions even at apotential where the substance of interest iselectroinactive. In this system, as shown inFigure 2.9 (this is for the oxidation process, butexactly analogous behaviour can be observedfor the reduction process), electron interchangebetween the mediator MRed (catalyst) and theelectrode generates a substance MOx that can inturn undergo an electron interchange with thesubstrate molecule to give the product. Thiskind of system is called a mediated

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electrocatalytic reaction and the reactionbecomes an indirect electrolysis [13].

Figure 2.9 Principle of a redox mediatoryreaction. This is for an oxidation process

The features of a mediated electrocatalyticreaction are as follows:

A catalytic amount of mediator is enough tocarry out indirect electrolysis.

The substrate undergoes a redox conversionat a potential lower than that required toeffective its direct electrolysis.

Passivation of the electrode as a result of theformation of non-conductive polymersduring direct electrolysis can be avoided.

Highly selective and efficient redoxconversion can be carried out.

Frequently used mediators for indirectelectrolyses are multivalent metal ions, halideions, polycyclic aromatic hydrocarbons such asnaphthalene and anthracene, triarylamines andnitroxyl radicals. In addition, many mediators

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have been synthesized for specific indirectelectrolyses. In this situation CV measurementsare usually used to select a suitable mediator inthe studied electrochemical reaction.

Since the direct electrochemical oxidation ofalcohols to carbonyl compounds is carried outwith great difficulty, oxidation has been afavourite testing ground for new electrocatalyticsystems. Triarylamines are well known asexcellent mediators for the indirectelectrochemical oxidation of various alcohols.For example, tris-(p-bromophenyl)amineexhibits a typical reversible redox response inCV (Eo′ = 0.8 V vs Ag/AgNO3), as shown inFigure 2.10a [14]. On the other hand, by adding4-methoxybenzyl alcohol as a substrate, theanodic peak increases while the cathodic peakbecomes smaller or disappears (Figure 2.10b).This behaviour suggests thattris-(p-bromophenyl)amine can play the role ofmediator and direct electrochemical oxidationof the substrate can take place smoothly.However, when benzyl alcohol is added to thetris-(p-bromophenyl)amine solution suchmediatory behaviour is never observed in CVand a reversible redox response of the amine ismaintained because the oxidation potential ofbenzyl alcohol (Epa = 2.0 V vs Ag/AgNO3) ismuch higher than that of 4-methoxybenzylalcohol (Epa = 1.2 V vs Ag/AgNO3). To observemediatory behaviour for benzyl alcoholoxidation, a more powerful mediator such as

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tris-(2,4-dibromophenyl)amine (Eo′ = 1.2 V vsAg/AgNO3) is required. Thus CV study is apowerful tool for selecting a suitable mediatorin this indirect electrochemical reaction.

Figure 2.10 Cyclic voltammograms for theoxidation of tris-(p-bromophenyl)amine in the

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(a) absence and (b) presence of4-methoxybenzyl alcohol

2.8.4 Voltammetry for Selection ofElectrode MaterialAs mentioned in section 2.1, the upper andlower limits of the potential window aredetermined by the working electrode material,and hence the selection of a working electrodematerial is critical not only to experimentalsuccess for voltammetry but also smoothprogress of the desired preparative scaleelectrolysis.

Although the electrochemical oxidation of furanin methanol solution usually gives thecorresponding methoxylated product, as shownin Eq. 2.14, the desired oxidation proceedsinefficiently at the Pt electrode. On the otherhand, an excellent yield of product can beobtained by using a glassy carbon (GC) workingelectrode. The influence of the workingelectrode material on the efficiency of theelectrochemical reaction can also be confirmedby the CV study [15].

(2.14)

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Figures 2.11a and 2.11b show cyclicvoltammograms for the oxidation of furan inmethanol solution recorded at GC and Ptanodes, respectively. An oxidation peak forfuran can be observed at a very positivepotential of 1.9 V vs. SCE using the GC plate asthe anode. In contrast, when the Pt plate is usedas the anode material, methanol solvent isdischarged more easily, a high backgroundcurrent is recorded and therefore no oxidationpeak for the oxidation of furan is observed. Itcan therefore be stated from the CVexperiments that GC is the better anodematerial for the oxidation of furan.

Figure 2.11 Cyclic voltammograms for theoxidation of furan in methanol solutionrecorded at (a) a GC plate anode and (b) a Ptplate anode. Dashed curves indicate theresponse in the absence of furan

As we have seen in the above example,voltammograms measured in this processprovide preliminary information about a

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suitable electrode material that will allow us touse the process efficiently in synthesis.

References1. Bard, A.J. and Faulkner, L.R. (2001)Electrochemical Methods, Fundamentals andApplications, 2nd edn, John Wiley & Sons, Inc.,New York, Chapter 2.



4. Bard, A.J. and Faulkner, L.R. (2001)Electrochemical Methods, Fundamentals andApplications, 2nd edn, John Wiley & Sons, Inc.,New York, Chapter 1.



7. Lund, H. and Hammerich, O. (eds) (2001)Organic Electrochemistry, 4th edn, MarcelDekker, New York, Chapter 1.

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8. Fry, A.J. (1989) Synthetic OrganicElectrochemistry, John Wiley & Sons, NewYork.

9. Compton, R.G. and Banks, C. (2000)Understanding Voltammetry, World ScientificPublishing, Singapore.

10. Cheng, P.-C. and Nonaka, T. (1989) J.Electroanal. Chem., 269, 223–230.

11. Atobe, M., Matsuda, K. and Nonaka, T.(1996) Electroanalysis, 8, 784–788.

12. Michielli, R.F. and Elving, P.J. (1968) J.Am. Chem. Soc., 90, 1989–1995.


14. Brinkhaus, K.-H.G., Steckhan, E. andSchmidt, W. (1983) Acta Chem. Scand., B37,499–507.

15. Horii, D., Atobe, M., Fuchigami, T. andMarken, F. (2006) J. Electrochem. Soc., 153,D143–D147.

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2Method for Study of Organic Electrochemistry:Electrochemical Measurements of OrganicMolecules

83

Toshio Fuchigami

Organic electrosynthesis is generally affected bymore complicated factors than ordinary organicsynthesis therefore we must choose suitableelectrolytic cells (divided or undivided),electrolytic methods (constant current orconstant potential), electrodes, supportingelectrolytes, solvents and so on [1–5]. In thischapter, the detail is described so that evenbeginners will be able to carry out organicelectrosynthesis.

3.1 Selection of Electrolytic CellsA proper choice of cell design is important inperforming the desired electrolytic reaction.Organic electrolytic reactions are achieved on alaboratory scale by using an undivided cell. Thesimplest cell design is shown in Figure 3.1, but acylindrical cell, shown in Figure 3.2, is therecommended design when anhydrousconditions or electrolysis under an inert gasatmosphere, like nitrogen, is required. In aninert gas atmosphere the solvent and substrateare injected by syringe into the cell through arubber septum.

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Figure 3.1 Beaker-type cell

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Figure 3.2 Undivided cell for anhydrouselectrolysis

When a species reduced at a cathode is oxidizedat an anode, and vice versa, a two-compartmentcell, namely a divided cell with a diaphragm(sintered glass or an ion-exchange membrane),should be used to prevent mixing of the anodicand cathodic solutions. An H-type cell dividedwith a sintered glass diaphragm (pore size φ =5–10 μm) is convenient, as shown in Figure 3.3.A suitable volume for each compartment is10–200 cm3, and the diameter of thediaphragm should be as large as possible todecrease the cell resistance. When startingsubstrate and/or products migrate to acounter-compartment through a sintered glass

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diaphragm, an ion-exchange membrane shouldbe used instead of the diaphragm. In this case, aseparable cell in which the membrane issandwiched between the compartments usingscrews is recommended. Such separable dividedcells are commercially available. When adiaphragm is used, the cell resistance usuallyincreases and consequently the cell voltage (thevoltage between the anode and cathode)increases. To decrease the cell voltage, thedistance between the electrodes should be keptas small as possible.

Figure 3.3 H-type cell with glass filterdiaphragm

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Even when a divided cell is necessary forcathodic reduction, we can achieve the desiredreduction using an undivided cell as follows.Use of sacrificial anodes like Mg and Zn oraddition of sacrificial organic compounds likeoxalic acid and acetic acid to the electrolyticsolution enables re-oxidation of cathodicproducts to be avoided. In the former case theanode metals are dissolved as metal ions in thesolution during the electrolysis, while in thelatter case the acids are anodically oxidized toform CO2 and ethane. Such sacrificialelectrolytic systems enable oxidation ofcathodic products and the starting material atthe anode to be avoided. On the other hand, inthe case of anodic oxidation requiring a dividedcell, a proton source should be added as asacrificial substance into the electrolyticsolution and cathode materials with lowhydrogen overpotential should be used. In thiscase, proton reduction at the cathode takesplace preferentially, which enables thereduction of anodic products and the startingmaterial at the cathode to be avoided.Consequently, we can achieve the desiredanodic oxidation selectively even in anundivided cell.

Crucibles made of conducting materials likegraphite and glassy carbon can be used asworking electrodes. In this case, the volume ofelectrolytic solution can be reduced and the

88

surface area of the working electrode can beincreased.

For scaling up for commercialization a filterpress type flow cell is recommended, as shownin Figure 3.4. In this case, the volume of theelectrolytic solution has no limit since thesolution is circulated by pump, and masstransport from the bulk to the surface of theworking electrode is promoted to increase theefficiency of the electrolytic reaction.

Figure 3.4 Filter press type flow cell

3.2 Constant Current Electrolysisand Constant PotentialElectrolysisIt is recommended that organic electrosynthesisbe carried out at a constant current at firstbecause the setup of the electrolytic system andthe operation of the power supply are simple.The product selectivity and yield can be

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improved by controlling current density and theamount of electricity passed. Since currentdensity is correlated to applied potential,changing the current density creates a potentialshift. The current density should be selecteddepending on the concentration of the startingsubstrate. When the concentration of thesubstrate is low, the current density should below and vice versa. The electricity passed isreadily calculated according to the followingequation: current (A) × time (s) = electricity(C). For instance, when the desired electrolyticreaction is a two-electron reaction, thetheoretical amount of electricity is 2 F (2 ×96480 C). When this electricity is divided byapplied current (A), one can easily calculate forhow many hours the electrolysis must becarried out. As shown in Figure 3.5a, theelectrode potential changes with theconsumption of the starting substrate (positiveshift in case of oxidation or negative shift incase of reduction), therefore the productselectivity and current efficiency sometimesdecrease, particularly in the late stage ofelectrolysis. However, highly selective andefficient organic electrosynthesis can often beachieved even at constant current electrolysis,hence commercialized electrode processes areoperated mainly by constant currentelectrolysis. Nevertheless, a constant potentialelectrolysis is suitable for achieving highselectivity and clarification of the reactionmechanisms. Moreover, based on constant

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potential coulometry, one can easily find thenumber of electrons involved in the electrolyticreaction. In order to carry out constantpotential electrolysis, the oxidation potential orreduction potential of the substrate should bemeasured in advance, but one can estimate theapplied potential at which rapid currentincrease is observed from a current-potentialcurve of the substrate. As shown in Figure 3.6, asalt bridge terminated either by a Luggincapillary or a plug of porous Vycor glass isplaced closed to the working electrode, and anappropriate constant potential relative to areference electrode such as an aqueous SCE isapplied using a potentiostat. The amount ofelectricity passed is measured by a coulometer.

Figure 3.5 Profile showing electrolysis timeduring constant current electrolysis (a) andconstant potential electrolysis (b)

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Figure 3.6 Setup for constant potentialelectrolysis

3.3 Direct Electrolysis andIndirect ElectrolysisElectrolysis is classified as direct electrolysis orindirect electrolysis. The former is based ondirect electron transfer between substratemolecule and electrode, which is a simple andcommon electrolytic method. The latter isbased on electron transfer using redoxmediators dissolved in an electrolytic solvent,as shown in Figure 3.7 (indirect cathodicreduction). (Note that indirect anodic oxidationis covered in Chapter 2, Figure 2.9.) Indirectelectrolysis is also classified into in-cell, whereelectrolysis is carried out in the presence of

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both substrate and mediator, and ex-cell, whereelectrolysis is used for only regeneration of themediator. In the former case, the redoxpotential of the mediator should be lower thanthat of the substrate. When the redox potentialof the mediator is higher than that of thesubstrate, the latter type is employed. In theformer case a catalytic amount of mediatorshould be enough, while in the latter case aquantitative or excess amount of mediator isnecessary. When the heterogeneous electrontransfer between mediator and electrode, aswell as the redox reaction with the substrate, isfast enough (Figure 3.7b), a significantlyenhanced catalytic current can be obtained dueto decreased electrolysis potential (decreasedactivation energy), as shown in Figure 3.8. Inother word, a large electrocatalytic current canbe obtained.

Figure 3.7 Principle of indirect electrolysisusing a mediator

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Figure 3.8 Current potential curve for directand indirect electrolysis

The characteristics and proper choice ofmediators and their synthetic applications aredescribed in Chapters 2 and 4, respectively.Many types of mediators are available and theredox potentials of mediators can be tuned bytheir molecular design. Moreover, somemediators can be applied to electrochemicalasymmetric synthesis [6–9].

3.4 Electrode Materials andReference ElectrodesThe choice of electrode material is one of themost important factors in electrolysis since thematerial is not only the interface for electrontransfer with the substrate molecule but alsoacts as an electrocatalyst. If the correct

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electrode material is not chosen, the desiredelectrolytic reaction does not proceed, thereforea suitable electrode material must be selectedcarefully. In an aqueous solution, oxygen gasand hydrogen gas are generated competitivelydue to discharge of water during the electrolyticreaction. In order to avoid such undesirablereactions, cathode materials with highhydrogen overpotential or anode materials withhigh oxygen overpotential should be used. Theorder of oxygen overpotential is Au > Pt, Pd,Cd, Ag > PbO2 > Cu > Fe > Co > Ni, and theorder of hydrogen overvoltage is Hg > Zn, Pb,Cd > graphite > Cu > Fe, Ni > Ag, Co > Pt, Pd. Amercury cathode was very often used forelectroreductive organic synthesis, but it is nolonger used except for electrochemicalmeasurements because of its toxicity. Aplatinum electrode is the first choice in aproticorganic solvents. Although various kinds ofcathode materials are available, usable anodematerials are limited. Most metals are easilyoxidized apart from noble metals like platinumand gold. Carbon, graphite and metal oxideslike PbO2 are commonly used as anodes.Carbon electrodes generally include a traceamount of metals like iron, therefore theirsurface is paramagnetic. Hence, such carbonanodes easily capture anodically generatedradicals to enhance further oxidation to formcationic intermediates. On the other hand,platinum anodes generally tend to generate

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radical intermediates selectively. Accordingly,platinum anodes are most suitable for Kolbeelectrolysis. It should be noted that when acarbon anode is used in a solution containingLiClO4, collapse of the electrode surface oftentakes place due to the oxidation of binder suchas coal tar pitch in the carbon electrode. Insharp contrast, glassy carbon (GC) electrodeshave enough durability for anodic oxidation,and anode passivation does not occur, which isquite different from other carbon electrodes.However, GC electrodes are costly, as areplatinum electrodes. Althoughnon-precious-metal electrodes are not suitablefor anode materials, Mg, Zn, Al and Cu areoften used as reactive electrodes or sacrificialanodes, as explained in section 3.1.

The following points should be noted whenorganic electrosynthesis is carried out. Incontrast to electrochemical measurements,electrochemical synthesis requires a relativelylarge current of 10–100 mA (∼10 mA cm−2)even on a laboratory scale. The use of a largerelectrode surface area is thereforerecommended to complete the electrolysis in ashorter time. As working electrode shape, plate,candy stick (baculiform) and mesh type arecommonly used. The surface area of the counterelectrode should be same as that of the workingelectrode. Depending on the kind of electrodes,pre-treatment of the electrode surface differs,

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but in general the electrode surface is polishedwith alumina abrasive or emery papers.

Reference electrodes are required whenmeasurement of current potential curves andconstant potential electrolysis are carried out.When aqueous and protic solvents are used aselectrolytic solvents, a SCE (Figure 2.2a) and asilver–silver chloride (Ag/AgCl) electrode(Figure 2.2b) should be used. For electrolysis innon-aqueous solution, an Ag/Ag+ referenceelectrode is convenient since it can be placednear the working electrode surface without asalt bridge. Reference electrodes are describedin further detail in Chapter 2.

3.5 Electrolytic Solvents andSupporting ElectrolytesElectrolytic solvents must have polarity, whichallows supporting electrolytes to be dissolved todissociate ions and provide sufficient ionconductivity for the electrolytic solutions. Theindex of solvent polarity is reflected by itsdielectric constant, and electrolytic solventsshould have low viscosity and toxicity. Inparticular, it should be noted that solventviscosity strongly affects diffusion of substratefrom bulk to electrode surface.

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3.6 StirringSince electrolytic reactions are typicallyheterogeneous, stirring of an electrolyticsolution controls mass transport of substrate,which greatly affects current efficiency andproduct selectivity. Even if a solution isvigorously stirred, the transport of the substratefrom bulk to the electrode surface is not alwaysenhanced. As shown in Figure 3.4, circulationof an electrolytic solution through a flow cell,particularly a filter press type cell, is muchmore efficient compared to a conventionalbeaker type cell, hence the filter press type cellis usually employed in industrial electrolysis.

3.7 Tracking of Reactant andProductEfficient conversion of substrate to desiredproduct is very important in organic synthesis.In the case of constant current electrolysis,discharge of solvent and/or supportingelectrolyte takes place simultaneously at a laterstage of the electrolysis. A theoretical amount ofcharge passed is not always enough forcomplete substrate conversion, hencemonitoring electrolysis (consumption ofstarting substrate and amount of product)should be carried out using TLC, GC, HPLC ormass spectrometry (MS) to confirm substrateconversion.

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3.8 Work-Up, Isolation andDetermination of ProductsWork-up of electrolysis is similar to ordinaryorganic synthesis, but the following pointsshould be noted.

Since the anolyte becomes acidic and thecatholyte becomes basic after electrolysisusing a divided cell, neutralization of thedesired electrolytic solution should be done.

Since a large amount of supportingelectrolyte is contained in the electrolyticsolution, its removal should be done first.For example, when the electrolytic solvent iswater soluble, the solvent must be removedby evaporation under reduced pressure. Theremaining residue or water-insolubleelectrolytic solution is mixed with water orbrine (saturated NaCl aqueous solution), ifnecessary, neutralization is performed andthe product is extracted with appropriateorganic solvents like diethyl ether. It shouldbe noted that some hydrophobic supportingelectrolytes like Bu4NBF4 are insoluble inwater, but soluble in organic solvents likeCH2Cl2.

Isolation and identification techniques aresimilar to those in ordinary organic synthesis.Product isolation is carried out usingdistillation, recrystallization or various types ofchromatography, and identification is

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performed using various spectroscopic analysismethods such as 1H NMR, IR and MS.

3.9 Current Efficiency and Effectof the Power UnitCurrent efficiency is the most important factorfor evaluating the results of electrolyticreactions. For instance, consider the followingtwo-electron reduction of acetone:

(3.1)

Electricity of 2 × 96,480 C is required for theformation of 1 mol of isopropyl alcohol. When nmol of isopropyl alcohol is formed from 1 mol ofacetone after passing Q coulombs, the currentefficiency (%) is (96,480 × 2n/Q) × 100.Current efficiency is usually below 100% sincethe solvent and/or supporting salt aredischarged simultaneously during theelectrolytic reaction.

The energy consumption for electrosynthesis isusually disregarded in basic research, but it isthe most important factor in industrialelectrolytic processes. The energy consumptionfor the production of the desired organiccompound is shown as kWh kg−1, which greatlydepends on cell voltage, therefore it is

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recommended that the distance betweenelectrodes should be kept as small as possible todecrease cell resistance.

References1. Bard, A.J. and Stratmann, M. (eds) (2007)Electrochemistry applied to organic synthesis:Principles and main achievements, inEncyclopedia of Electrochemistry, Vol. 5 (edsD. D. Macdonald and P. Schmuki), Wiley-VCHVerlag GmbH, Weinheim, Chapter 6.

2. Bard, A.J. and Stratmann, M. (eds) (2002)Methods to investigate mechanisms ofelectroorganic reactions (Chapter 1) andPractical aspects of preparative scaleelectrolysis (Chapter 2), in Encyclopedia ofElectrochemistry, Vol. 8 (ed. H. J. Schäfer),Wiley-VCH Verlag GmbH, Weinheim.

3. (a) Lund, H. and Hammerich, O. (eds)(2001) Organic Electrochemistry, 4th edn,Marcel Dekker, Inc., New York, Chapters 1 and2. (b) Hammerich, O. and Speiser, B. (eds)(2014) Organic Electrochemistry, 5th edn,CRC/Taylor & Francis.

4. Grimshaw, J. (2000) ElectrochemicalReactions and Mechanisms in OrganicChemistry, Elsevier, Amsterdam.

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5. Fry, A.J. (1989) Synthetic OrganicElectrochemistry, Wiley Interscience, NewYork.

6. Tanaka, H., Kawakami, Y., Goto, K. andKuroboshi, M. (2001) Tetrahedron Lett., 42,445–448.

7. Kashiwagi, Y., Kurashima, F., Chiba, S.,Anzai, J., Osa, T. and Bobitt, T.M. (2003)Chem. Commun., 1124–1125.

8. Shiigi, H., Tanaka, H., Demizu, Y. andOnomura, O. (2008) Tetrahedron Lett., 49,5247–5251.

9. Demizu, Y., Shiigi, H., Mori, H., Matsumoto,K. and Onomura, O. (2008) TetrahedronAsymmetry, 19, 2659–2665.

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3Methods for Organic Electrosynthesis

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Toshio Fuchigami

In this chapter, the general features ofelectrolytic reactions and differences betweenelectrolytic reactions and ordinary chemicalones will be explained. Typical characteristicssuch as umpolung (polarity inversion) andselectivity control together with electrontransfer control are discussed. Reaction typesand various electrochemically generatedreactive species will also be explained [1–6].

4.1 General Characteristics ofElectrode ReactionsOrganic electrode reactions have the followingfeatures, many of which cannot be achieved byother methods:

Electrode reactions are typicallyheterogeneous reactions, and the reactionfields are specific since oxidation andreduction take place separately at differentfields.

Umpolung (polarity inversion) is readilyperformed without the use of any reagents.

The selectivity of electrode reactions is oftendifferent from that of ordinary organicreactions.

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Since electrons are used as a reagent, theuse of hazardous reagents can be avoided,i.e. electrode reactions are low-emissionprocesses.

Electrode reactions proceed under mildconditions such as room temperature andnormal pressure.

Electrode reactions can be started orstopped readily by on-off switch of thepower supply, i.e. electrode reaction controlis easy.

The scale effect is generally small.

Electrode reactions are redox reactions throughelectron transfer between a substrate moleculeand an electrode. The main reaction field is anelectrode surface (solid–liquid interface) ornear the electrode surface, and the surface hasan extremely large electrical field, which is quitedifferent from ordinary redox reactions onheterogeneous catalysts. Electrode reactionstherefore take place in highly unique fields.

Using a chemical redox reaction as an example,the difference between an ordinary chemicalreaction and an electrode reaction will beexplained in detail. Figure 4.1a shows thereduction of substrate B by reducing reagent A.When an activated complex is formed or A andB approach each other closely enough forelectron transfer, electron transfer from A to Btakes place. Next, reductant A is transformed to

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oxidized C while substrate B is transformed toreduced product D. The former electrontransfer through the complex is calledinner-sphere electron transfer or bondedelectron transfer, while the latter one withoutany complex is called outer-sphere electrontransfer or non-bonded electron transfer. Thus,both oxidation and reduction occur at the sameplace in the case of an ordinary chemicalreaction, while oxidation and reduction occur atdifferent places, such as an anode and cathode,respectively, in the case of an electrochemicalreaction (Figure 4.1b). In other words, electrodeelectron transfer takes place separately due tothe existence of the electrode interfaces, whichis a significant characteristic feature that isdifferent from ordinary chemical reactions.

Figure 4.1 Difference between chemicalreaction (a) and electrochemical reaction (b)

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4.2 Mechanism of OrganicElectrode ReactionsIn the case of organic electrode reactions,electron transfer generally does not take placecleanly, and pre and/or post reactions usuallyaccompany the transfer. This is quite differentfrom inorganic electrode reactions. An organicelectrode reaction consists of an electrontransfer step as well as several chemical andphysical steps.

Figure 4.2 illustrates each elementary reactionstep of substrate S forming product P viaintermediate I. In step (a), mass transport of Sfrom the bulk of an electrolytic solution to theelectrode surface takes place by diffusion ormigration. In step (b), pre-reactions such asdesolvation, dissociation and/or deprotonationof S take place to form intermediate I.However, such pre-reactions do not always takeplace. In step (c), the intermediate I adsorbs onthe surface of the electrode to formintermediate Iad. In step (d), electron transferbetween Iad and an electrode generatesintermediate I′ad. In step (e), desorption of I′adfollowed by subsequent chemical reaction (f)proceeds to provide a product P that diffuses tothe bulk of the electrolytic solution, and thenthe sequential reaction is completed.Intermediate I may undergo an electrontransfer reaction without an adsorption step (c)and also the order of sequential steps (e) and (f)

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may be reversed, i.e. I′ad undergoes subsequentreaction and then desorption of the resultingproduct occurs. Thus, the electrode reaction istypical in a heterogeneous system, and masstransfer steps (a) and (g) as well as adsorptionand desorption steps (c) and (e) are involved,which is quite different from homogeneousreactions.

Figure 4.2 Elementary processes of electrodereactions

If the electrode process (electron transferprocess) is abbreviated to E and the chemicalprocess is abbreviated to C, the organicelectrode reaction can be shown using theseabbreviations. For example, the electrochemical

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reaction illustrated in Figure 4.2 can be shownby the sequence CEC (pre-chemical reaction →electrode process → follow-up chemicalreaction), and adsorption and desorption stepsare usually disregarded unless they areimportant.

In order to clarify the reaction mechanism,electrochemical analyses such as coulometryand voltammetry in addition to ordinaryorganic mechanistic studies are necessary toobtain information such as the number ofelectrons transferred, redox potentials anddetection of the reaction intermediates. Thedetails of the electrochemical analyses aredescribed in Chapters 2 and 3.

4.3 Characteristics of OrganicElectrolytic Reactions

4.3.1 UmpolungThe polarity inversion of chemical bonding canbe readily carried out in electrolytic reactions[6]. In other words, electrophiles can beconverted electrochemically to nucleophileswithout use of any reagents, and vice versa.Such polarity inversion is widely used fororganic synthesis. For example, alkyl halidesare inherently electrophilic reagents. In order toconvert them to nucleophilic reagents, theyhave to be transformed to Grignard reagents orlithium compounds using Mg or Li metal.

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However, such polarity inversion (umpolung)can be readily achieved in one step by cathodicreduction of the alkyl halides, as shown in Eq.4.1. When the basic products are isolated afterGrignard reaction, the work up can betroublesome, and it may be rather difficult toseparate the product from aqueous alkalinesolution due to insoluble Mg(OH)2. In sharpcontrast, in the case of electrolytic reactions,the product isolation is rather easy and thesevere waste problem does not occur becausethere is no use of metals. In addition,electrolytic reactions do not require easilyflammable ethereal solvents, and alternativesolvents like acetonitrile can be used aselectrolytic solvents.

(4.1)

When alkyl substituents are introduced tophosphine, silicon and stannum compounds,their corresponding chloro compounds (R3ZCl,R2PCl) and alkyl Grignard reagents are usuallyused. However, electrochemical reduction ofthese chloro compounds generates the anionicintermediates, which can readily react withalkyl halides (R′X) to provide thecorresponding alkyl-substituted products (Eq.4.2).

(4.2)

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Allylsilanes usually react with electrophiles, buttheir electrochemical oxidation generates allyliccations, which react with nucleophiles as shownin Eq. 4.3.

(4.3)

Electron-rich benzene derivatives usually reactwith electrophiles, while they can react withnucleophiles by electrochemical oxidation (Eq.4.4).

(4.4)

Furthermore, industrialized electroreductivehydrodimerization of acrylonitrile, shown inEq. 4.5, is also a typical example of umpolungusing cathodic reduction.

(4.5)

4.3.2 SelectivityThe selectivity of electrolytic reactions is rathercomplicated since it is controlled by many

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factors, such as electrode materials, appliedpotential, current density, electrolytic solvents,supporting electrolytes, electric field,adsorption orientation of substrate orintermediate species at the electrode surfaceand so on [4–7]. Heterogeneous electrolyticreactions therefore often exhibit differentselectivity from ordinary homogeneouschemical reactions. In particular, when anelectrochemically generated reactive species orintermediate reacts with reagents before itdiffuses from the electrode into the solution,the stereo- and regioselectivities of the productare often quite different from those of ordinarychemical reactions. Typical examples of theselectivity of electrode reactions are describedin the following sections.

4.3.2.1 Chemoselectivity

Chemoselectivity in ordinary chemical reactionsis controlled by the choice of reagents, but it isquite difficult to achieve high chemoselectivitywhen multiple similar functional groups exist ina single molecule. On the other hand,chemoselectivity can be achieved by the controlof applied potential based on the differencebetween the redox potentials of functionalgroups. Since the applied potential is adjustedprecisely using a potentiostat, high selectivitycan be readily achieved. For instance, eventhough phenylimino and alkylimino groupsexist in the same molecule, as shown in Eq. 4.6,

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the phenylimino group can be predominantlyreduced by constant potential electrolysis. Thisis because the phenylimino group is more easilyreduced than the alkylimino group as a result ofthe electron-withdrawing phenyl group.However, such selective reduction cannot beachieved by ordinary reducing reagents.

(4.6)

Similarly, in the case of the molecule with threehalogen atoms shown in Eq. 4.7, the halogenatom at the α-position to the carbonyl group ismost easily reducible, thereby this halogen canbe predominantly reduced at constant potentialelectrolysis.

(4.7)

4.3.2.2 Reaction Pathway Selectivity

It is known that the reaction pathway is greatlychanged depending on applied potentials. Asshown in Eq. 4.8, one-electron andtwo-electron reduction products are obtained

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selectively depending on the applied cathodepotentials.

(4.8)

4.3.2.3 Regioselectivity

(a) Kinetically Controlled andThermodynamically ControlledRegioselectivity

Regioselectivity is often controlled by either thestability of a reactive intermediate(thermodynamic control) or the reaction rate ofan intermediate with a reagent as well as theelimination rate of a leaving group (kineticcontrol). As a typical example of the former, it iswell known that anodic benzylic substitutionseasily take place and are attributable to a stablebenzylic cation intermediate.

On the other hand, whenN,N-dimethylbezylamine is anodically oxidizedin methanol, a methoxy group is selectivelyintroduced to the methyl group, as shown inFigure 4.3 [8]. This regioselectivity is notcontrolled by the stability of the cationicintermediate, i.e. this reaction is not

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thermodynamically controlled. Since theanodically generated radical cationintermediate seems to be adsorbed on theanode, deprotonation takes place preferentiallyfrom a less hindered methyl group.Consequently, this reaction is kineticallycontrolled.

Figure 4.3 Regioselective anodicmethoxylation

In a similar manner, anodic methoxylation ofN-ethyl-N-methylaniline, the correspondingcarbamate, and amide derivatives also takesplace at the methyl group selectively (Eq. 4.9)[9].

(4.9)

However, the regioselectivity can also beexplained by the difference in deprotonationrates of radical cation intermediates (so-calledkinetic acidity [10]). For example, in the case ofanodic methoxylation of an aniline derivative

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having a N-fluoroalkyl (Rf) group and anN-alkyl group, as shown in Figure 4.4,methoxylation takes place at the adjacentposition to the Rf group preferentially. Theregioselectivity increases with an increase in theelectron-withdrawing ability of the Rf group,i.e. the selectivity increases in the followingorder: CH3 < CH2F < CHF2 < CF3 [9]. Themechanism for this regioselectivity outcome iscalled kinetic control.

Figure 4.4 Mechanism of regioselective anodicmethoxylation (kinetic control)

The regioselectivity of electrolytic fluorinationis also controlled by kinetic acidity. Forexample, in the case of electrochemicalfluorination of heterocyclic compounds, asshown in Eq. 4.10, fluorination proceedspredominantly via the unstable cationintermediate adjacent to the carbonyl grouprather than via the stable benzylic cation. Thiscan be explained in terms of enhanced faciledeprotonation of the anodically generated

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radical cation by an electron-withdrawingcarbonyl group [10–12].

(4.10)

(b) Regioselectivity Controlled by SelectiveAdsorption of a Substrate to an Electrodedue to its Dipole Moment

As shown in Figure 4.5, tetrachloropicolinicacid is regioselectively dechlorinated bycathodic reduction. The high regioselectivity isattributed to the controlled orientation of thesubstrate at the cathode surface owing to thedipole moment of the molecule [13].

Figure 4.5 Regioselective cathodicdechlorination

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4.3.2.4 Stereoselectivity

(a) Stereoselectivity Controlled by SelectiveAdsorption of Reactive Intermediate toElectrode

As shown in Figure 4.6, the lithium salt of anunsaturated amine derivative adsorbs on theanode due to coulombic interactions.One-electron transfer then takes place togenerate the corresponding aminyl radicalintermediate, which undergoes subsequentintramolecular cyclization in an adsorbedmanner or near the anode to result inpredominant formation of thethermodynamically less stable cis-form product[14]. Interestingly, in this reaction thethermodynamically favoured trans-formproduct is not generated at all.

Figure 4.6 Stereoselective anodic cyclization

On the other hand, a similar reaction using achemical oxidant such as HgCl2 providesmainly the thermodynamically favourable transproduct. Such high stereoselectivity inelectrochemical reactions is mainly attributableto the adsorption effect. Thus, it can be statedthat the electrode contributes greatly to

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stereocontrol. In the cases of cathodichydrogenation and anodic addition of cyclicolefins, cis-form products are preferentiallyformed. This is also attributable to addition ofprotons and nucleophiles to the anionic andcationic intermediates adsorbed on theelectrode, respectively.

(b) Stereoselectivity Controlled by StericHindrance between Substrate and Electrode

Simple stereocontrol is achieved by sterichindrance between substrate and electrode. Aswe can easily see in Figure 4.7, the orientationof bicyclic gem-dibromocyclopropane at thecathode surface as shown in (a) is favoured overthat of the substrate shown in (b) owing tosteric repulsion between the cathode and the 6-or 8-membered ring fused to cyclopropane.Therefore, the exo-bromide near the cathode ismore easily reducible than the endo-bromide,and consequently the endo-bromide product ismainly formed [15].

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Figure 4.7 Stereoselective reduction based onorientation of substrate on the cathode

4.3.2.5 Selectivity Depending on ElectrodeMaterials

There are many examples of electrode materialsthat greatly affect product selectivity andstereoselectivity.

(a) Product Selectivity

Acetone is reduced at a lead cathode in anacidic aqueous solution to give thecorresponding alcohol, isopropyl alcohol, whilethe reduction with zinc and copper cathodesprovides the corresponding alkane, propane[16].

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(b) Anode Material Dependence of KolbeElectrolysis

It is well known that Kolbe electrolysis with aplatinum anode generates a radicalintermediate, while that with a carbon anodegenerates a cationic intermediate [1].

(c) Stereoselectivity

The reduction of 2-methylcyclohexanone at zincand copper cathodes in aqueous NaOH solutionprovides mainly the correspondingtrans-alcohol, while that with a tin cathodeprovides cis-alcohol as a major product [17].

4.4 Molecular Orbitals andElectrons Related to ElectronTransferThe electrons involved in electrode electrontransfer are not characteristic of electrolysis.From the viewpoint of molecular orbitals, theoxidation process can be explained by electrontransfer from the highest occupied molecularorbital (HOMO) of a substrate molecule to theanode, while the reduction process is explainedby electron transfer from the cathode to thelowest unoccupied molecular orbital (LUMO) ofa substrate, as shown in Figure 4.8 [18].

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Figure 4.8 Molecular-orbital diagram forelectron transfer

In the case of a hydrocarbon, electron transfertakes place from unsaturated bonds andσ-bonds of strained compounds likecyclopropane or ionic species. However,ordinary saturated hydrocarbons rarelyundergo redox reactions. On the other hand,even in the cases of saturated heteroatomcompounds containing heteroatoms like N, Sand O atoms, their oxidation is rather easysince electron transfer from lone pairedelectrons of a heteroatom readily takes place.Furthermore, depending on the heteroatom (Z),C–Z and Z–Z bonds can be reductively cleaved.Isolated olefins are generally difficult to oxidize,but olefins with a directly attached heteroatom,for instance enamines, enol ethers and silyl enolethers (Figure 4.9), are electron-rich olefins,therefore their oxidation potentials decreasesignificantly and they are easily oxidized.Moreover, heteroatoms greatly contribute to

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the stability of electrochemically generatedadjacent reactive intermediates, as shown inFigure 4.10. These effects are importantcharacteristic properties of heteroatomcompounds.

Figure 4.9 Easily oxidizable electron-richolefins

Figure 4.10 Stabilization effect of heteroatomson electrogenerated reactive species

4.5 ElectroauxiliariesElectroauxiliaries are functional groups thatfacilitate electron transfer and control thereaction pathways of electrogenerated reactivespecies to provide the desired productsselectively. Synthetic applications ofelectroauxiliaries in anodic oxidation arewidespread while those for cathodic reductionare rare [18].

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4.5.1 Electroauxiliaries Based onMolecular Orbital InteractionsThe orbital interaction is highly effective inincreasing the HOMO level. In principle, theinteraction of the HOMO with a high-energyfilled orbital increases its energy level. Forinstance, the energy level of a C–Si σ orbital isgenerally much higher than that of C–H andC–C σ orbitals therefore the C–Si σ orbitalinteracts effectively with a non-bonding porbital of a heteroatom like N, O or S (σ–ninteraction) if two orbitals align in the sameplane (Figure 4.11) [18,19]. Accordingly, a silylgroup at the α position activates a heteroatomcompound towards anodic oxidation.One-electron oxidation gives the radical cation,in which the C–Si σ orbital interacts with ahalf-vacant p orbital of the heteroatom tostabilize the system. Such interaction alsoweakens the C–Si bond, and hence the C–Sibond is cleaved selectively. The resulting carbonradical undergoes further oxidation to give thecarbocation, which is trapped by a nucleophileto give the desired product (Eq. 4.11). Thus, thesilyl group not only activates substrates towardsoxidation but also controls the reactionpathway [18,19].

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(4.11)

Figure 4.11 Interaction between the C–Si sorbital and the non-bonding 2p orbital of anoxygen atom to increase the HOMO level

In general, the electroauxiliary effect of a silylgroup increases S < N < O, and the oxidationpotential of a silyl compound with an αheteroatom decreases in the same order, asshown in Figure 4.12. In other words, themagnitude of the silicon effect for oxygencompounds is much greater than that fororgano nitrogen and sulfur compounds. This is

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in accord with the better overlap of thenon-bonding p orbital of oxygen with the C–Siσ orbital than that of the orbitals of nitrogenand sulfur [18,20,21].

Figure 4.12 Example of decrease in oxidationpotentials owing to molecular orbitalinteraction

The interaction of the C–Si σ orbital is alsoeffective for raising the energy level of theadjacent π systems (σ–π interaction), and theC–Si bond is cleaved selectively. The silyl grouptherefore serves as an electroauxiliary for theoxidation of π systems. A stannyl group alsoserves as an electroauxiliary for theelectrochemical oxidation of heteroatomcompounds and π systems [18].

The anodic cyanation of N-benzyl-substitutedpiperizines usually provides a mixture of two

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regioisomeric products. However, theintroduction of a silyl group as anelectroauxiliary directs the reaction pathway toform a single product, as shown in Eq. 4.12[22]. The silyl group also decreases theoxidation potential of the amine.

(4.12)

4.5.2 Electroauxiliaries Based onReadily Electron-TransferableFunctional GroupsEasily oxidizable functional groups like thearylthio (ArS) group also work aselectroauxiliaries, i.e. the introduction of an ArSgroup at the α position of ethers andcarbamates decreases their oxidationpotentials. Importantly, the oxidation of α-ArSsubstituted heteroatom compounds results inthe selective cleavage of the C–S bond togenerate a cation intermediate stabilized by theadjacent heteroatom. The resulting cationintermediate reacts with a nucleophile toprovide the product selectively. The use of anArS group as an electroauxiliary expands thescope of nucleophiles, which enables in situ use

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of carbon nucleophiles like allylsilane, as shownin Eqs. 4.13 and 4.14, even if such nucleophilesare easily oxidized [23]. In these cases, owing tothe ArS group, selective C–S bond cleavagetakes place and an allyl group is introduced tothe α or benzylic carbon selectively.

(4.13)

(4.14)

On the other hand, it is known that theelectroauxiliary moiety and reactive site aredifferent in the case of cathodic reduction. Forinstance, as shown in Eq. 4.15, the first electrontransfer takes place at the easily electronacceptable carbonyl group, and then adjacentcarbon–heteroatom bond cleavage takes placeselectively [24].

128

(4.15)

Based on this principle, selective C–O bondcleavage at the benzylic and allylic positions canbe easily achieved, as shown in Eq. 4.16.

(4.16)

Although the C–F bond is not so easily cleavedby reduction owing to its larger bond energycompared to that of the C–H bond, fluorineattached to a benzylic position or α to carbonyland imino groups is readily removed bycathodic reduction. This is also quite similar tothe cases shown in Eqs. 4.15 and 4.16. As shownin Eq. 4.17, electron transfer first takes place atthe aromatic ring or carbonyl and imino groupsfollowed by β-elimination of the fluoride ion[25]. When the cathodic reaction shown in Eq.4.17 is carried out in the absence of Me3SiCl,further cathodic reduction takes place to resultin successive elimination of fluorine atoms,providing complicated products.

129

(4.17)

4.5.3 Electroauxiliaries Based onIntermolecular Coordination EffectsThis effect is quite important from an electrontransfer aspect. The electron transfer reactionin solution is generally facilitated by thestabilization of the resulting radical ion andionic intermediates by the coordination ofsolvent molecules or counter ions in thesolution. For instance, the reduction potentialof the metal cation usually becomes morenegative in solvents with large donor numbersbecause the positive nature of the metal cationis decreased by solvation in such solvents.

It is well known that the reduction potentials ofketones, imines, cyano and nitro compoundsshift in the positive direction by protonaddition. The polarography of the reduction ofnitro compounds has been intensively studied.When the pH value of the solution is mademore acidic by one pH unit, the reductionpotential becomes more positive by about 58mV. The positive shift of the reduction potentialis due to the protonation of unsaturatedfunctional groups resulting in a positivelycharged form, as shown in Figure 4.13.

130

Figure 4.13 Anodic shift of reduction potentialowing to protonation

Furthermore, the reduction potentials ofketones often shift to positive in the presence ofLewis acids. This is due to the coordination ofthe Lewis acid to the oxygen atom of thecarbonyl group resulting in a decrease in theelectron density of the carbonyl group orcoordination of the Lewis acid to the anionradical intermediate generated by one-electronreduction of the carbonyl group resulting instabilization of the intermediate, as shown inFigure 4.14.

131

Figure 4.14 Anodic shift of reduction potentialowing to coordination with Lewis acid andhydrogen bonding

It is also known that the second reductionpotentials of diketones like anthraquinone anddinitrobenzenes shift greatly in the positivedirection in the presence of alcohols. This is dueto the formation of hydrogen bonds betweendianion intermediates generated bytwo-electron reduction and the alcohols.

4.5.4 Electroauxiliaries Based onIntramolecular Coordination EffectsIf a substrate molecule has a specificcoordinating site such as a functional group orheteroatom to stabilize the electrogeneratedionic intermediate, the electron transfer isenhanced by intramolecular coordination, asshown in Figure 4.15. As such a functionalgroup, pyridyl, carbonyl and ether groups areeffective, and they decrease the oxidationpotential appreciably. Such coordination wouldfacilitate subsequent chemical steps like bondfission. When bond cleavage accompanieselectron transfer, as shown in Figure 4.15, theoxidation potential decreases significantly. Thisis quite a new concept and methodology forelectron transfer control [26]. Furthermore, ithas been demonstrated that a combination ofthe orbital interaction and intramolecularcoordination is highly effective for controlling

132

the oxidation potential as well as the reactionpathway.

Figure 4.15 Cathodic shift of oxidationpotential owing to intramolecular coordinationwith a suitable functional group (Y)

4.6 Reaction Pattern of OrganicElectrode ReactionsAlthough electrode reactions are limited tooxidation and reduction, various follow-upchemical reactions often take place afterorganic electron transfer. Organic electrodereactions are therefore quite different fromthose of inorganic compounds.

From a synthetic viewpoint, organic electrodereactions are classified into various reactiontypes, and examples of cathodic reduction andanodic oxidation are given here.

4.6.1 Transformation Type of FunctionalGroupCathodic Reduction

(4.18)

133

(4.19)

(4.20)

(4.21)

(4.22)

(4.23)

(4.24)

134

(4.25)

(4.26)

(4.27)

(4.28)

(4.29)

Anodic Oxidation

(4.30)

(4.31)

(4.32)

(4.33)

135

(4.34)

(4.35)

(4.36)

Equations 4.18–4.36 are shown usingstoichiometric formula including elementaryreaction steps, but only final products areshown and elementary reaction steps areomitted in the following examples. It shouldalso be noted that the reagents shown under thearrows react with electrochemically generatedintermediates, but they are not subjected todirect electron transfer reactions.

4.6.2 Addition TypeCathodic Reduction

(4.37)

136

(4.38)

(4.39)

(4.40)

Anodic Oxidation

(4.41)

(4.42)

(4.43)

137

4.6.3 Insertion TypeCathodic Reduction

(4.44)

Anodic Oxidation

(4.45)

4.6.4 Substitution TypeCathodic Reduction

(4.46)

(4.47)

Anodic Oxidation

138

(4.48)

(4.49)

(4.50)

(4.51)

4.6.5 Substitutive Exchange TypeCathodic Reduction

(4.52)

139

In Eq. 4.52, the transformation type offunctional group also occurs.

Anodic Oxidation

(4.53)

(4.54)

4.6.6 Elimination TypeCathodic Reduction

(4.55)

(4.56)

Anodic Oxidation

(4.57)

140

4.6.7 Dimerization TypeCathodic Reduction

(4.58)

(4.59)

(4.60)

(4.61)

(4.62)

141

Anodic Oxidation

(4.63)

(4.64)

(4.65)

(4.66)

4.6.8 Crossed Dimerization

(4.67)

In Eq. 4.67, the ratio of crossed dimerization tonon-crossed dimerization can be increased, butthis is not so easy in the case of Eq. 4.68.

(4.68)

142

4.6.9 Cyclization Type

(4.69)

(4.70)

4.6.10 Polymorphism Formation Type

(4.71)

4.6.11 Polymerization Type

(4.72)

The reaction shown in Eq. 4.72 is initiated bycathodic reduction of activated olefin and acatalytic amount of electricity is enough tofacilitate the polymerization in a chain reactionmechanism, while the following reaction is

143

anodic oxidative polymerization consumingstoichiometric amount of electricity.

(4.73)

4.6.12 Cleavage Type

(4.74)

(4.75)

4.6.13 Metalation TypeMetal used as an electrode is incorporated inthe product.

(4.76)

(4.77)

144

4.6.14 Asymmetric Synthesis Type

(4.78)

(4.79)

(4.80)

As a chiral source, chiral supporting salts, chiralsolvents, chiral adsorbants and chiral modifiedelectrodes are used.

4.7 Electrochemically GeneratedReactive SpeciesElectrochemically generated reactive speciesoften undergo follow-up reactions in anadsorbed state or near the electrode surface toprovide products. Since these reactive speciesare affected by the electrode itself and a strong

145

electric field (approximately 107–108 V cm−1),their reactivity and behaviour are often quitedifferent from the same reactive speciesgenerated by other methods. Useful syntheticreactions can be developed utilizing suchunique electrogenerated reactive species. Avariety of ionic and radical species aregenerated by electrolysis. Electrochemicallygenerated reactive species are classified intocarbon species and heteroatom ones, and theirgeneration methods are explained below.

4.7.1 Carbon Species

4.7.1.1 Anodically Generated CarbonSpecies

Oxidation of Carboxylic Acid: Anodicoxidative decarboxylation of carboxylic acidgenerates an alkyl radical and/or alkyl cation[27]. A carboxylic acid with a straight alkylchain is oxidized at the Pt anode in weak acidicsolution to generate mainly the correspondingalkyl radical. On the other hand, anodicoxidation of carboxylic acid with an α-branchedalkyl chain at a carbon anode in neutral oralkaline solution generates mainly thecorresponding alkyl cation.

(4.81)

Oxidation of Carbanion and ActiveHydrogen Compounds: The oxidation

146

potentials of carbanions are extremely low andtheir one-electron oxidation generates radicals.

(4.82)

(4.83)

Oxidation of Alkyl Halides: Alkyl iodidesare more easily reduced and oxidized comparedto the corresponding bromides. Theirone-electron oxidation generates a radicalcation intermediate followed by elimination ofthe halogen molecule to generate the alkylcation.

(4.84)

Oxidation of Olefins, Ketones, Imines,and Strain and Cage Compounds: Theoxidation potential of ketone is generally high,but even aliphatic ketone is oxidizable.Although alkane is difficult to oxidize, straincyclopropanes and cage-type adamantanes arerelatively easy to oxidize. One-electronoxidation of those compounds generates radicalcations.

(4.85)

147

(4.86)

Oxidation of Aromatic Compounds:Initially, one-electron transfer from thearomatic ring occurs to generate a radicalcation on the ring. When an alkyl chain existson the aromatic ring, elimination of theα-proton from the alkyl chain generates thecorresponding benzyl radical, which undergoesfurther one-electron oxidation to generatebenzyl cation. On the other hand, in the case ofdifficult elimination of the α-proton oraromatics devoid of α-proton, the aromaticradical cation is attacked by nucleophiles,resulting in ring substitution with thenucleophiles. In the absence of nucleophiles,homo-coupling of radical cations or radicalintermediates takes place.

(4.87)

(4.88)

4.7.1.2 Cathodically Generated CarbonSpecies

Reduction of Alkyl Halides: The ease ofreduction of alkyl halides is related to the bondenergy of C–X, and hence iodide compounds

148

are the most easily reduced, while chloridecompounds are the most difficult to reduce.One-electron and two-electron reduction ofalkyl halides generate alkyl radicals and alkylanions, respectively.

(4.89)

Reduction of Ketone and Imine: In thecase of cathodic reduction of ketone and iminein aqueous solution, the generated reactivespecies as well as the reaction mechanism arechanged by the pH of the solution. In an acidicsolution, the oxygen atom of the ketone and thenitrogen atom of the imine are protonated,therefore their reduction potentials shifts to thepositive side, and their one-electron reductiongenerates neutral radicals. In contrast, in analkaline solution the protonation of ketone andimine does not occur due to low protonconcentration. In this case, the radical anion isgenerated first, and then the dianion is formed.

(4.90)

Reduction of Activated Olefin andConjugated Olefin: Activated olefin is readilyreduced because the electron-withdrawinggroup attached to the double bond decreasesthe electron density of the double bond.

149

Isolated olefin is not so easy to reduce, butconjugated olefin is reducible. One-electronreduction of the olefin generates a radicalanion.

(4.91)

Reduction of Active HydrogenCompounds: One-electron reduction of anactive hydrogen compound generates thecorresponding anion, eliminating the activehydrogen atom as hydrogen gas.

(4.92)

Reduction of gem- and vic-DihalogenoCompounds: Two-electron reduction of thesecompounds generates carbene and benzyne.

(4.93)

150

(4.94)

4.7.2 Heteroatom SpeciesDifferent from hydrocarbons, heteroatomcompounds are oxidizable since they have lonepaired electrons on the heteroatom, from whichelectron transfer occurs. In general, ease ofoxidation has the order N > S > O, and a varietyof nitrogen active species are generatedelectrochemically [28].

4.7.2.1 Nitrogen Species

One-electron oxidation of amine generates aradical cation at the nitrogen atom. The radicalcation of aromatic amine is relatively stable,while that of aliphatic amine is so unstable thatthe α-proton is immediately eliminated, andthen the active site shifts to the α-carbon. In thecase of ordinary aliphatic amines, iminium ionsare so unstable that cleavage of the C–N bondoccurs predominantly. On the other hand,iminium ions of aromatic amines, carbamatesand amides are stable, therefore nucleophilicsubstitution reactions like methoxylation,acetoxylation and cyanation occur efficiently atthe α-position to the nitrogen atom, as shown in

151

Eq. 4.95 [6]. These anodic substitutions are oneof the characteristic electrolytic reactions andthey are useful for construction of a C–C bondat the adjacent position to the nitrogen atom.Anodic substitution at the adjacent position tothe nitrogen atom of imines is also possible, asshown in Eq. 4.96 [29].

(4.95)

(4.96)

Depending on the molecular structures ofamines, the active site is retained at the originalnitrogen, resulting in the generation of variousreactive nitrogen species, and eventuallyreactions occur at the nitrogen atom, as shownin Figure 4.16 [28]. Two-electron reduction ofN,N-dihalo compounds generates thecorresponding nitrenes [30], while two-electronoxidation of aziridine and medium-sized cyclicamines followed by deprotonation of NHmoiety generates nitrenium ions [31]. Anodic

152

oxidation of hydrazine derivatives generatesamino nirenes [32]. In particular, anodicoxidation of N-phthalimide in the presence ofolefin provides the corresponding aziridinederivatives in high yields, as shown in Figure4.16 [33]. The reaction proceeds via anodicallygenerated aminonitrene.

(4.97)

Figure 4.16 Electrochemically generatedreactive nitrogen species

Furthermore, anodic oxidation of bicyclicamines and aliphatic amines devoid ofα-protons generates aminyl radicals efficiently.

153

In the former case, anodic elimination ofα-proton cannot occur due to Bredt's rule.

(4.98)

(4.99)

4.7.2.2 Oxygen Species

Oxidation of Alcohol and CarboxylicAcid: Radical species are mainly generatedfrom these compounds, but cationic speciescould also be generated.

(4.100)

(4.101)

Oxidation of Ether: Similarly to the case oforganonitrogen compounds, anodic oxidationof ethers generates a cation α to the oxygenatom, which undergoes nucleophilicsubstitution. However, the scope of itsapplication is limited because of the highoxidation potentials of ethers.

154

(4.102)

Reduction of Alcohol and CarboxylicAcid: One-electron reduction of thesecompounds generates the correspondinganions, eliminating a hydrogen molecule.

(4.103)

Reduction of Dioxygen: One-electronreduction of dioxygen generates superoxide ion,which is called active oxygen and works as bothoxidant and reductant as well as base andradical (see section 5.2).

(4.104)

4.7.2.3 Calcogeno (Sulfur, Selenium,Tellurium) Species

Carcogen compounds are easily oxidized andoxidation of the corresponding anion generatesradicals that are further oxidized to cations, asshown in Eq. 4.105. These oxidation processesare usually reversible electron transfersalthough the reversibility depends on theelectrolytic conditions.

(4.105)

In the case of sulfides, the anodically generatedradical cation at the sulfur atom is attacked by

155

nucleophiles (nucleophilic addition). However,in the case of sulfide with a strongly acidicα-hydrogen, α-deprotonation of the radicalcation occurs predominantly and eventuallyα-nucleophilic substitution takes placeselectively. Among the substitution reactions,fluorination is of great importance in syntheticorganic chemistry. However, in the case ofselenides, even selenides having anelectron-withdrawing group do not alwaysundergo nucleophilic substitutions, and anucleophilic addition reaction also occurs at theselenium atom. In contrast, anodic substitutionof organotelluides is not known andnucleophilic addition at the tellurium atompredominantly occurs.

(4.106)

4.7.2.4 Halogen Species

Halogen is easily oxidized and also readilyreduced. Since the redox reaction of the halideion is reversible, it is widely used as a mediatorfor indirect electrochemical oxidation (seeChapters 3 and 5).

(4.107)

156

4.7.2.5 14-Family and 15-Family ElementSpecies

Reactive species are generated predominantlyat heteroatoms by electron transfer, as shown inEqs. 4.108–4.110.

(4.108)

(4.109)

(4.110)

References1. Torii, S. (2006) Electroorganic ReductionSynthesis, Vols 1 and 2, Kodansha andWiley-VCH Verlag GmbH, Weinheim.

2. Bard, A.J. and Stratmann, M. (eds) (2002)Organic electrochemistry, in Encyclopedia ofElectrochemistry, Vol. 8 (ed. H. J. Schäfer),Organic Electrochemistry Wiley-VCH VerlagGmbH, Weinheim.

3. Lund, H,. and Hammerich, O. (eds) (2001)Organic Electrochemistry, 4th edn, MarcelDekker, Inc., New York.

4. Grimshaw, J. (2000) ElectrochemicalReactions and Mechansims in OrganicChemistry, Elsevier, Amsterdam.

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5. Fry, A. J. (1989) Synthetic OrganicElectrochemistry, Wiley Interscience, NewYork.

6. Shono, T. (1984) Electroorganic Chemistryas a Tool in Organic Synthesis,Springer-Verlag, Berlin.

7. Fuchigami, T., Nonaka, T. and Schäfer, H.J.(2003) Encyclopedia of Electrochemistry, Vol.8 (eds A. J. Bard and M. Stratmann),Wiley-VCH, Verlag GmbH, Weinheim.

8. Fussing, I., Hammerich, O., Hussain, A.,Nielsen, M.F. and Utley, J.H.P. (1998) ActaChem. Scand., 52, 328–171.

9. Fuchigami, T., Ichikawa, S. and Konno, A.(1994) J. Org. Chem., 59, 607–615.

10. (a) Nelsen, S.F. and Ippoliti, J.T. J. Am.Chem. Soc., 108, 4879–4881. (b) Yoon, V.C.and Mariano, P.S. (1992) Acc. Chem. Res., 25,233–240.

11. Fuchigami, T., Narizuka, S. and Konno, A.(1992) J. Org. Chem., 57, 3755–3757.

12. Higashiya, S., Narizuka, S., Konno, A. andFuchigami, T. (1992) J. Org. Chem., 57,3755–3757.

13. Edamura, F., Kyriyacou, D., Love, J. (1980)US Patent 4217185; Chem. Abstr. (1981), 94,22193.

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15. (a) Fry, A.J. and Moor, R.H. J. Org. Chem.,33, 1283–1284. (b) Erickson, R.E., Annino, R.,Sainlor, M.D. and Zon, G. (1969) J. Am. Chem.Soc., 91, 1767–1770.

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18. (a) Yoshida, J., Kataoka, K., Horcajada, R.and Nagaki, A. (2008) Chem. Rev., 108,2265–2299. (b) Yoshida, J. and Nishiwaki, K.(1998) J. Chem. Soc., Dalton Trans.,2589–2596. (c) Yoshida, J., Maekawa, T.,Murata, T., Matsunaga, S. and Isoe, S. (1990) J.Am. Chem. Soc., 112, 1962–1970.

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22. Gall, E.L., Hurvois, J.P. and Sinbandhit, S.(1999) Eur. J. Org. Chem., 2645–2653.

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26. (a) Yoshida, J. and Izawa, M. (1997) J. Am.Chem. Soc., 119, 9361–9365. (b) Watanabe, M.,Suga, S. and Yoshida, J. (2000) Bull. Chem.Soc. Jpn, 73, 243–247. (c) Yoshida, J., Suga, S.,Fuke, K. and Watanabe, M. (1999) Chem. Lett.,251–252.

27. Schäfer, H. (1990) Recent Contributions ofKolbe Electrolysis to Organic Synthesis, inTopics in Current Chemistry, 152.Electrochemistry, IV (ed. E. Steckhan),Springer-Verlag, Berlin, pp. 91–151.

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29. Baba, D. and Fuchigami, T. (2003)Tetrahedron Lett., 44, 3133–3136.

30. Fuchigami, T., Iwata, K. and Nonaka, T.(1976) J. Chem. Soc., Chem. Commun.,951–952.

31. Gassman, P.G., Nishiguchi, I. andYamamoto, H. (1975) J. Am. Chem. Soc., 97,1600–1602.

32. Fuchigami, T., Sato, T. and Nonaka, T.(1986) Electrochim. Acta, 31, 365–369.

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4Organic Electrode Reactions

162

Toshio Fuchigami and Shinsuke Inagi

Development of new methodology to achievehighly selective reactions is one of the mostimportant areas in organic synthetic chemistry.As already described, electrode reactions havetheir own specific factors for controllingselectivity, therefore both electrochemical andordinary chemical factors make the control ofelectrochemical reactions more complicated.Electrodes are of great importance for bothelectron transfer interfaces and reaction fields.As described earlier, an electrode has a functionto control a chemical reaction pathway throughadsorption and orientation of the substratemolecule to the electrode surface. Althoughhydrogen and oxygen overpotentials could becriteria for the selection of suitable electrodematerials to achieve the desired electrochemicalreaction in an aqueous solution, theseoverpotentials are not proper criteria in aproticsolutions. Hence, it is not so easy to predictsuitable electrode materials for desiredelectrochemical reaction in aprotic solvents.However, many novel electrolyticmethodologies have been developed in order toachieve high selectivity for the desiredreactions. In this chapter, relatively newelectrolytic methodologies, which have alreadybeen established and are widely used, will bedescribed in detail. Although there are many

163

applications of such electrochemical reactionsto organic syntheses, limited examples aregiven in this chapter. It is recommended thatreaders also study other more detailed books[1].

5.1 ElectrocatalysisAlthough indirect electrochemical reactionsusing mediators, i.e. electrocatalytic reactions,have already been explained in Chapter 2,various examples of synthetic applications aredescribed in this chapter [2]. Some othersynthetic examples are also demonstrated inChapters 6 and 7.

5.1.1 Classification and Kinds ofMediatorsMediators are classified into two groups, asshown in Eqs. 5.1 and 5.2: outer sphere typemediators involving electron transfer betweenthe mediator and a substrate molecule (Eq. 5.1),and inner sphere type mediators involvingredox reaction through ordinary chemicalreactions (Eq. 5.2), as shown in Table 5.1.

164

(5.1)

(5.2)

Table 5.1 Classification of mediators

Mediator

Outer sphere Inner sphere

Multivalent metal ionTriarylamineViologenPolycyclic aromatichydrocarbon

HalogenN-OxylcompoundSulfide

Mediators are also classified into oxidative andreductive mediators, and into organic andinorganic mediators. Various redox mediatorsare known, for example (i) inorganiccompounds such as multivalent metal ions,transition metal complexes and halide ions, (ii)

165

organic compounds such as polycyclic aromaticcompounds, triarylamines and2,2,6,6-tetramethylpiperidine nitroxyl(TEMPO), (iii) anomalous valence compoundsand (iv) hypervalent compounds. Quiterecently, novel mediators such as carboranes,which have the characteristics of boron atoms,have been developed for highly efficientcathodic reductive dehalogenation [3].Triarylimidazole mediators have also recentlybeen developed for selective anodic oxidation[4].

Examples of oxidative and reductive mediatorsare as follows:

Oxidative Mediators

Ru4+/Ru2+, Co3+/Co2+, IO4−/IO3

−,

Mn3+/Mn2+, Ni3+/Ni2+, Ce4+/Ce3+,S2O8

2+/SO42+, Cr6+/Cr3+, Os8+/Os6+,

halogen (X+/X−), NO3√/NO3

−, CAN,

Ar3N+/Ar3N, N-Oxyl derivatives (R1R2N =

O), sulfides (R1SR2).

Reductive Mediators

Ni2+/Ni(0), Co2+/Co+, Cr3+/Cr2+,Mo2+/Mo(0), Ti3+/Ti2+, Sn4+/Sn2+,Sn2+/Sn(0), Mg2+/Mg(0), Pd2+/Pd(0),naphthalene, anthracene, phenathrene,pyrene, fullerene C60, benzonitrile,

166

phthalonitrile, antraquinone, nitrobenzene,viologen, superoxide ion.

The reduction potentials of aromatic mediatorsfor indirect cathodic reduction are shown inTable 5.2.

Table 5.2 Reduction potentials of aromaticmediators for indirect cathodic reduction

Compound E1/2 (V vs. SCE)

Phthalonitrile −1.7

4-Methoxybenzophenone −1.8

Anthracene −2.0

Methyl benzoate −2.2

Benzonitrile −2.2

Chrysene −2.5

Naphthalene −2.5

5.1.2 Organic Electrolytic ReactionsUsing Mediators

5.1.2.1 Electrosynthesis Using MultivalentMetal Ion Mediators

Polyvalent metal ions have been used for a longtime and some are still used for industrialelectrolysis. Synthetic examples using suchmediators are illustrated in Eq. 5.3. Directelectrolysis of toluene derivatives provides amixture of aldehyde and carboxylic acidderivatives, while each product can be obtained

167

selectively by the choice of appropriatemediator.

(5.3)

5.1.2.2 Electrosynthesis Using HalogenMediators

Anodic oxidation of halide ions (X−) generatesvarious kinds of reactive cationic species, forexample X+, OX−, X2, X3

− etc., which arewidely utilized as mediators for variousoxidative molecular conversions, as shown inFigure 5.1. Oxidative power increases in theorder I < Br < Cl, and positive Cl is too strongoxidant to give good selectivity. Iodide andbromide ions are therefore mainly employed asmediators for selective indirect oxidation. Thesynthetic applications of such electrogeneratedactive halogen species as mediators have beendemonstrated by highly selectivefunctionalization of olefins, e.g. epoxidation,halohydroxylation, 1,2-dihalogenation andene-type chlorination, heteroatom–heteroatom

168

bond formation and carbon–heteroatom bondcleavage [5–8].

Figure 5.1 Indirect anodic oxidation ofalcohols using iodine mediator

5.1.2.3 Electrosynthesis Using TriarylamineMediators

Triarylamines, which are well known as outersphere electron transfer reagents, were firstused as mediators for anodic oxidativedeprotection of a protective group likedithiolane for a carbonyl group by Steckhan(Eq. 5.4) [2]. The oxidation ability of thismediator can be tuned by substitution withelectron-withdrawing groups such as thebromine atom, as shown in Table 5.3.Triarylamine mediators can be applied tooxidation of alcohols, amines and theside-chain of aromatic compounds, togetherwith fluorodesulfurization. Thus, they arehighly useful and widely applicable mediators.

169

(5.4)

Table 5.3 Standard oxidation potentials oftriarylamines

Triarylamine Substituent Eo (V vs.NHE)

X Y Z

A Br H H 1.30

B Br Br H 1.74

C Br Br Br 1.96

5.1.2.4 Electrosynthesis UsingMulti-Mediatory Systems

Various combinations of different kinds ofmediators have been developed to expand thescope of electrosynthesis. As shown in Figure5.2, a double mediator consisted of sulfide and

170

bromide ions enables the oxidation of alcoholsat much lower oxidation potential [9].

Figure 5.2 Indirect anodic oxidation ofalcohols using double mediator consisting ofsulfide and bromide ion

5.1.2.5 Electrosynthesis Using HypervalentCompounds as Mediators

Anodic oxidation of p-methoxyiodobenzene inthe presence of fluoride ions provides thecorresponding hypervalent difluoroiododerivative, which is a useful fluorinatingreagent. Thus, p-methoxyiodobenzene has beendemonstrated to be a highly efficient mediatorfor anodic fluorodesulfurization of dithioacetalsas shown in Figure 5.3 [10]. This is the firstsuccessful example of the catalytic use ofhypervalent compounds for organic synthesis aswell as organic electrosynthesis.

171

Figure 5.3 Electrochemicalfluorodesulfurization using hypervalentiodobenzene derivative mediator

Anodic α-fluorination of α,γ-diketones andα-ketoesters can be also achieved usingp-iodotoluene as a mediator [11].

5.1.2.6 Electrosynthesis Using TransitionMetal Complex Mediators

Transition metal complex mediators havevarious reactivities and many advantages. Theirredox potentials and the selectivity of thedesired reaction can be controlled by changingligand. Their synthetic application is mainlybased on the reactivity of the low valent stategenerated by cathodic reduction of themediators [2]. Typically, Co(III) complex(vitamin B12) is readily reducible, opticallyactive, non-toxic and inexpensive. It is reducedat −0.9 V vs. SCE to form Co(I) complex, whichundergoes oxidative addition to alkyl halide toform alkyl Co(III) complex as an intermediate.The resulting intermediate is reduced at more

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negative potential, −1.5 V vs. SCE, to generatean alkyl radical or anion, and Co(I) complex isregenerated simultaneously (reductiveelimination). Since the resulting alkyl radical oranion undergoes conjugate addition, efficientMichael addition can be achieved under neutralconditions, as shown in Figure 5.4 [12]. Thismediatory reaction is widely applicable tovarious halogeno compounds, such as allylhalides, vinyl halides, α-halo ethers and so on.

Figure 5.4 Electrosynthesis using Co(III)complex, vitamin B12 as mediator

As shown in Eq. 5.5, even halogeno compoundswith non-protected hydroxyl and carbonylgroups can be used, which is one of theadvantages of this mediatory system [13].

(5.5)

On the other hand, β-bromodiester andω-bromoalkyl acrylate undergo1,2-rearrangement and intramolecular

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cyclization, respectively, to form large cycliclactones by their cathodic reduction usinghydrophobic vitamin B12 mediator under UVirradiation [14].

Homo-coupling products are obtained fromaromatic halides using Pd(0) complex as well asNi(0) complex as a mediator [15]. The yieldsand turnover of the Pd(0) complex are generallysuperior to those using Ni(0) complex. Themechanism proposed is shown in Figure 5.5.Aromatic halide reacts with Pd(0) complex togenerate an aryl Pd intermediate, which isreduced cathodically followed by reaction withone more aryl halide molecule to form diaryl Pdcomplex, resulting in reductive elimination togive a homo-coupling product. When thisreaction is performed in the presence of CO2,aromatic carboxylic acids are obtained in highyields [16].

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Figure 5.5 Homo coupling of aryl halide usingPd(0) complex

5.1.2.7 Electrosynthesis Using MediatorImmobilzed on Solid

N-Oxyl radicals exemplified by2,2,3,3-tetramethylpiperidinyl-N-oxyl(TEMPO) are useful mediators for efficientoxidation of alcohols and amines. Themediators have never been recovered andreused after electrolysis, but recyclablemediators have recently been developed froman atom economical aspect. Mediators areimmobilized on the surface of silica gel throughsilane coupler, and are readily recovered andrecycled after electrolysis. Thus, thecombination of halide ions andTEMPO-immobilized silica gel or polymerparticles as a disperse phase enables a mediatedelectrocatalytic reaction in aqueousNaBr-NaHCO3 as a disperse media [17].Asymmetric oxidation of alcohols is alsopossible using a chiral N-oxyl mediator [18].

5.2 Electrogenerated Acids andBases

5.2.1 Electrogenerated BasesAcids and bases play very important roles inorganic synthesis. It is well known that whenaqueous solution containing a neutral

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supporting electrolyte is electrolysed in adivided cell, the anolyte becomes acidic whilethe catholyte becomes alkaline. This is becausehydroxide ions are consumed at the anodewhile protons are consumed at the cathode. In asimilar manner, some acid is generated in theanolyte while some base is generated in thecatholyte during electrolysis in an organicsolvent. Even in an undivided cell, the vicinityof an anode becomes acidic while that of acathode becomes basic during electrolysis.

Anionic species generated cathodically act notonly as nucleophiles but also as bases, and haveinteresting reactivities in organic synthesis. Theinventor of the cathodic hydrodimerizationprocess of acrylonitrile, Baizer, demonstratedthat the cathodically generated anion radical ofhindered azobenzene (Figure 5.6) could be auseful base for various organic synthesis, andhe named such bases electrogenerated bases(EGBs) [19].

Figure 5.6 Electrogenerated base of hinderedazobenzene

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There are two main methods for the generationof EGBs, as shown in Eq. 5.6. One method iscathodic reduction of compounds with anunsaturated moiety in aprotic solventscontaining quaternary ammonium salt as asupporting electrolyte to generate the radicalanions or dianions, and the other one iscathodic reductive deprotonation of activehydrogen compounds to generate thecorresponding anions.

(5.6)

Cathodic reductive deprotonation of2-pyrrolidone, a hindered phenol like2,6-t-butyl-4-methylphenol, andtriphenylmethane generates the correspondinganions. Since EGBs have quaternaryammoniumcations (Q+), the anions formed bythe treatment with EGB have high reactivity(Figure 5.7) and hence this methodology ishighly useful for organic synthesis.

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Figure 5.7 Reactive anion derived fromelectrogenerated base (Q+ = Et4N+)

In particular, the EGB derived from2-pyrrolidone is a versatile base and applicableto organic synthesis such as Stevensrearrangement, selective α-monoalkylation ofα-(aryl)acetate esters and C-monoalkylation of1,3-diketones [20,21]. This EGB has also beendemonstrated to be a highly efficient base forthe synthesis of organofluorine compounds. Forinstance, it is known that trifluoromethyl anionis so unstable that it undergoes α-elimination offluoride anion to generate fluorocarbene, butstable trifluoromethyl anion can be generatedby the treatment of fluoroform with this EGB,and consequently trifluoromethylation ofaromatic aldehydes and ketones is realized togive the trifluoromethylated alcohols, as shownin Eq. 5.7 [22].

(5.7)

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Generally, it is quite difficult to generate theenolate anion with an α-CF3 group since theenolate anion readily undergoes decompositionsuch as β-elimination of fluoride ion. However,pyrrolidone-derived EGB enables thegeneration of stable trifluoromethylated enolateanion, and alkylation can be performed in goodyield without elimination of the fluorine atom,as shown in Eq. 5.8 [23].

(5.8)

Furthermore, this EGB catalyzes ring-openingpolymerization of N-carboxyanhydrides ofα-amino acid (NCA) to provide poly(aminoacids) in a short time in excellent yield, asshown in Eq. 5.9. The yield is much higher andthe reaction time relatively short compared toreactions using conventional base with a metalcation such as Na+ [24].

179

(5.9)

EGB derived from hindered phenol catalyzes aselective double aldol condensation reaction, asshown in Eq. 5.10 [25].

(5.10)

It is well known that the substrate moleculeitself also serves as an EGB. For instance, asshown in Eq. 5.11, a catalytic amount of EGBefficiently catalyzes aldol condensation. In thisreaction, aldehyde itself is cathodically reducedto generate a trace amount of its radical anion,which acts as a base to abstract the α-protonfrom unreacted aldehyde, and aldolcondensation proceeds successively [26].Finally, eliminating hydroxyl ion would act asan EGB in a manner similar to a chain reaction.

180

(5.11)

Treatment of alcohols with sodium metalgenerates hydrogen gas and sodium alkoxide isformed. This can be explained as follows. Theperipheral electron of sodium metal transfers tothe alcohol to generate a hydrogen radical,forming hydrogen gas, and the alcohol istransformed to the alkoxide, in other wordssodium metal acts as a reducing reagent for thealcohol. When the alcohol is reduced at acathode with a low hydrogen overpotential, likeplatinum, hydrogen gas evolves and thecorresponding alkoxide is readily formedwithout any reducing reagent. It can thereforebe considered that the cathode would act assodium metal. This is quite a safe method forthe preparation of alkoxide because harmfulsodium metal is not required. Moreover, thecathodic reduction of carboxylic acid, phenol,thiophenol and alcohol is performed in thepresence of quaternary ammonium salt as asupporting electrolyte to generate thecorresponding highly reactive anions withquaternary ammonium cation (Q+). By using

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such reactive anions, highly selectiveesterification, etherification and carbonylationcan be achieved at room temperature, as shownin Eq. 5.12 [27,28].

(5.12)

Cathodic reduction of dioxygen generatessuperoxide ion (O2

−√). Superoxide ion acts asnucleophile, oxidant, reductant, radical andbase (EGB), therefore this reactive species ishighly useful for organic synthesis. Althoughsuperoxide ions are available from KO2 andNaO2, these are insoluble in aprotic solvents. Inorder to dissolve these salts in aprotic solvents,costly crown ethers are required therefore theirsynthetic applications are limited. On the otherhand, the electrolytic method can generatesuperoxide ion in situ, which is better than theconventional chemical method. As shown in Eq.5.13, electrogenerated superoxide ion acts as anEGB to eliminate the α-proton of malonic esterderivative, followed by reaction with O2 to givethe α-hydroxy product in good yield [29]. Highcurrent efficiency suggests that this reactionmay involve a chain reaction.

182

(5.13)

5.2.2 Electrogenerated AcidsIn contrast to EGBs, the vicinity of an anodebecomes acidic during electrolysis. The acidthus formed is called an electrogenerated acid(EGA), which has unique reactivity compared toconventional chemical acids. For instance,electrolysis of a solution containing LiClO4 anda trace amount of water generates acid, HClO4,as shown in Figure 5.8 [30].

Figure 5.8 Principle of electrogenerated acid

EGA generated in this way is assumed to beanhydrous HClO4, which is a stronger acid thancommercially available aq. HClO4. Such EGA

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acts as a kind of Lewis acid and is widelyapplicable to various organic reactions, forinstance isomerization, transformation offunctional groups, carbon–carbon bondformation, including Diels–Alder reaction, andso on [31]. As shown in Eq. 5.14, a combinationof supporting salts and solvents allows threekinds of products to be obtained selectivelyfrom the same starting material [30].

(5.14)

Furthermore, EGAs are also highly effective forthe molecular transformation of organofluorinecompounds. For instance, as shown in Eq. 5.15,the α-cation attached to the CF3 group iscatalytically generated by the treatment oftrifluoromethylated O,S-acetal with an EGA,and subsequently carbon nucleophiles arereadily introduced to the α-position [32].However, use of other conventional Lewis acidsresults in no formation of the desired product.

184

(5.15)

5.3 Electrochemical AsymmetricSynthesisAsymmetric synthesis is a highly importantarea in organic chemistry and a number ofasymmetric catalytic syntheses have recentlybeen developed. However, electrochemicalasymmetric synthesis is still immaturecompared with well-established chemical,catalytic and enzymatic methods. Variousmethods for electrochemical asymmetricsynthesis have been proposed [33,34]: (i)intramolecular asymmetric induction, (ii)utilization of chiral solvents, (iii) use of chiralsupporting salts, (iv) use of chiral electrodeadsorbance, (v) use of electrodes chemicallymodified with chiral substances, (vi) use ofchiral polymer-modified electrodes, (vii) use ofchiral mediators. Among these, (i) and (ii) arenot unique methodology characterized byelectrochemistry. However, a number of papersdealing with these methodologies have beenreported. Although (iii) is a methodologycharacterized by electrolysis, a large amount ofchiral supporting salt is required, therefore it is

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not useful. Here, methods (iv)–(vi) areexplained.

In 1967, Grimshaw first reported theasymmetric reduction of 4-methylcumarineusing optically active alkaloids, and a maximumasymmetric yield (17%) was obtained using asmall amount of sparteine (1 mM) as a chiralelectrode adsorbant [35]. After 10 years, Millerobtained 48% asymmetric yield in the cathodicreduction of 2-acethylpyridine to thecorresponding alcohol in the presence of 0.5mM of strychnine salt [36]. Asymmetricinduction seems to be attributable to the chiralreaction fields constructed by the physicaladsorption of optically active alkaloids on thecathode surface. However, such asymmetricelectrosynthesis is very sensitive to electrolyticconditions such as stirring, concentration,solvent, cathode potential and so on.

Miller and co-workers found that prochiralcarbonyl compounds like ethyl phenylglyoxylatewere reduced to chiral alcohols in 10%asymmetric yield on a cathode chemicallymodified with (S)-phenylglycine [37]. They alsoreported asymmetric oxidation of p-tolylmethyl sulfide to the sulfone with 2.5% ee byusing a similar electrode modified with(+)-camphoric acid [38]. These are alsopioneering works on chemically modifiedelectrodes. The low asymmetric yields seem tobe due to a low density of chiral compound on

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the electrode surface, and other researchershave pointed out that Miller's results are notalways reproducible. Later on, in order to solveproblems such as the instability of electrodesmodified with chiral adsorbance and the lowdensity of modified chiral compounds, chiralpolymer-modified electrodes were developed.For instance, Nonaka, Fuchigami andco-workers prepared various electrodes coatedwith optically active poly(amino acid)s andapplied these to the asymmetric reduction ofolefins. They obtained 43% ee in the reductionof 4-methylcumarin at a cathode coated withpoly(L-valine) as shown in Figure 5.9 [39].

Figure 5.9 Asymmetric reduction using chiralpolymer-modified electrode

Subsequently, a graphite felt electrode modifiedwith 2,2,6,6-tetramethylpiperidin-1-yloxyl(TEMPO) was developed and applied toenantioselective oxidative coupling of2-naphthol, 2-methoxynaphthalene and10-hydroxyphenanthrene in the presence of(−)-sparteine as a chiral base. Theenantioselectivity of the coupling products wasvery high (98%) [40,41]. It has been consideredthat high asymmetric yield cannot be expected

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since electrolytic reactions usually require polarsolvents. It should therefore be noted that suchexcellent results of asymmetric electrosynthesiswere realized even in polar solvents.

The combination of enzymes such as lactic aciddehydrogenase and mediators or chiralmediators is also effective for electrochemicalasymmetric synthesis and kinetic resolution toprovide products with high asymmetric yields[42–44].

5.4 Modified ElectrodesAs described previously, electrode surfaces andthe vicinities of electrodes are the main reactionfields for electrode reactions, therefore if theelectrode surface is modified with desiredfunctional substances such as electrocatalyst,and chiral compound, the efficiency, desiredselectivity, asymmetric synthesis, and etc.would be realized. Such electrodes are calledmodified electrodes and their use enables theamount of functional materials required to bereduced and the work-up for electrolyticsolutions to be simplified [45]. Excellentdurability is needed for the modified electrodesfor electrosynthesis since both a largeelectrolytic current and long electrolysis timeare required, which is different from modifiedelectrodes for sensors. Modified electrodes areclassified as follows depending on modificationmethods and modifiers.

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5.4.1 Electrodes Modified withAdsorbantsWhen the electrode is immersed in a solutioncontaining redox compounds like flavins andphenazines, and then pulled up from thesolution, an electrode modified with the redoxcompounds is readily obtained. Themodification is not very strong, the durability ofthe modified electrode is not great and themodification density is low. On the other hand,an electrode modified with strongly adheringand homogeneous films with redox catalystsusing the Langmuir–Blodgett technique issuperior to an adsorbant-modified electrodesince both the modification density (density ofredox catalyst) and durability of theLangmuir–Blodgett film modified electrode aremuch higher. Based on hydrophilic/hydrophobic control of the electrode surfaceusing Langmuir–Blodgett film containingquaternary ammonium salt, product selectivitycontrol has been achieved [46].Langmuir–Blodgett film-coated electrodes alsoallow electrocatalytic reactions to be achieved[47].

5.4.2 Foreign Metal Adatom ModifiedElectrodesBase electrodes can be modified by depositionof foreign metal adatoms at potentials severalhundred millivolts positive to the reversible

189

potential for metal deposition. A submonolayerof adatoms covers the base electrode surfaceuniformly, making it into an adatoms modifiedelectrode [48]. Such electrodes are highly usefulfor electrocatalysis of oxygen reduction andoxidation of methanol, therefore the electrodesare currently of great importance in thedevelopment of fuel cells.

These electrodes also allow interesting productselectivity as follows. Cathodic reduction ofdinitrobenzenes at a silver cathode in an acidicmethanol solution mainly providesdiaminobenzenes, while the use of a silvercathode modified by deposition of lead adatomsforms phenylene dihydroxyamines selectively,as shown in Eq. 5.16 [49]. This is considered tobe due to the inhibition of adsorption of theintermediate phenylene dihydroxyamines onthe electrode surface by deposited lead atoms,which interfere with further reduction.

(5.16)

5.4.3 Chemically Modified ElectrodesAs described above, Miller's work on chiralcompound modified electrodes has stimulated

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intense and fruitful research in the field ofchemically modified electrodes. Functionalsubstances are immobilized on base electrodesthrough covalent bonds like ester, ether andamide with desired functional substances.When noble metal electrodes like gold andplatinum are immersed in a solution of thiol ordisulfide for a while, the electrode surface isreadily modified with a self-assembled thiolmonolayer. Since various functional groups canbe immobilized on the electrode surface and themethod of modification is versatile, this methodis widely used. However, only a few examples ofits application to electrosynthesis are known[50].

Pinson, Saveant and co-workers developed anefficient and general procedure for covalentbonding of aryl groups to an electrode bycathodic reduction of an aryldiazonium ion. Thearyl radicals thus generated bind efficiently tothe electrode surface. The resulting modifiedelectrode is highly stable. Thus, various arylgroups can be attached to carbon, silicon andmetal electrode surfaces by this procedure[51a]. Such modified electrodes are widelyapplicable, for instance in sensors andelectrocatalysts, but application toelectrosynthesis has been limited so far [51b].

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5.4.4 Polymer-Modified (Coated)ElectrodesPolymer-modified (coated) electrodes have anadvantage that a large number of functionalsubstances, such as optically active compoundsand electroactive compounds like redoxcatalysts, can be incorporated in the polymermatrix coated on the electrode surface, asshown in Figure 5.10 [45].

Figure 5.10 Polymer-modified electrode withredox catalyst

The polymer used for coating must be soluble insome solvent and insoluble in the solventcontaining the supporting electrolyte used forelectroanalytical studies and/or electrolyticreactions. Modification methods are shownbelow: simple dip coating and subsequentdrying, polymer coating directly frommonomers by chemical or electrochemicalinitiated polymerization, conducting polymercoating and a confirmed chemical binding like

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an amido bonding of the polymer layer usingelectrode surface anchor groups.

The following polymer materials are used forthe modification of electrodes:

redox polymer (polyviologene,polyvinylferrocene, etc.)

polymers (poly-4-vinylpyridine,polyvinylsulfonic acid, polyacrylonitrile,etc.) that can form metal complexes andimmobilize redox active species throughcolumbic interaction

polymers (polycarbonate, polyurethane,etc.) that enable selective ion/gaspermeation

conducting polymers.

Although examples of inorganic polymer-coatedelectrodes are limited, Prussian blue,heteropolyacid, clay film etc. can be utilized asmodification substances. The polymer-modifiedelectrodes have wide applications for fuel cells,capacitors, display materials, biosensors, ion/gas sensors and asymmetric synthesis, andtherefore they are currently being intensivelystudied.

5.5 Paired ElectrosynthesisElectrosynthesis involves the simultaneousoccurrence of both reduction at a cathode andoxidation at an anode. In a typical inorganic

193

electrolysis of an aqueous solution of sodiumchloride, sodium hydroxide is produced at thecathode, while chlorine gas is formed at theanode. Thus, both electrode reactions areefficiently utilized. In organic electrolysis, thesynthesis of desired products is performed byeither cathodic or anodic reaction. Hence, inmost cases, the reaction products at the counterelectrode are wasted, therefore thesimultaneously synchronous utilization of bothcathode and anode reactions for efficientsynthesis of desired products is preferable froma practical point of view. Pairedelectrosynthesis is based on this methodologicalconcept. Baizer first formulated anddemonstrated approaches to the above concept,and introduced the term of pairedelectrosynthesis [19,52]. At most 200% currentefficiency could be expected in the ideal case ofpaired electrosynthesis using cathodic andanodic processes to provide the same product.Recently, BASF Co. in Germany commercializedpaired electrosynthesis for the first time.Phthalide and t-butylbenzaldehydedimethylacetal are produced simultaneously inan undivided cell. The details are described inChapter 8.

Paired electrosynthesis is classified in thefollowing ways based on reaction modes[19,52].

Parallel paired electrosynthesis(electrosynthesis of different

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products from different startingsubstrates): A typical example iselectrosynthesis of dihydrophthalic acid andacetylene dicarboxylic acid from phthalicacid and butynediol at a cathode and anode,respectively [52]

Divergent paired electrosynthesis(electrosynthesis of differentproducts from the same substrate):Details and an example are described inChapter 7 (Figure 7.8).

Convergent paired electrosynthesis:This is the electrosynthesis of a singleproduct from different starting substrates,for instance electrosynthesis of propyleneoxide via chlorohydrine from propylene andchloride ion as well as electrosynthesis ofsulfeneimines from amine and disulfideusing bromide ion mediator [53]. As shownin Figure 5.11, aliphatic ester is reduced withreactive Mg metal cathodically derived fromMg ion and alkoxide ion is formed. On theother hand, tetrahydrofuran as a solvent isoxidized at the anode to generate thecorresponding cation intermediate, whichreacts with the cathodically formed alkoxideion to give α -alkoxytetrahydrofuran [54]. Inthis reaction, both anodic and cathodereactions participate to produce a singleproduct, thereby the total currentefficiencies do not exceed 100%. Theelectrolytic reaction shown in Figure 5.12 is

195

another example [55]. Thetrifluoromethylation of fumaronitrilerequires both anodic and cathodic reactions,hence this reaction is a kind of pairedelectrosynthesis.

Linear paired electrosynthesis(electrosynthesis of single productthrough anodic oxidation followed bycathodic reduction or vice versa): Asshown in Eq. 5.17, oxidation of2,3-butandiol provides acetoin, andafterwards acetoin is reduced at an Hg/Zncathode to ethyl methyl ketone [56]. The useof an undivided cell is essential in this case.

(5.17)

Paired electrosynthesis using EGBregenerated at the anode and cathodesuccessively or vice versa: Figure 5.13illustrates an example of this kind of pairedelectrosynthesis. An azobenzene is reducedto give the corresponding dianion (EGB),which then reacts with a mixture of ethylacetate and butyl bromide as the startingsubstrate to give the final product (ethylα-(acetyl)hexanoate) and EGBH2. TheEGBH2 is oxidized at the anode toregenerate the EGB [57].

196

Paired electrosynthesis usingdifferent mediators regenerated atthe anode and cathode: Dioxygen (O2) iscathodically reduced in aqueous andnon-aqueous solutions to reactive oxygenspecies such as H2O2 and O2

−√,respectively, which are oxidizing reagentsfor organic compounds. Hence, cathodicoxidation of organic substrates is realizedusing such cathodically generated oxygenspecies. However, the concentration ofcathodically generated H2O2 is so low thatthe oxidation rate of organic compoundswith H2O2 is not enough for a practicalelectrolytic process. Catalysts are thereforenecessary for the practical cathodicoxidation to be paired with the anodicoxidation. Nonaka and his co-workersdeveloped a unique cathodic oxidationsystem using a tungstate/pertungstate redoxmediator and applied this to the pairedelectrosynthesis, as shown in Figure 5.14[58]. N-Hydroxyamine derivative as a singlestarting substrate is indirectly oxidized tothe nitrone as a single product with HWO5

−

derived from HWO4− and H2O2 in a

cathodic chamber, and with halogen (X2)

anodically formed from X− in an anodicchamber. The total cathodic and anodiccurrent efficiencies for the nitrone were veryhigh (190%).

197

In addition, paired electrosynthesis was alsodeveloped by the combination of anodicoxidation and cathodic reduction using ahydroxyl radical derived from H2O2 andvarious metal ion redox mediatory catalysts,such as Fe2+/Fe3+, V4+/V3+, Cu+/Cu2+ andNi2+/Ni3+.

Parallel paired electrochemicalpolymer reaction: Quite recently, Inagi,Fuchigami and co-workers achieved the firstpaired electrochemical polymer reactions ofconducting polymers, such as alternatingcopolymers of 9-fluorenol and9,9-dioctylfluorene adsorbed on an anodeand cathode to form a 9-fluorenone moietyat the anode and a fluorene moiety at thecathode, respectively [59]. Since thereactions are solid phase, product isolationis very easy.

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Figure 5.11 Convergent pairedelectrosynthesis (a)

Figure 5.12 Convergent pairedelectrosynthesis (b)

Figure 5.13 Paired electrosynthesis usingregenerated EGB

Figure 5.14 Paired electrosynthesis usingcathodically generated hydrogen peroxide as anoxidant

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5.6 Reactive ElectrodesIn order to avoid the decomposition of productor intermediate once it has been formed at thecounter electrode, a divided cell is required.However, the divided cell increases the cellvoltage (the voltage between anode andcathode) and maintenance is also troublesome.When the desired reaction is cathodicreduction, anodically dissolving metalelectrodes are used as anodes to avoid both thedecomposition of product or intermediate at theanode and increase the cell voltage. Suchelectrodes are called sacrificial electrodes,sacrificial anodes or sacrificial metal anodes[60]. For instance, as shown in Eq. 5.18,cathodic carboxylation of organic halides usinga magnesium anode provides carboxylic acidselectively in high yield [61]. In this case, thecarboxylate anion formed is trapped withmagnesium ion to precipitate. Both the anodicoxidation of the formed carboxylate ion and itsesterification with unreacted organic halide cantherefore be avoided. In such cathodiccarboxylation, aluminium and zinc anodes, aswell as magnesium anodes, are often used.

200

(5.18)

Recently, a sacrificial metal anode was used inelectrosynthesis. The resulting metal ionsderived from the anode, e.g. Mg, Zn, Al and, Cu,often play important roles in electrochemicalreactions. These metal ions and cathodicallygenerated reactive species form highly reactiveintermediates new reagents or trap halide ionsgenerated by cathodic reduction of silylchlorides to polysilanes (see section 5.8.6) [62].Moreover, such anodically dissolving metal ionssignificantly affect regiochemistry,stereochemistry and product selectivity by theircoordination and catalytic effects as well asformation of a new reagents in situ [63]. Forinstance, as shown in Figure 5.15, zinc iongenerated from the anode and trifluoromethylanion cathodically derived from CF3Br forms anorganometallic compound as an intermediate,which undergoes a Reformatsky type reaction

201

with aldehydes and ketones to providetrifluoromethylated alcohols. In contrast, thecathodic reduction of CF3Br in the presence ofzinc ions does not provide any desired product[64]. Hence, anodically generated zinc ions arevery specific and quite different from ordinaryzinc ions.

Figure 5.15 Electrosynthesis using reactiveelectrode

Other typical examples are the cross-couplingreaction of activated halogenated compoundsand carbonyl compounds, and thecross-coupling reaction betweennon-halogenated compounds [65].

5.7 Electrochemical FluorinationOrganofluorine compounds are classified intotwo groups: perfluoro compounds and partiallyfluorinated compounds. The compounds in theformer class are widely utilized as functionalmaterials, while those in the latter family findbiological uses as pharmaceuticals andagrochemicals. Perfluoro compounds aremanufactured by converting all C–H bonds toC–F bonds using electrochemical fluorination

202

in anhydrous liquid HF as a solvent with anickel anode. This process is calledelectrochemical perfluorination and has alreadybeen commercialized (see Chapter 8) [66–69].Electrochemical perfluorination in KF-2HFmelts at a carbon anode has also beendeveloped for the preparation of perfluorinatedlow-molecular-weight organic compounds [69].

Electrochemical partial fluorination, orselective electrochemical fluorination, is a newmethod [69–72]. Since the discharge potentialof the fluoride ion is extremely high (>+2.9 Vvs. SCE at Pt anode in MeCN), the fluorinationproceeds via a (radical) cation intermediate, asshown in Eq. 5.19, which is the general pathwayfor anodic nucleophilic substitutions.

(5.19)

In this method, fluorine gas is not generatedand no hazardous reagents are required,therefore this electrochemical fluorination ismuch safer than the conventional chemicalmethod, which often requires hazardous and/ordifficult-to-handle reagents.

Selective electrochemical fluorination cancommonly be achieved in aprotic solvents suchas acetonitrile (MeCN), dichloromethane,

203

dimethoxyethane (DME), nitromethane andsulfolane containing fluoride ions to providemostly mono- and/or difluorinated products[72]. Electrolyses are conducted at constantpotentials slightly higher than the firstoxidation potential of the substrate by using aplatinum or graphite anode. Constant currentelectrolysis is also effective for selectivefluorination in many cases. The choice of thecombination of a supporting fluoride salt andan electrolytic solvent is important toaccomplish efficient selective fluorinationbecause competitive anode passivation (theformation of a non-conducting polymer film onthe anode surface that suppresses the Faradaiccurrent) takes place very often during theelectrolysis. Pulse electrolysis is in many caseseffective in order to avoid such passivation,therefore difficult-to-oxidize fluoride salts,which do not cause the passivation of the anodeand have strongly nucleophilic F–, are generallyrecommended as the supporting fluoride salts.Thus, room temperature molten salts such asR3N-nHF (n = 3–5), R4NF-nHF (n = 3–5) andpyridine poly(hydrogen fluoride) salt (Py-nHF)are most often used and even R4NBF4 andR4NPF6 salts are effective in some cases[70–72]. Particularly when HF supporting saltsand low hydrogen overpotential cathodes suchas platinum are used, the reduction of protons(hydrogen evolution) occurs predominantly atthe cathode during the electrolysis. A divided

204

cell is therefore not always necessary forfluorination under such conditions.

In aprotic solvent, F− becomes morenucleophilic, but the reactivity of F− is quitesensitive to the water content of the electrolysissystem because a hydrated F− is a weaknucleophile. Drying of both the solvent andelectrolyte is therefore necessary to optimizethe formation of fluorinated products.

A few examples are given below, and moreexamples have been reported in the literature.

5.7.1 Electrochemical Fluorination ofAromatic RingsAromatic compounds such as benzene,substituted benzenes and naphthalene areselectively fluorinated by constant potentialanodic oxidation in Et4N-3HF/MeCN orEt4NF-4HF without solvent (Eq. 5.20) [68,69].Fluorination proceeds via addition with twofluoride ions followed by elimination of HF toprovide monofluorinated aromatic compounds.Notably, ipso-substitution with fluorine alsotakes place in high yield, as shown in Eq. 5.21.

205

(5.20)

(5.21)

5.7.2 Electrochemical Fluorination ofOlefinsElectrochemical fluorination of olefins providesmono- and/or difluorinated products. Theselectivity depends on the molecular structureof the substrate and the electrolytic solvent, asshown in Eq. 5.22 [72,73]. For α-acetoxystyreneand 1-acetoxy-3,4-dihydronaphthalene, thecorresponding α-fluoroketones are formed asshown in Eq. 5.23 [72,74].

(5.22)

206

(5.23)

5.7.3 Benzylic ElectrochemicalFluorinationAlthough anodic benzylic substitution reactionstake place readily, anodic benzylic fluorinationdoes not always occur. The major competitivereaction is acetamidation when MeCN is usedas a solvent. For example, electrochemicalbenzylic fluorination in MeCN proceededselectively when the p-position of the phenylgroup was substituted by an electron-donatinggroup, as shown in Eq. 5.24 [75]. In contrast,α-acetoamidation became the major reactionwhen the phenyl group had noelectron-donating substituent.

(5.24)

Mono- and difluorination can be performedselectively depending on applied potential.

207

5.7.4 Electrochemical Fluorination ofSulfidesFuchigami and co-workers found that anodicfluorination of sulfides havingα-electron-withdrawing groups proceeded quitewell to provide the corresponding α-fluorinatedproducts in good yields, as shown in Eq. 5.25[72,76]. The fluorination proceeds by way of aPummerer-type mechanism via thefluorosulfonium cation (A), as shown in Eq.5.26 [77,78]. Thus, when R is anelectron-withdrawing group, the deprotonationof A is significantly facilitated, andconsequently the fluorination proceedsefficiently.

(5.25)

(5.26)

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5.7.5 Electrochemical Fluorination ofHeterocyclic CompoundsMany heterocyclic compounds have specificbiological activities, while it is known thatintroduction of fluorine atom(s) to organicmolecules dramatically changes or enhancestheir biological activities. However, selectivefluorination of heterocyclic compounds usingconventional fluorinating reagents is not easy.Many successful examples of selective anodicfluorination of heterocycles containing sulfur,nitrogen, oxygen and phosphine have beenreported to date [72,79]. Some examples areshown in Table 5.4.

Table 5.4 Electrochemical fluorination ofheterocyclic compounds

Substrate Salt/solvent

Product Yield(%)

Et3N-3HF/MeCN

74(translcis= 74/26)

209

Substrate Salt/solvent

Product Yield(%)

Et3N-3HF/MeCN

88

Et4NF-4HF/DME

66

Et4NF-4HF/DME

72

Et4NF-4HF/DME

81

210

The selectivity of fluorination is also stronglyinfluenced by supporting fluoride salts, asshown in Eq. 5.27 [80]. Since Et3N-3HFcontains the free base Et3N, the difluorinatedproduct, once formed, is dehydrofluorinated tothe monofluoro product.

(5.27)

Furthermore, anodic difluorinationaccompanied by C–C double bond cleavage isalso known, as shown in Eq. 5.28 [81].

(5.28)

Anodic fluorination of heterocyclic compoundsderived from optically active α-amino acidsproceeds in good yields and with highdiastereoselectivity, as shown in Eqs. 5.29 and5.30. Notably, fluorination does not occur at thebenzylic position and fluorinationpredominantly takes place α to the carbonylgroup, as shown in Eq. 5.31 [82].

211

(5.29)

(5.30)

(5.31)

5.7.6 Electrochemical Fluorination ofHeterocyclic Compounds with PhSGroup as ElectroauxiliaryHeterocyclic compounds having aphenylsulfenyl group are also anodicallyfluorinated in good yields, as shown in Eqs.5.32 and 5.33 [83,84].

212

(5.32)

(5.33)

It is noted that fluorination product selectivityis dramatically changed depending onelectrolytic solvents, as shown in Eq. 5.34 [85].

(5.34)

As mentioned earlier, anode passivation takesplace often, resulting in poor yield and lowcurrent efficiency. In order to avoid suchpassivation, various mediators, such as halideions, triarylamines and iodoarenes, can be used,as shown in Figure 5.16 [86].

213

Figure 5.16 Electrocatalyticfluorodesulfurization of β-lactams

Moreover, ethereal solvents such as DME aremuch more suitable than MeCN for the anodicfluorination of various heterocyclic sulfides (Eq.5.35) [87]. The pronounced solvent effect ofDME could be explained in terms of thesignificantly enhanced nucleophilicity offluoride ions as well as the suppression ofanode passivation and overoxidation offluorinated products.

(5.35)

214

5.7.7 Electrochemical Fluorination UsingInorganic Fluoride SaltsInorganic fluoride salts such as alkali-metalfluorides (MFs) are stable, easy to handle andinexpensive. They are therefore strongcandidates for reagents in nucleophilicfluorination as well as supporting electrolytes inchemical and electrochemical fluorination. Thechallenge is to overcome problems such as poorsolubility and low nucleophilicity of MFs inorganic solvents. Phase-transfer catalysts suchas crown ethers and quaternary ammonium orphosphonium salts are known to reduce thecoulombic interactions of MFs and arecommonly used for this purpose. Fuchihgamiand co-workers reported successful anodicfluorination in combination with anelectrochemical method using a poly(ethyleneglycol)/MF system where the MF is either KF orCsF, as shown in Figure 5.17 [88].

215

Figure 5.17 Poly(ethylene glycol) with twoterminal hydroxy groups serves as amultifunctional additive for anodic fluorinationusing KF

5.8 ElectrochemicalPolymerizationElectrochemical polymerization useselectrogenerated substrates for polymerizationas monomers or initiator. In the former case,the electrogenerated aromatic monomercouples in a polycondensation process to give aπ-conjugated polymer, in which the p-orbitalsof the aromatics overlap throughout thepolymer main chain. Such conjugated polymersare intrinsically conductive and therefore arecalled conducting polymers [89]. Generally theyare semi-conducting materials but chemical orelectrochemical doping imparts electricalconductivity to them. The conducting polymersare obtained on the surface of the workingelectrode as films because the electron transferof monomer and its coupling reaction proceedsnear the surface of the electrode and insolublepolymeric product is deposited on it. Althoughchemical electron transfer of monomers insolution is also available to produce thecorresponding conducting polymers, theinsoluble polymer powder obtained is difficultto process for application. The easy filmformation during electrochemical

216

polymerization is useful for application inelectrochemical devices such as sensors anddisplays [90]. This chapter deals withelectro-oxidative polymerization,electro-reductive polymerization andelectrosynthesis of polysilane. Polymerizationof vinyl monomers using an electrogeneratedinitiator system is summarized elsewhere [91],although recent developments in this processare included in this chapter.

5.8.1 Electro-oxidative Polymerization ofAromatic MonomersThe electron-rich aromatic and hetero-aromaticmonomers listed in Figure 5.18 can be easilyoxidized on the anode surface to form theirradical cation states. The radical cations coupleto form C–C bonds, and followingdeprotonation this leads to a neutral dimer (Eq.5.36). The generated conjugated dimer has alower oxidation potential than that of themonomer, so further oxidation of the dimerleads to oligomerization and polymerization.The highly conjugated polymers are no longersoluble in electrolytic medium and therefore aredeposited on the anode surface as a film. Thedeposited polymer is oxidatively doped duringthe application of potential for polymerization,thus the film formed is not passive but stillconductive to carry out a continuouselectrochemical reaction on the electrode. Inthis procedure, the coupling position of

217

monomer can be predicted by the spin-densitydistribution on the aromatic ring [92]. Theintroduction of electro-auxiliaries such as a silylgroup can enable dominant coupling at aspecific position [93].

(5.36)

Figure 5.18 Aromatic monomers for oxidativepolymerization

5.8.2 Electrochemical PolymerizationThere are several methods of electrochemicalpolymerization, for example the potentialsweep, potentiostatic (constant-potential) andgalvanostatic (constant-current) methods. Asuitable method should be chosen dependingon the monomers and conditions used. Cyclicvoltammetry (CV) measurement of a monomerin a three-electrode setup gives information onthe oxidation potential for electrochemicalpolymerization. To prevent overoxidation of aproduct polymer, the application of a higher

218

potential to a working electrode for a longperiod should be avoided since the productpolymer with extended π-conjugation is easilyoxidized compared to the correspondingmonomer. The potential sweep andpotentiostatic methods require milderpolymerization conditions than thegalvanostatic method.

In the potential sweep method, polymerizationis carried out with CV analyzer. A workingelectrode, a counter electrode and a referenceelectrode are put together in an electrolytic cellcontaining electrolyte and monomer. Therepeating potential sweep across the potentialrange determined from the advanced CVmeasurement produces product polymer filmon the working electrode and thiselectron-transfer process can be monitored byCV, as shown in Figure 5.19, which showstypical cyclic voltammograms of pyrrole duringelectropolymerization. The first scan representsthe oxidation current of pyrrole monomer andthen polymerization proceeds at the surface ofthe working electrode, resulting in anirreversible voltammogram. In the course ofdeposition of the product polypyrrole duringrepeated scans, anodic and cathodic currentsderived from redox of polypyrrole at a lowerpotential range gradually increase.

219

Figure 5.19 Typical cyclic voltammograms ofpyrrole monomer

In the potentiostatic method a three-electrodesystem is used and the applied potential to theworking electrode is controlled by apotentiostat with a monitoring charge. In thegalvanostatic method a two-electrode systemconsisting of a working electrode and a counterelectrode is used, and constant current ispassed between them by a DC power source.This process cannot control potential on the

220

working electrode, which may cause problemssuch as overoxidation of the product polymers.

5.8.3 Conditions for ElectrochemicalPolymerizationThe concentration of monomer is oneimportant factor in electrochemicalpolymerization. If the polymerization does notproceed at the electrode surface, the use of a lotof monomer may be effective to promote filmformation of the conducting polymers. Thechoice of material for the working electrode isalso important in terms of not onlyoverpotential but also compatibility withconducting polymer films.

In addition to conventional electrolyticsolutions such as the supporting salt/solvent,room temperature ionic liquids are known to besuitable as polymerization media [94]. Thereare several advantages of electropolymerizationin ionic liquids: (i) electropolymerizationproceeds rapidly, (ii) the compatibility of theproduct film with the electrode surface is goodand (iii) product polymers have a dense andsmooth surface, high electric conductivity, highelectrochemical capacitance and goodreversibility of redox cycles.

Recently boron trifluoride–ether complex hasbeen shown to be attractive as a polymerizationmedium [95]. Boron trifluoride acts as a Lewisacid to form a complex with aromatic

221

monomer, which reduces the oxidationpotential of the monomer. This method istherefore effective for polymerization ofaromatic monomers with relatively highoxidation potentials. The reduction inpolymerization potential can avoidoveroxidation of product polymers.

5.8.4 Electrochemical DopingIf a polymer film on a working electrode isobtained by electropolymerization, it should beput into electrolytic solution without monomer(monomer-free electrolyte) in order to studythe redox behaviour of the conducting polymer.Typically a pair of broad redox responses isobserved in the voltammogram. The injectionor removal of electrons to or from conductingpolymers results in the formation of polaronsand bipolarons in the repeating structure(commonly known as doping), producingconsiderable variations in the physicalproperties of the polymer itself and impartingfeatures such as drastic colour changes andelectrical conductivity [89]. The charges thusgenerated along the polymer must becompensated for by the addition ofneighbouring ions (or dopants) and theinsertion and release of such dopants mayinduce volume changes in the conjugatedpolymer. The doping ratio can be estimatedfrom the amount of charge generated in theconducting polymers.

222

5.8.5 Electro-reductive Polymerizationof Aromatic MonomersAromatic dihalides are representativemonomers for electroreductive polymerization.Cathodic reduction of an aromatic dihalidegenerates a radical anion, followed bypolymerization accompanying elimination ofhalide (Eq. 5.37). Bond formation takes place atthe specific position of the monomer where thehalogen atom is substituted.

(5.37)

On electroreductive polymerization of1,4-dibromopyridine, addition of a catalyticamount of nickel(II) complex is effective andthe complex works as a mediator.Electrogenerated nickel(0) promotesdehalogenation of the dibromopyridinefollowed by its polymerization [96].Electro-oxidative polymerization of pyridine isdifficult because of its high oxidation potential,but this reductive polymerization enablespolypyridines to be produced.

Poly(p-phenylene vinylene) is also synthesizedby electroreductive polymerization. As shown inEq. 5.38, the electrochemical reduction ofα,α,α′,α′-tetrabromo-p-xylene generates aquinodimethane intermediate and its

223

subsequent polymerization gives apoly(xylylene). This is further reduced topoly(p-phenylene vinylene) [97].

(5.38)

5.8.6 Applications of ConductingPolymersThe physical properties of conducting polymersare drastically changed by the doping process.Table 5.5 summarizes the application ofconducting polymers. The undoped (neutral)state of conducting polymers is generallysemi-conducting, and it can be applied tomaterial for transistor, photovoltaic cell andelectroluminescence devices. On the otherhand, electrical conductivity appears in thedoped state of conducting polymers, which isuseful in conducting materials, anti-statics andcapacitor materials. The doping/dedopingprocess of conducting polymers is also usefulfor electrochromic devices, sensors andactuator applications. Application of conductingpolymers for electronics will be described inChapter 7.

Table 5.5 Applications of conducting polymerfilms

224

Property Application

Conductivity(doped state)

Conducting materials,antistatics, capacitormaterials

Doping/dedopingprocess

Electrochromic materials,sensors, actuators, memory

Semi-conductivity(undoped state)

Transistors, photovoltaiccells, electroluminescentmaterials

5.8.7 Electrochemical Synthesis ofPolysilanesPolysilanes composed of linearly connectedSi–Si bonds are a class of σ-conjugatedpolymers and show unique optical properties[98]. The Kipping method, a well-knownsynthetic method of obtaining polysilanes froma dichlorosilane, is powerful but the use of Nametal is indispensable. Electrochemicalreduction of chlorotrimethylsilane affordshexamethyldisilane without the use of areducing reagent. Although polymerization ofdichlorosilanes by electrochemical reduction israther difficult, the use of an Mg sacrificialanode results in the effective formation ofpolysilanes (Eq. 5.39) [99]. The mechanismproposed is that Mg2+,once it has beenanodically dissolved from the anode, is reducedat the cathode to give reactive Mg. This involvesSi–Si bond formation of monomers. Cu and Al

225

are also available as sacrificial anode materialsfor reductive polymerization of polysilanes.

(5.39)

Reductive polymerization of dichlorosilanes hasled to a variety of copolymers, such as randomcopolymers of silane monomers,poly(carbosilane)s, and random copolymers ofsilane monomer and germane monomer(Figure 5.20) [100].

Figure 5.20 Structures of copolymers basedon silane and germane prepared by reductivepolymerization

5.8.8 Chain Polymerization Initiated withElectrogenerated Reactive SpeciesElectrogenerated reactive species of smallmolecules can work as polymerization initiatorsfor ionic and radical polymerization of vinylmonomers [91]. For example, a methyl radicalgenerated by Kolbe electrolysis of acetate can bean initiator for radical polymerization of olefins.The heterogeneity of initiator concentration in

226

the electrochemical setup makes it difficult toobtain high molecular weight polymers.

Recent progress in controlled radicalpolymerization using transition metal catalystsis remarkable both in academia and industry[101]. As shown in Eq. 5.40, the equilibrium(ka/kda) between domant species (A) and activespecies (B) controlled by redox of a transitionmetal catalyst can control the rate of monomerconsumption, resulting in polymerization witha controlled manner. The external stimuli byelectrochemical oxidation and reduction of themetal catalyst change the equilibrium andconsequently more precise control ofpolymerization is possible (Eq. 5.40) [102].

(5.40)

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79. Dawood, M. (2004) Tetrahedron, 60,1435–1451.

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81. Shaaban, M.R., Inagi, S. and Fuchigami, T.(2009) Electrochim. Acta, 54, 2635–2639.

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83. Narizuka, S. and Fuchigami, T. (1993) J.Org. Chem., 58, 4200–4201.

84. Cao, Y., Hidaka, A., Tajima, T. andFuchigami, T. (2005) J. Org. Chem., 70,9614–9617.

85. Ishii, H., Yamada, N. and Fuchigami, T.(2000) Chem. Commun., 1617–1618.

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5Organic Electrosynthesis

242

Toshio Fuchigami, Mahito Atobe and ShinsukeInagi

In the 21st century, environmentally friendlyprocesses are becoming much more important.The concept and significance of greensustainable chemistry (GSC), which wasdeveloped in the USA, has been recognizedthroughout the world, and nowadays newprocesses cannot be developed withoutconsideration of GSC. Since the latter stages ofthe last century, much attention has been paidto organic electrosynthesis as a typicalenvironmentally friendly process. Furthermore,in this century, electrolytic reactions with muchlower emission processes and electrochemicalmethodologies based on new concepts havebeen developed. These related studies havebecome an active research area [1].

In this chapter, new methodologies for organicelectrolysis are described. Even though somemethodologies are not very new, they arementioned because they are highly useful fromthe viewpoint of GSC.

6.1 SPE Electrolysis and ItsApplicationsIn contrast to organic synthesis, electrolyticreactions require supporting salts. It is

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troublesome and time-consuming to separatethe desired product from an electrolyticsolution containing a large amount ofsupporting salts. Moreover, supporting salts areusually not recovered and recycled, and becomewaste after electrolysis. In general, supportingsalts are very soluble in polar solvents, but,depending on the starting organic substrates,some are not soluble in polar solvents, only innon-polar solvents. As mentioned previously,the correct choice of supporting salt andelectrolytic solvent is not easy. In sharpcontrast, electrolysis using solid polymerelectrolytes (SPE) does not require anelectrolytic solution, therefore these problemsare readily solved [2,3].

6.1.1 Principle of SPE ElectrolysisBy chemical plating, fine metal particles aredeposited on the surface of ion-exchangemembranes like Nafion®, and then a wiremesh electrode or porous electrode made of thesame metal is pressed on the opposite surface ofthe membrane to provide an SPE compositeelectrode, which has dual roles of electrode andsupporting salt.

The general advantages of the SPE electrolysissystem for organic synthesis can be described asfollows: (i) to economize the separation andrecycling of a supporting salt and (ii) to avoidany contamination or side reaction with asupporting electrolyte. There are two types of

244

SPE systems. In one an electrode is pressedonto only one side of the surface of theion-exchange membrane and in the other twoelectrodes are pressed on both surfaces. Figure6.1 illustrates the latter SPE system. SubstrateAH2 is oxidized at the anode and the resultingprotons migrate through the cation exchangemembrane to the cathode. Thus, ionicconductivity, namely proton conductivity, isavailable. The arriving protons are reduced atthe cathode to generate hydrogen gas solely inthe absence of substrate (Figure 6.1a), whilesubstrate B is reduced to BH2 in the presence ofsubstrate B (Figure 6.1b). In the latter case, twoproducts, A and BH2, are obtained from twostarting substrates, AH2 and B, respectively,therefore this electrolysis can be considered tobe a kind of paired electrosynthesis.

Figure 6.1 Principle of electrolysis using anSPE system with platinum electrodes

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In the case of the SPE electrolysis system inwhich an electrode is pressed onto only a singleside of the surface of an ion-exchangemembrane, the counter-electrode chamber isfilled with electrolytic solvent. In SPE systems,even non-polar solvents like hexane can be usedas electrolytic solvents. Furthermore, when thestarting substrate is a liquid or gas, electrolysiscan be performed without any solvents.Moreover, the amount of redox mediator usedcan be reduced by immobilizing it on the SPEcomposite electrode.

Various organic electrosyntheses using SPEsystems have been reported to date, e.g.hydrogenation of olefins, reduction ofnitrobenzene, Kolbe electrolysis,dimethoxylation of furan and ketone synthesisusing a halogen mediator, etc. [2–4]. SPEelectrosynthesis is an energy-efficientelectrolytic method since the cell voltage is lowowing to the small distance between electrodes.However, the durability of SPE compositeelectrodes is not high and contamination of theSPE membrane decreases the current efficiencyand yield. Those problems have not yet beensolved and a commercialized SPE organicelectrosynthesis has not been established todate. The original concept for SPE organicelectrosynthesis is a polymer electrolyte fuel cell(PEFC). Thus, the development of newmaterials such as membranes and increased

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knowledge of fuel cell research should stimulatethe progress of SPE electrosynthesis.

6.1.2 SPE Electrolysis withCogeneration (Chemicals ProductionUsing Fuel Cell Reactions)The SPE electrolysis system has been applied tocommercialized water electrolysis (watersplitting) and fuel cells. A fuel cell is a device toconvert Gibbs free energy in a chemical reactioninto electricity through electrochemicalreactions. In an H2–O2 fuel cell, electricity isobtained through the formation of water fromoxygen and hydrogen. This principle suggeststhat catalytic oxidation and reduction inchemical synthesis can be converted to fuel cellreactions at an anode and cathode. Forinstance, the Wacker oxidation of ethylene iscatalyzed by redox species like Pd2+/Pd0 andCu2+/Cu+, as shown in Figure 6.2. Based on theflows of protons and electrons in the reactionscheme, it can be expected that the Wackeroxidation of ethylene to acetaldehyde withoxygen would be possible using fuel cellreactions, as shown in Figure 6.3.

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Figure 6.2 Reaction mechanism of the Wackeroxidation reaction

Figure 6.3 Fuel cell reaction

In fact, ethylene together with water vapourand oxygen are carried into the left and rightcompartments, respectively, as shown in Figure6.3, and then the circuit is closed. Under theseconditions, current flows and acetaldehyde isformed highly selectively (95%) at 100°C. Thus,in this system, both flow of electric current andautoxidation of ethylene take placespontaneously even if potential is not applied.

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This fuel cell system enables cogeneration ofelectricity and chemicals [5].

6.2 Electrolytic Systems UsingSolid Bases and AcidsOrdinary soluble bases are readily oxidizedanodically, but insoluble solid-supported basesare not oxidized at the anode. Based on thisfact, new electrolytic systems have recentlybeen developed using recyclable and reusablesolid-supported bases and acids as supportingelectrolytes [5–8]. For instance, as shown inFigure 6.4, solid (silica gel or porouspolystyrene)-supported bases like piperidinedissociate protic organic solvents like methanoland acetic acid to protons and anions, and theresulting protons act as the main carriers ofelectronic charge.

Figure 6.4 Dissociation of protic solvents bysolid base

This system is widely applicable to variousanodic nucleophilic substitutions, such asmethoxylation and acetoxylation of variouscompounds (Eq. 6.1). Moreover, it is notablethat after the electrolysis, solid-supported bases

249

are easily separated by only filtration and theproduct is readily isolated from the filtrate. Therecovered solid-supported bases can be reusedmany times, as shown in Figure 6.5.

(6.1)

Figure 6.5 Recovery and recycle system ofanodic methoxylation and acetoxylation usingsolid base

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6.3 Solid-Supported MediatorsAlthough mediators have many advantages asexplained in Chapter 2, 3, and 5, they have to beremoved from the electrolytic solution afterelectrolysis. However, mediators have neverbeen recovered after electrolysis for a long time.Quite recently, disposal-type mediators havebecome a problem from GSC, particularly atomeconomical aspect. In order to overcome suchproblems, easily separable and recyclablemediators have been developed.

At an early stage of this approach, across-linked poly-4-vinylpyridinehydrobromide as a solid-supported mediatorysystem was developed [9]. As shown Eq. 6.2,Br− derived from the solid base is anodicallyoxidized in an aqueous electrolytic solution togenerate hypobromide ion, BrO−, whichoxidizes alcohols to give ketones, and then Br−

is regenerated. Thus, the mediator can be easilyrecovered as the HBr salt of the solid base.

(6.2)

Since N-oxyl radicals as represented by TEMPOgenerally show reversible redox properties, asshown in Eq. 6.3, they are useful mediators for

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the oxidation of alcohols. N-Oxyl adsorbed onthe surface of silica gel disperse mediatorysystem has been developed (Figure 6.6) [10].

(6.3)

In this dispersive system, Br− is at firstanodically oxidized to generate Br+, whichoxidizes N-oxyl on the silica gel to formoxoammonium ion (N=O+) followed byoxidation of alcohols to produce ketones. In thiscase, the oxidation can be achieved withoutorganic solvent. One of advantages of thedispersed system is as follows. Afterelectrolysis, electrolytic solution is filtered andthe remaining silica gel is washed withappropriate solvents like acetone to provide theproduct. However, during washing, themediator also dissolves into the organic solvent.In order to avoid dissolution of the mediator,N-oxyl-immobilized silica gel was developed asa readily recyclable mediator [11].N-Oxyl-immobilized polymer particles havealso been developed. Furthermore, thecombination of electrodes modified withTEMPO-immobilized polymer and chiral basesis known to be highly useful for asymmetricsynthesis [12] (see also section 5.3).

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Figure 6.6 Indirect oxidation of alcohols withN-oxyl-immobilized silica gel

6.4 Biphasic Electrolytic SystemsIn general, electrochemical reactions arecarried out in homogeneous electrolyticsolutions. However, the electrochemicalreaction can be conducted even inheterogeneous electrolytic solutions unless bothelectron transfer and mass transfer processesare not inhibited by the reaction system. In thissection, we will discuss biphasic electrolyticsystems such as emulsion electrolysis,suspension electrolysis, electrolysis using phasetransfer catalysis and the thermomorphilicbiphase electrolytic reaction system [13–15].

6.4.1 Emulsion ElectrolysisAs shown in Figure 6.7, when the product is anoxidizable and reducible compound such as analdehyde, an organic phase in a biphasic

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electrolytic system (in general, a systemcomposed of organic and water phases in whichorganic droplets are finely dispersed into water(aqueous electrolyte)) is often used to extractthe product in order to prevent itsoveroxidation or over-reduction at theelectrode.

Figure 6.7 Emulsion electrolysis

On the other hand, immiscible organiccompounds are often electrolyzed in theiremulsion. However, in these cases, theemulsion contributes to maintain the saturatedconcentration of immiscible organic substratein an aqueous electrolyte and the emulsifieddroplet itself is not directly electrolyzed at theelectrode.

Unlike the above emulsion electrolytic systems,the direct electrolysis of the emulsified dropletscan also take place at the electrode if thedroplets contain supporting electrolytes andtheir presence allows the formation of an

254

electric bilayer inside the droplets. However, inthis case, droplets should be miniaturized to thesub-micrometre range by the use of specialsurfactants or ultrasonication in order to obtaina practical reaction rate [13].

6.4.2 Suspension ElectrolysisThe direct electrolysis of suspensions ofnon-electronconductive solid materials such asmost organic compounds cannot take place inprinciple, since electrochemical electrontransfer between a solid electrode and anon-conductive solid particle hardly ever occurseven when they contact with each other. On theother hand, the indirect electrolysis ofsuspended solid particles using soluble redoxmediators may be able to take place, since thisconsists of a series sequence of two possibleelectrochemical and chemical electron-transfersteps between solid and liquid phases (electrode(solid)–dissolved mediator (liquid)–suspendingparticle (solid)) (Figure 6.8) [14]. Similarly tothis case, the electrolysis of emulsified liquiddroplets can be conducted by usingelectron-transfer mediators.

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Figure 6.8 Suspension electrolysis

6.4.3 Electrolysis Using Phase-TransferCatalysisAnodic aromatic substitutions have beencarried out using dispersions of an organicsolvent (usually methylene chloride) in aqueousmedia. Current flow takes place mainly throughthe relatively conducting aqueous phase(thereby lowering power costs) while thesynthetic reactions have been assumed to beconfined to the organic phase; the function ofthe phase-transfer catalyst is to transfer withanions into the organic phase both to conferadequate conductivity to this phase and toprovide the coupling agent (e.g. CN− orCH3COO−) for the intermediates generatedfrom the aromatic substrates (Figure 6.9).

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Figure 6.9 Example of electrolysis usingphase-transfer catalysis

6.4.4 Thermomorphic BiphasicElectrochemical Reaction SystemChiba and co-workers constructedthermomorphic biphasic electrochemicalreaction systems composed of organic solventsof different polarity (e.g.cyclohexane–nitroalkanes) [15]. As shown inFigure 6.10, in the thermally immixedhomogeneous electrolyte solution, the anodicdesulfurization proceeds smoothly to giveo-quinone methide, and then reacts withterpene to give the cycloadduct as a finalproduct. After the completion of theelectrolysis, the product as the cyclohexanesolution can be separated from the polarelectrolytic solution (nitroalkane phase) simplyby on-cooling phase separation. In addition, theseparated polar electrolytic solution can bereused for the next cycle of the reaction.

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Figure 6.10 Thermomorphic biphasicelectrochemical reaction system

6.5 Cation Pool MethodElectro-oxidatively generated carbocations canreact with nucleophiles to form covalent bonds.Because of the short life-time of carbocations, itis necessary to generate carbocations in thepresence of nucleophiles, followed byimmediate reaction. In the above system, thenucleophiles used should have higher oxidationpotential than precursor molecules ofcarbocation. Since the carbon nucleophile iseasily oxidized, the formation of carbon–carbonbonds using carbocations generated by anodicoxidation is generally difficult.

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The cation pool method can preservecarbocations stabilized by neighbouringheteroatoms or aromatics at low temperature(–78°C) without decomposition. Addition ofnucleophiles to the cation pool results indesirable nucleophilic reaction (Figure 6.11)[16,17].

Figure 6.11 Nucleophilic substitution reactionby the cation pool method

At low temperature, the viscosity of theelectrolytic solution increases and thisdecreases the ionic conductivity of the solution.Dichloromethane is a suitable solvent for thecation pool method that has sufficientconductivity for electrolysis. With a dividedelectrolytic cell, the substrate is injected intothe anode chamber and trifluoromethanesulfonic acid (the proton source) is injected intothe cathodic chamber to promote a sacrificialcathodic reaction.

A variety of cation pools are successfullygenerated, as shown in Figure 6.12. The cationpool of alkoxycarbenium ions is obtained byanodic oxidation of α-silyl ethers at lowtemperature, and the addition of allylsilanes ascarbon nucleophiles yields the desired products

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efficiently. Similarly, an N-acyliminium ionpool and a diarylcarbenium ion pool areavailable for the resulting chemicaltransformations. Figure 6.13 shows examples ofnucleophilic reactions of accumulatedN-acylcarbenium ion species.

Figure 6.12 Examples of cation pools

Figure 6.13 Various reactions of theN-acyliminium cation ion pool

The concept of the cation pool method can beextended to a cation flow method incombination with a micro-reactor system (see

260

section 6.8). This method makes it possible notonly to survey short-life intermediatesgenerated electrochemically but also to realizethe combinatorial synthesis of useful materials.

6.6 Template-Directed MethodsIn general, a regioselective intermolecularcoupling reaction is difficult compared to anintramolecular one. Template-directedreactions via intramolecular reactions enableselective coupling reactions that are otherwisedifficult to achieve. As shown in Figure 6.4,regioselective anodic coupling of phenols can beachieved using a boron template [18]. First, thetemplate, tetraphenoxyborate, is prepared in aone-pot-procedure and then its anodicoxidation followed by hydrolysis provides anortho-ortho coupled product selectively.

(6.4)

A silicon template is also known [19].

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6.7 Electrolysis in SupercriticalFluidsThe commercialization of electrosyntheticprocesses has been restricted by the limitedsolubility of substrates and products inconventional electrolytic solutions, the poorinterphase mass transport characteristicsassociated with the two-phase system in whichthe reaction occurs at solid (electrode)–liquid(electrolyte) interfaces, the low selectivity of thedesired reaction products and the complexprocessing schemes often used to recoverproducts.

Supercritical fluid solvents can overcome manyof the limitations associated with conventionalsolvents such as water and organic solvents.Supercritical fluids have traditionally beenapplied to chromatography, extraction, cleaningand so on [20]. Moreover, they are becomingwidely recognized as useful media for organicand polymer syntheses in a range of laboratoryand industrial processes because of their lowtoxicity, ease of solvent removal, potential forrecycling and variation of reaction rates [21,22].

A supercritical fluid can be defined as anysubstance that is above its critical temperature(Tc) and critical pressure (Pc), and exists as asingle phase (Figure 6.14). Thephysico-chemical properties of a supercriticalfluid are between those of liquids and gases,

262

and they can vary between those exhibited bygases to liquid-like values by small changes inpressure and temperature [20]. For example,supercritical fluids exhibit larger diffusivilityand lower viscosity than conventional liquids.Consequently, interphase mass transferresistance is also lower relative to the liquidsolvent. On the other hand, substrates andproducts that are sparingly soluble in the liquidphase can become significantly more soluble insupercritical fluids. Such unique and usefulproperties of the fluids make them viablesolvent media for electrosynthesis.

Figure 6.14 Pressure–temperature phasediagram

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Conventional supercritical solvents, such ascarbon dioxide and ethane, have mild criticalconditions, but they lack the ability to solvatepolar electrolytes. A reaction medium shouldtherefore generally be designed by mixing withco-solvent such as methanol [23], ethanol [24],acetone [24], acetonitrile [25] or DMF [26] toproduce a suitable electrolytic solution.

Dombro et al. reported that dimethyl carbonatehas been electrosynthesized from carbonmonoxide and methanol in a supercriticalcarbon dioxide–methanol medium (Eq. 6.5)[27]. They added 0.35 mole fraction ofmethanol to supercritical carbon dioxide toproduce a mixture capable of dissolving anammonium salt. Such a mixture is capable ofionic conductivity and the electrolysisproceeded smoothly to afford dimethylcarbonate in an excellent current efficiency(near 100%).

(6.5)

In another example, Tokuda et al. reportedelectrochemical carboxylation of benzylchloride, cinnamyl chloride and2-chloronaphtalene in supercritical carbondioxide–DMF media (Eqs. 6.6–6.8) [26]. Amixture used in this work also exhibited theability to solvate polar electrolytes likeammonium salts. In these reactions, since

264

carbon dioxide plays the role of a reactant,CO2-rich conditions such as supercriticalconditions are suitable for improving productyields.

(6.6)

(6.7)

(6.8)

As mentioned above, conventional supercriticalfluids like supercritical carbon dioxide are thelow dielectric constant of the fluid. Hence,many supporting electrolytes are usually hardto dissolve in the fluid. A polar organic solventshould therefore be added to the fluid toproduce a suitable electrolytic solution.However, the inclusion of a co-solvent limitsthe accessible potential range and introducescomplications. The use of supercritical

265

fluorinated hydrocarbons is an attractivealternative which avoids these problems. Forexample, supercritical fluoroform exhibitsrelatively high solubility and its dielectricconstant can be controlled from 1 to 7 bymanipulating either the temperature or thepressure of scCHF3 without any additives likepolar solvents [28]. Supercritical fluoroformhas therefore been used as the reaction mediumin electrochemical syntheses.

Atobe et al. reported the electrochemicalsynthesis of polypyrrole and polythiophene insupercritical fluoroform [29]. Both thecorresponding monomers (pyrrole andthiophene) can be electropolymerized faster insupercritical fluoroform than in a conventionalorganic media like acetonitrile solution, and thefilms obtained have a highly uniform structure(Figure 6.15).

Figure 6.15 SEM photographs of polypyrrolefilms polymerized in (a) acetonitrile and (b)supercritical fluoroform solutions

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Moreover, Atobe et al. successfully prepared apolythiophene nano-brush using templateelectrochemical polymerization in supercriticalfluoroform (Figure 6.16) [30]. In this work,nanoporous alumina membranes (60 µm thick,200 nm pore size) coated on one side withevaporated Pt (about 500 nm thick) wereemployed as a template electrode forpolythiophene electrodeposition into pores. Theuse of the special properties of the fluids suchas higher diffusivility and lower viscosityenabled effective monomer transport into theporous template and nanoprecise filling withthe polymers. Consequently, the solidpolythiophene nano-brush was obtained afterremoval of the alumina membrane.

Figure 6.16 Preparation ofpolythiophenenano-brush using templateelectrochemical polymerization in supercriticalfluoroform

6.8 Electrolysis in Ionic LiquidsIonic liquids consist of cations and anionswithout any solvent, and they are in a liquid

267

state around room temperature. Since ionicliquids have unique properties likenon-flammability, thermal stability,non-volatility and reusability, they have beenintensively studied as a green solvent frombasic and applied aspects [31–35]. Since theyhave also good electroconductivity, muchattention has been paid to their application inorganic electrochemical devices. In fact,intensive studies on applications torechargeable lithium batteries, electricdouble-layer capacitors, wet solar cells and fuelcells have recently been carried out.

In this section, the structures and physicalproperties of ionic liquids are explained andtheir application to electrolytic reactions isdescribed.

6.8.1 Structures of Ionic LiquidsIonic liquids are roughly classified into twofamilies based on kinds of cations. One consistsof nitrogen-containing heteroaromatic cationsand the other consists of aliphatic ammoniumcations. Aliphatic ionic liquids are subdividedinto open-chain and cyclic. Based on kinds ofanions, ionic liquids are divided into fiveclasses: chloroalminate, fluoroinorganic,fluoroorganic, poly(hydrogen fluoride) andnon-fluoroionic. In addition, recentlyphosphine-containing ionic liquids likephosphonium salts have been prepared, andvarious novel types of ionic liquids with other

268

new anions have been continuously developed(Figure 6.17).

Figure 6.17 Typical examples of the structuresof ionic liquids and their abbreviations

6.8.2 Hydrophilicity and Hydrophobicityof Ionic LiquidsIonic liquids can also be classified ashydrophilic or hydrophobic, as shown in Table6.1. Regardless of cation type, ionic liquidshaving BF4

− and CF3SO3− as an anion are

hydrophilic while those with PF6− and

(CF3SO2)2N− are hydrophobic. As acharacteristic property, hydrophobic ionicliquids are miscible with neither water norordinary organic solvents like ether and hexane,and hence they make phase separation resultingin formation of three phases. The products cantherefore be separated by liquid–liquid

269

extraction, which is one of the big advantages ofionic liquids.

Table 6.1 Hydrophobic and hydrophilicproperties of imidazolinium salt ionic liquids

Ionic liquid Property

[emim]BF4 Hydrophilic

[bmim]PF6 Hydrophobic

[bmim]BF4 Hydrophilic

[emim](CF3SO2)2N Hydrophobic

[bmim](CF3SO2)2N Hydrophobic

[bmim]CF3SO3 Hydrophilic

[emim]CH3OSO3 Hydrophilic

[emim]CF3SO3 Hydrophilic

[emim]NO3 Hydrophilic

In the case of the widely used imidazolium ionicliquids, the relationship between theirmolecular structures and physical properties,such as melting point, viscosity andelectroconductivity, is as follows.

With increasing alkyl chain length theviscosity increases while the melting pointand electroconductivity decrease. However,the relationship between melting point andalkyl chain length is not always observed, asshown in Figure 6.18. In order to decreasethe melting point an unsymmetrical or bulky

270

cation should be used, which results in aweak columbic interaction between cationand anion.

Figure 6.18 Relationship between alkylchain length of imidazolinium salt ionicliquids and physical properties (viscosity,melting point and electroconductivity)

When a methyl group is introduced to the2-position of the imidazolium ring,hydrogen bonding at the 2-position cannotoccur. The melting point does not decreaseunexpectedly but increases. Thus, therelationship between hydrogen bonding andmelting point has not yet been clarified.

When an alkyl group is introduced to the2-position of the imidazolium ring, thereduction of the ring becomes difficult butoxidation becomes easier. Moreover, whenthe imidazolium ion as cation is changed toan aliphatic ammonium ion, the oxidativeresistance increases, and the viscosity andmelting point also increase. Regarding theviscosity, imidazolium ionic liquids have thelowest viscosity among various types of ionicliquids at the present time (Figure 6.19).

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Figure 6.19 Relationship between cationicmoieties of ionic liquids and physical properties(viscosity, melting point andelectroconductivity)

6.8.3 Polarity of Ionic LiquidsThe polarity of ionic liquids can be estimated bythe energy difference, between the groundstate and the excited state of Reichrdt dye,which is commonly used for the polarityevaluation of ordinary solvents. The polarity ofimidazolium ionic liquids and pyridinium ionicliquids is similar to that of ethanol, and is alittle lower than that of methanol. However,their polarity is much higher than that of MeCNand DMF, therefore it is equivalent to that ofprotic solvents. The polarity of ionic liquids andorganic solvents can be illustrated as follows:

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6.8.4 Electrochemical Properties of IonicLiquidsAlthough the electroconductivity of anon-aqueous electrolyte is lower than that of anaqueous one, as shown in Table 6.2,non-aqueous electrolytes have the advantage ofa wide usable potential window. Accordingly,ionic liquids belong to non-aqueouselectrolytes.

Table 6.2 Classification of ionic liquids andelectroconductivity

Electrolyte Electroconductivity(at 25°C) (mScm−1)

Aqueoussystem

Acid 35 wt%H2SO4/H2O

848

Alkaline 30 wt%KOH/H2O

625

Neutral 30 wt%ZnCl2/H2O

105

Non-aqueoussystem

Organic 1 MLiPF6/EC+EMC

9.6

Inorganic 2 MLiAlCl4/SOCl2

20.5

Ionic liquid [emim]BF4 13.6

As shown in Table 6.3, imidazolium ionicliquids generally have relatively good

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electroconductivity, while poly(hydrogenfluoride) salt ionic liquids exhibit rather goodelectroconductivity regardless of cation (Table6.4). 1-Ethyl-3-methylimidazolium (emim) saltsshow the highest electroconductivity at roomtemperature among the series of ionic liquids.Furthermore, since the electroconductivity ofionic liquids is correlated with the mobility ofions, it greatly depends on the viscosity of ionicliquids. Accordingly, the molecular design ofionic liquids with low viscosity is of greatimportance. Aliphatic ammonium ionic liquidsexhibit lower conductivity since they generallyhave a higher viscosity compared toimidazolium ionic liquids. In contrast, ionicliquids with poly(hydrogen fluoride) as theanion have extremely low viscosity, andtherefore have good low-temperatureproperties.

Table 6.3 Physical and electrochemicalproperties of imidazolinium salt ionic liquids at25°C

Ionic liquid Meltingpoint(°C)

Density(gcm−1)

Viscosity(mPa)

Conductivity(mS cm−1)

Potentialwindow

Ered Eox

(V vs.Li+/Li)c

[emim]AlCl4 8 1.29 18 22.6 1.0 5.5

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Ionic liquid Meltingpoint(°C)

Density(gcm−1)

Viscosity(mPa)

Conductivity(mS cm−1)

Potentialwindow

Ered Eox

(V vs.Li+/Li)c

[emim]H2.3F3.3 −90 1.14 5 100 1.5 5.3

[emim]BF4 11 1.24 43 13 1.0 5.5

[emim]CF3CO2 −14 1.29b 35a 9.6a 1.0d 4.6d

[emim]CH3SO3 39 1.25 160 2.7 1.3d 4.9d

[emim]CF3SO3 −10 1.38 43 9.3 1.0 5.3

[emim](CF3SO2)2N −15 1.52b 28 8.4 1.0 5.7

[emim](C2F5SO2)2N −1 61 3.4 0.9 5.8

[emim](CF3SO2)3C 39 181 1.7 1.0 6.0

a20°C.b22°C.cGC (glassy carbon), 1 mA cm−2, 20 mV s−1.dPt, 50 mV s−1.

Table 6.4 Electroconductivity ofpoly(hydrogen fluoride) salt ionic liquids

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Ionic liquid Electroconductivity (mScm−1)

Me4NF-4HF 196.6

Et4NF-4HF 99.2

n-Pr4NF-4HF 33.6

Et3N-3HF 32.6

[emim]H2.3F3.3 100

In the relationship between the molecularstructure and conductivity of imidazolium ionicliquid, viscosity increases with increase inbulkiness of the N-alkyl group attached to theimidazolium ring, resulting in a drasticdecrease in electroconductivity. On the otherhand, even though the N-alkyl group is replacedwith a hydrogen atom, which is smaller than themethyl group, the viscosity increases owing tohydrogen bonding with the counter anion. Theconductivity is therefore not improved. TheWalden role, in which the product of mobilityand viscosity is constant, can be applied to ionicliquids, and molar electroconductivity againstviscosity of ionic liquids obeys the followingequation:

Accordingly, the conductivity of ionic liquidsincreases with decreasing viscosity. It isreasonable that the viscosity changes dependingon temperature, and the electroconductivity of

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ionic liquids (emim salts) drastically decreasesas temperature is decreased, as shown in Figure6.20. This is due to the rapid increase in theirviscosity with decreasing temperature. Thistrend causes a big problem whenelectrochemical devices like batteries andcapacitors that use ionic liquids as electrolytesare utilized during winter in a cold area. Thedevelopment of ionic liquids with excellentconductivity is eagerly anticipated.

Figure 6.20 Temperature dependency ofelectroconductivity of imidazolinium salt ionicliquids

277

Ionic liquids are electrochemically stable andtheir usable potential region, i.e. their potentialwindow where the ionic liquid itself can beneither oxidized nor reduced, is very large. Thisis one of the characteristics of ionic liquids. Ingeneral, oxidative stability depends on theanion while reductive stability depends on thecation. However, although the imidazolium ionhas a positive charge, it is more easily oxidizedcompared to BF4

− owing to the unsaturatedbonds of the imidazole ring. Accordingly, theoxidative stability of imidazolium ionic liquidsis not always attributable to the oxidativedecomposition of anions. Among organicanions, trifluoroacetate ion is the most easilydecomposed oxidatively, while (CF3SO2)2N−

and (CF3SO2)3C− have oxidation resistivity andare not easily decomposed oxidatively. Theorder of oxidation resistivity is as follows, andthis order agrees with that of oxidationpotentials measured in a solution:

On the other hand, reductive stability generallydepends on the reductive decomposition of thecation and the aliphatic ammonium ion is moredifficult to reduce compared to the imidazoliumion. 2-Methylimidazolium ion is more difficultto reduce by 0.3–0.5 V compared anon-substituted imidazolium ion. As shown inFigure 6.21, ionic liquids consisting of an

278

aliphatic ammonium ion and (CF3SO2)2N−

(TFSA) have wide potential window of about 6V.

Figure 6.21 Linear sweep voltammograms invarious ionic liquids

Phosphonium ionic liquids are superior toaliphatic ammonium ionic liquids in oxidationresistance, and they have a wide potentialwindow (about 6.5 V) as well as thermalstability.

Poly(hydrogen fluoride) salt ionic liquids,however, have a narrow negative potentialwindow because of the reduction of acidicproton, as shown in Figure 6.22, but such easyproton reduction assists electrochemicalfluorination preferentially (see sections 5.7 and5.8.6). Et3N-3HF is easily oxidized because of

279

contaminated free amine, while Et4NF-4HFand Et3N-5HF have excellent oxidationresistance, and their protons are readilyreduced at the cathode to generate hydrogengas.

Figure 6.22 Potential windows ofpoly(hydrogen fluoride) salt ionic liquids

6.8.5 Voltammetry in Ionic LiquidsWhen ionic liquids are used as a solvent for themeasurement of the cyclic voltammetry oforganic compounds, the observed redoxcurrents are generally extremely small. This isdue to the high viscosity of ionic liquids, whichdecreases the diffusion coefficient. For instance,

280

the diffusion coefficient for Ni(II) salen in ionicliquid [bmim]BF4 at room temperature is 1.8 ×

10−8 cm2 s−1, which is more than 500 timessmaller than that in a typical organicsolvent–electrolyte system like 0.1 MEt4NClO4/DMF [36].

6.8.6 Organic ElectrochemicalReactions in Ionic LiquidsAlthough ionic liquids are considered to be anideal medium for organic electrolytic reactionsbecause of their non-flammability and sufficientelectroconductivity as well as wide potentialwindows, there have not yet been many reportsof organic electrochemical reactions in ionicliquids.

6.8.6.1 Organic Electrosynthesis

Cathodic reduction of carbonyl compounds likebenzaldehyde and acetophenone has beeninvestigated in ionic liquids, and notably theirdimerization proceeded predominantly [37,38].For acetophenone, the corresponding pinacolwas formed as a diastereomeric mixture, andthe diastereoselectivity is greatly affected by theionic liquids used, as shown in Eq. 6.9 [37]. As asolvent, aliphatic ionic liquids result in higherdiastereoselectivity compared to aromatic ionicliquids.

281

(6.9)

Electrocatalytic homocoupling of PhBr andPhCH2Br can be carried out in the presence ofNiCl(bpy) complex in [bmim]Tf2N, as shown inEq. 6.10 [39].

(6.10)

Interestingly, Pd nanoparticles generatedcathodically in ionic liquid were shown to be ahighly effective ligand-free catalyst for thecoupling of aryl halides [40].

Electroreductive dehalogenation ofvic-dihalides using a Co(II)salen complex in[bmim]BF4 has also been achieved, as shown inEq. 6.11 [41]. The product isolation is mucheasier compared to the similar dehalogenation

282

in ordinary molecular solvents since theCo(II)salen complex remains in the ionic liquidphase during product extraction with non-polarorganic solvents like diethyl ether.Furthermore, the recyclability of the catalyst/ionic liquid system was demonstrated.

(6.11)

Cyclic carbonates are prepared by the reductionof CO2 at −2.4 V vs. Ag/AgCl in the presence ofepoxides in various ionic liquids like[emim]BF4, [bmim]PF6 and [BPy]BF4(BPy=n-butylpyridinium) using a Cu cathodeand a Mg or Al anode [42].

Electrochemical synthesis of carbamates wasalso achieved by the electrolysis of a solution ofCO2 and amine in [bmim]BF4 followed by theaddition of alkylating agent, as shown in Eq.6.12 [43].

283

(6.12)

Furthermore, a kinetic study on the anodiccoupling of aromatic compounds has beencarried out in ionic liquids. Anodic oxidation of1,2-dimethoxybenzene led to the correspondingtrimer, as shown in Eq. 6.13 [44]. In thisreaction, the dimerization rate in ionic liquid isfive to ten times smaller than that inacetonitrile because of the high viscosity of theionic liquid.

(6.13)

N-Heterocyclic carbenes can be generated bycathodic reduction of imidazolinium-basedionic liquids [45–47]. The resulting carbenesare stable bases that are able to catalyze theHenry reaction as shown in Eq. 6.14 [47].

284

(6.14)

Since ionic liquids generally have much higherviscosities, mass transport is quite slow, asdescribed before. This is a disadvantage forelectrosynthesis in ionic liquids. However, itwas found that cathodic reduction ofN-methylphthalimide was promoted underultrasonication, resulting higher conversionand current efficiency, as shown in Eq. 6.15[48]. This is due to facilitated mass transport ofthe substrate under ultrasonication.

(6.15)

6.8.6.2 Electrochemical Fluorination

Poly(hydrogen fluoride) salts consisting ofamine and ammonium fluoride and hydrogenfluoride (Et3N-nHF, n = 3–5; Et4NF-nHF, n =3–5) are ionic liquids that have good

285

conductivity because of their low viscosity[49,50]. They become more anodically stablewith increasing HF content (increasing n). Asdescribed in Chapter 5, conventionalelectrochemical fluorination has been carriedout in organic solvents containing fluoride salts,such as Et3N-3HF and Et4NF-3HF [49–51].However, anodic passivation (the formation ofa non-conducting polymer film on the electrodesurface that suppresses Faradaic current) takesplace often, resulting in poor yield and lowcurrent efficiency. Moreover, acetoamidationalso occurs preferentially when MeCN is usedas an electrolytic solvent. In order to overcomesuch problems, solvent-free electrochemicalfluorination is an alternative method ofpreventing anode passivation andacetoamidation. Solvent-free selectiveelectrochemical fluorination of benzenes,naphthalene, olefins, furan, benzofuran andphenanthroline was first achieved in less than50% yield using the ionic liquid Et3N–3HF asthe reaction medium, supporting electrolyteand a fluorine source [52].

Although Et3N-3HF and Et4NF-3HF are easily

oxidized (lower or around 2 V vs. Ag/Ag+),anodically very stable poly(hydrogen fluoride)salts, Et4NF-mHF (m > 3.5) and Et3N-5HF (3 V

vs. Ag/Ag+) have also been developed [50].Using ionic liquid Et4NF-mHF (m > 3.5),solvent-free anodic partial fluorination of

286

arenes such as various substituted benzenes,toluene and quinolines was successfully carriedout at high current densities with good currentefficiencies (66–90%) [53,54]. As shown in Eq.6.16, monofluorotoluene and difluorotolueneare formed successively with increasingelectricity, however, even when the electricity isincreased further trifluorotoluene is not formedand ring fluorination starts [54].

(6.16)

Selective electrochemical formylhydrogen-exchange fluorination of aliphaticaldehydes affords acyl fluorides usingEt3N–5HF [55]. Selective anodic fluorination ofcyclic unsaturated esters in Et3N–5HF is alsoaccomplished to provide ring-expansionfluorinated products, as shown in Eq. 6.17[56,57].

(6.17)

287

Furthermore, anodic fluorination of variousphenol derivatives can be performed inEt3N-5HF using carbon fibre cloth as an anodeto form 4,4-difluorocyclohexadienonederivatives in good yields (Eq. 6.18) [58].

(6.18)

Electrochemical fluorination of adamantanes isalso possible in Et3N–5HF. Mono-, di-, tri- andtetrafluoroadamantanes were selectivelyprepared from adamantanes by controllingoxidation potentials, and the fluorine atomswere introduced selectively at the tertiarycarbons, as shown in Eq. 6.19 [59].

(6.19)

Anodic fluorination of cyclic ethers, lactonesand open-chain and cyclic carbonates can beachieved by anodic oxidation of a mixture of alarge amount of the liquid substrate and a small

288

amount of Et4NF–4HF (only 1.5–1.7 equiv. of

F− to the substrate) at a high current density(150 mA cm−2) (Eqs. 6.20 and 6.21) [60]. Inthis method, the substrate was selectivelyoxidized to provide the correspondingmonofluorinated product in good yield and withgood current efficiency. In sharp contrast, theuse of organic solvents or a large amount ofEt4NF-4HF resulted in no formation or lowyield (about 10%) of the desired fluorinatedproduct. Isolation of the fluorinated products iseasy: fluorinated lactone and carbonates can beisolated by extraction with solvent, whilefluorinated tetrahydrofuran can be easilyisolated by distillation of the reaction mixtureafter electrolysis. In these cases, the substratesare predominantly oxidized to result in efficientfluorination because only a small amount ofsupporting fluoride salt is used. Since platinumwith a low hydrogen overpotential is used as acathode, the acidic protons of the ionic fluoridesalt are predominantly reduced at the cathodeto generate hydrogen gas, hence a separator forthe electrolysis is not necessary. As explained, asmall amount of fluoride salt is enough for thefluorination, therefore this fluorination methodis desirable from an atom economicalperspective.

289

(6.20)

(6.21)

Since phthalide is hardly oxidized (vs. SCE), anodic fluorination does not proceedin either organic solvent or a solvent-freesystem. However, fluorination takes placehighly efficiently in a double ionic liquid systemconsisting of [emim]OTf and Et3N-5HF, as

290

shown in Eq. 6.22 [61]. In this double ionicsystem, the cationic intermediate generatedfrom the phthalide was expected to have a TfO−

counter anion (activated cation A in Eq. 6.22),which readily reacted with F− to provide thefluorinated phthalide in good to excellent yield.

(6.22)

The viscosity of ionic liquid fluoride salts ishigher than that of ordinary organic solvents,therefore the mass transport of substrates tothe anode surface from the bulk liquid is slowerthan in organic solvents, which is unfavourablefor electrolytic reactions.

As mentioned in section 6.11, it is known thatultrasonication greatly enhances masstransport from bulk to electrode surface. Thiseffect can also be applied to an ionic liquidsystem as the efficiency of anodic fluorinationin ionic liquid fluoride salts is markedlyincreased, as shown in Eq. 6.23 [62]. Notably,anodic difluorination of ethylα-(phenylthio)acetate proceeds efficiently evenin readily oxidizable ionic liquid, Et3N-3HF,which is not suitable for difluorination.

291

(6.23)

In neat ionic liquid fluoride salts, thenucleophilicity of fluoride ions is rather low,resulting in poor fluorination yields. It has beendemonstrated that ether solvents like DMEenhance the nucleophilicity of fluoride ions, butDME is rather easily decomposed [63]. Incontrast, PEG and even its oligomer are stableagainst anodic oxidation, and it was found thatthe addition of only about 3% PEG oligomer tothe reaction system greatly improved the yielddue to its ability to coordinate the countercations of fluoride ions, as shown Eq. 6.24 [64].

(6.24)

Severe passivation of the anode often occurseven in ionic liquid HF salts. Use of mediators

292

is effective to solve the problem, but productseparation becomes complicated. Fuchigamiand co-workers have developed a novel indirectanodic fluorination system employing atask-specific ionic liquid with an iodoarenemoiety as the mediator in HF salts [65]. Themediator improved the reaction efficiency for avariety of electrochemical fluorinations (Figure6.23) and remained intact in the ionic liquidsafter the product extraction process for reuse insubsequent runs.

Figure 6.23 Electrochemical fluorinationusing task-specific ionic liquid with mediator

In addition, a polymer-supported iodobenzene(PSIB) mediator is also effective for indirectanodic fluorination in HF salts [66]. In thiscase, the iodobenzene moiety pendent from thesolid polymer support cannot be directlyoxidized, therefore a double mediator system isnecessary. As shown in Figure 6.24, anodicoxidation of Cl− gave Cl+, which reacted with

293

the iodobenzene moiety to form PhI+Cl. Thisspecies then captures a fluoride ion to give thehypervalent [(chloro)(fluoro)iodo] benzenemoiety. The hypervalent iodine moiety thusgenerated oxidizes the substrate andconsequently the starting PSIB is recovered.The recovered PSIB mediator is reused insubsequent runs, maintaining a good yield(about 90%) of the fluorinated product.

Figure 6.24 Electrochemical fluorinationusing a polymer-supported mediator

6.8.6.3 Electropolymerization

Mattes's group and Fuchigami's groupindependently achieved the electrooxidativepolymerization of pyrrole, thiophene andaniline in different ionic liquids [67–69].Mattes's group used1-butyl-3-methylimidazolium tetrafluoroborateand hexafluorophosphate ([bmim]BF4 and[bmim]PF6) for electropolymerization ofpyrrole and aniline. They found that theπ-conjugated polymers thus obtained are highlystable and can be electrochemically cycled in

294

ionic liquids up to a million times [67]. Inaddition, because the polymers havecycle-switching speeds as fast as 100 ms, theycan be used as electrochromic windows andnumeric displays. Furthermore, it wasdemonstrated that polyaniline prepared in thisway is highly useful for electrochemicalactuators. A 10-mm length of 59-µm diameterwet-spun polyaniline emeraldine base (EB)fibre is treated with trifluoromethanesulfonic(triflic) acid to form the correspondingemeraldine salt (ES) fibre, which has highconductivity of 300 S cm−1. The fibre with ESstructure is reduced with two electrons at about−0.4 V vs. Ag/Ag+ in ionic liquid [bmim]BF4 toform leucoemeraldine (LE), and LE is oxidizedat about +0.8 V vs. Ag/Ag+ to form original ES,as shown in Figure 6.25. Thus, by oxidation andreduction, the cation of the ionic liquid,[bmim], is expelled and incorporated,respectively. The strain is therefore contractilewhen the polymer is oxidized from the fullyreduced LE state to ES, and expansive uponreduction (Figure 6.25). In lifetime tests of thefibre, both electroactivity andelectromechanical actuation continue withoutsignificant decrease (<1%) in either stress orstrain for 10,000 redox cycles becauseanhydrous ionic liquid is used as a solvent. Theπ-conjugated polymer in an ionic liquidelectrolyte system is therefore highly promisingfor electrochemical mechanical actuators.

295

Figure 6.25 Principle of an electrochemicalactuator

Fuchigami's group employed [emim]OTf forelectrochemical polymerization [68,69]. Theyfound that the polymerization of pyrrole inionic liquid proceeds much faster than that inconventional media like aqueous andacetonitrile solutions containing 0.1 M[emim]OTf as a supporting electrolyte.Interestingly, as shown in Table 6.5, the surfaceof the polypyrrole film prepared in neat[emim]OTf is so smooth that no grains areobserved, and both the electrochemical capacityand the electroconductivity are markedlyincreased when the polypyrrole andpolythiophene films are prepared in the ionicliquid. This may be attributable to theextremely high concentration of anions asdopants, which results in a much higher dopinglevel. As described above, polymer filmsprepared in the ionic liquid have a higher

296

electrochemical density and highly regulatedmorphological structures, and therefore theyhave possible uses as high-performanceelectrochemical capacitors, ion-sieving films,ion-selective electrodes, matrices for hostingcatalyst particles and so on.

Table 6.5 Physical properties of polypyrrol andpolythiophene films prepared electrochemicallyin various media

Polymer Media Roughnessfactora

(dimensionless)

Electro-chemicalcapacitance (Ccm−3)

Electroconductivity(S cm−1)

Dopinglevel(%)

Polypyrrole H2O 3.4 77 1.4 × 10−7 22

Polypyrrole CH3CN 0.48 190 1.1 × 10−6 29

Polypyrrole [emim]OTf 0.29 250 7.2 × 10−2 42

Polythiophene CH3CN 8.6 9 4.1 × 10−8 –

Polythiophene [emim]OTf 3.3 45 1.9 × 10−5 –aStandard deviation of thickness.

Anodic polymerization of3-(p-fluorophenyl)thiophene was also carriedout in [emim]NTf2 and the resulting polymerfilm was also found to be very smooth [70].Furthermore, electrosynthesis ofpoly(3,4-ethylenedioxythiophene) (PEDOT)and polyphenylene in ionic liquids has beenreported [71,72].

297

An ionic liquid is a recyclable medium fororganic synthesis, which is one of thepronounced characteristics of ionic liquids.However, when an ionic liquid is used aselectrolytic medium, decomposition of the ionicliquid itself often occurs at a counter electrode.This is a big problem when the ionic liquid isrecycled. In the case of electrochemicalpolymerization, however, this problem can besolved using cyclic potential-scanning oxidativepolymerization. In this polymerization method,monomer is oxidized to form polymer film,which deposits on the electrode. Additionally,the deposited polymer itself is oxidized andreduced repeatedly during alternately anodicand cathodic scanning. During reduction at theworking electrode, anodic oxidativepolymerization of monomer and oxidation ofthe polymer itself (so-called doping) thereforeoccur at the counter electrode, while duringoxidation at the working electrode, reduction ofthe polymer (so-called dedoping) occurs at thecounter electrode, hence decomposition of theionic liquid does not occur. Ionic liquid istherefore easily recovered simply by extractingthe remaining monomer with appropriatesolvents, and recovered ionic liquid can be usedmany times [68,69].

6.8.6.4 Others

Although the following synthetic application ofionic liquids is not for organic substances,

298

electrochemical deposition of various metals inionic liquids is possible. For instance, Zn, Gaand In can be electrochemically deposited frommixtures of [emim]Cl and ZnCl2, GaCl3 andInCl3, respectively. A variety of typical elementsand transition metals can be deposited fromionic liquids other than chloride ionic liquid.Furthermore, it is known that one-electronelectrochemical reduction of oxygen moleculesin ionic liquids generates superoxide ionsefficiently.

6.9 Thin-Layer Electrolytic CellsAlthough electro-organic syntheses have beenestablished as a powerful tool in organicsyntheses, they still are in development to fulfiltheir potential as a ‘green’ methodology. Inconventional electrosynthetic processes, a largeamount of supporting electrolyte has to beadded to the solvent to give sufficient electricalconductivity, but the presence of a large amountof the supporting electrolyte might causeseparation problems in the reaction mixturework-up and additional waste problems.

The capillary gap cell developed at BASF for thecommercial electrosynthetic process (pairedelectrosynthesis of phthalide andt-butylbenzaldehyde) allows the electrolysis tobe conducted even in a dilute electrolytesolution. The cell consists of circular diskelectrodes with a small interelectrode gap (1–2

299

mm) to minimize the ohmic voltage drop in theelectrolyte (Figure 6.26) [73].

Figure 6.26 Schematic representation of thecapillary gap electrochemical cell of BASF SE

On the other hand, in laboratory-scaleexperiments, the use of a simple thin-layerflow-cell geometry with working and auxiliaryelectrodes directly facing each other allowselectrosynthetic processes to be conducted inflow-through mode in the absence ofsupporting electrolyte (Figure 6.27) [74]. Forexample, by using this type of cell, themethoxylation of furan is conducted anodicallyin methanolic solution even in the absence ofadded supporting electrolyte [75].

300

Figure 6.27 Schematic representation of athin-layer flow cell

6.10 Electrochemical MicroflowSystemsRecently, microflow systems have receivedsignificant research interest from bothacademia and industry. The fundamentaladvantages and potential benefits of microflowtechnology are (i) an extremely largesurface-to-volume ratio, (ii) precisetemperature control, (iii) precise residence timecontrol, (iv) strict laminar flow control, (v)extremely fast molecular diffusion and (vi)improvement of reaction process safety. Asintroduced above, microflow systems havemany advantages that can be applied in a wide

301

range of organic synthesis and organic massproduction processes. Electron transfer is oneof the most common driving forces for organicreactions, and organic electrosynthesis servesas a straightforward and powerful method oforganic electron-transfer processes. Theintegrated use of microflow technology withorganic electrosynthesis is one of the mostsophisticated processes in organic chemistry. Inaddition, novel systems that are realized only byusing electrochemical microflow reactors havebeen developed.

Yoshida et al. reported that a microflowelectrochemical system serves as a quiteeffective method for oxidative generation ofunstable organic cations at low temperatures[76]. This method is called the cation-flowmethod. An electrochemical reactor for thecation-flow method is equipped with a carbonfelt anode and a platinum wire cathode (Figure6.28). The anodic chamber and the cathodicchamber are separated by a diaphragm of PTFEmembrane. A solution of a cation precursor isintroduced to the anodic chamber and asolution of trifluoromethanesulfonic acid(TfOH) as a proton source is introduced to thecathodic chamber. The organic cation that isgenerated is immediately transferred to a vesselin which a nucleophilic reaction takes place togive the desired coupling product.

302

Figure 6.28 Schematic representation of amicroflow electrochemical reactor

Atobe and co-workers reported that the use ofparallel laminar flow in a microflowelectrochemical reactor enables the effectivegeneration of an N-acyliminium ion followed bytrapping with an easily oxidizable carbonnucleophile such as allyltrimethylsilane (Figure6.29) [77]. A solution of a cation precursor anda solution of allyltrimethylsilane are introducedin a parallel manner. The laminar flow preventsthe oxidation of allyltrimethylsilane at theanode. Only the precursor is oxidized togenerate the N-acyliminium ion. TheN–acyliminium ion that is generated diffusesand reacts with allyltrimethylsilane. Althoughthe efficiency of the process is very low for theBu4NBF4/CH3CN system, use of theBu4NBF4/TFE (2,2,2-trifluoroethanol) system(59% yield) or an ionic liquid (62–91% yield)gave rise to the formation of the desiredproduct.

303

Figure 6.29 Schematic representation of (a) atwo-inlet microflow reactor system and (b)parallel laminar flow in the reactor. Theillustrated model reaction is anodic substitutionreaction of N-(methoxycarbonyl)pyrrolidinewith allyltrimethylsilane

The use of parallel laminar flow in a microflowelectrochemical reactor also enableschemoselective cathodic reduction to controlproduct regioselectivity in electrochemicalcarbonyl allylation [78]. Electrochemicalcarbonyl allylation can produce either γ- orα-adducts depending on whether the aldehydeor allylic halide is reduced by the cathode. If thealdehyde has a higher reduction potential, theallylic halide is predominantly reduced to givethe γ-adduct, but if the reduction potential ofthe allylic halide is higher, the aldehyde isreduced preferentially and the α-adductgeneration is favoured. Control of productselectivity (regioselectivity in this reaction)therefore requires that either the allylic halidesor the aldehydes should be reduced

304

chemoselectively, regardless of their reductionpotentials (Eq. 6.25). To performchemoselective cathodic reduction, the authorsemployed a liquid–liquid parallel laminar flowformed in an electrochemical microflowreactor. As shown in Figure 6.30, when twosolutions (allylic chloride solution and aldehydesolution) are introduced through inlets 1 and 2of the microflow reactor, a stable liquid–liquidinterface is formed and mass transfer betweenthe input streams occurs only by means ofdiffusion. The substrate introduced throughinlet 1 can therefore be predominantly reduced,whereas the reduction of the inlet 2 substrate(inflow 2) can be avoided. Consequently,chemoselective cathodic reduction proceedsand an intentional cross-coupling product isobtained regioselectively. In other words,product selectivity control is realized by simplyswitching the reagent flows.

(6.25)

305

Figure 6.30 Chemoselective cathodicreduction using parallel laminar flow in atwo-inlet microflow reactor. (a) Flow mode forthe selective reduction of benzaldehyde. (b)Flow mode for the selective reduction of1-chloro-3-methyl-2-butene

6.11 Electrolysis UnderUltrasonicationThe many benefits of ultrasound in chemicalprocesses are well known and have beeninvestigated in a variety of chemical fields, butperhaps the most striking influence ofultrasound concerns heterogeneous reactionsystems, particularly those with a solid–liquidinterface where particle size modification, themodification of particle dispersion, theenhancement of mass transport, the cleaning ofsurfaces or the formation of fresh surfaces areamong the beneficial processes [79].

Since an electrochemical synthetic process is atypical heterogeneous one in a solid(electrode)–liquid (electrolytic solution)interface, various effects of ultrasound,particularly promotion of mass transport,would be induced by ultrasonication. A majorcontribution to mass transport is the micro-jetstream resulting from the asymmetric collapseof a cavitation bubble (Figure 6.31) [80].Suslick et al. reported that the velocity of the

306

stream reaches in excess of 100 m s−1 in awater–solid interface [81].

Figure 6.31 Collapse of cavitation bubble. (a)Symmetrical collapse of a cavitation bubble inbulk solution. (b) Asymmetrical collapse of acavitation bubble in a liquid–solid interface

Such a mass transfer promotion byultrasonication provides an increase in thecurrent efficiency for a variety ofelectrosyntheses. For example, Atobe et al.reported that a significant ultrasonic effect oncurrent efficiency was found in theelectrochemical reduction ofp-methylbenzalaldehyde (Eq. 6.26) [82]. Asshown in Table 6.6, current efficiency for thereduction of p-methylbenzalaldehyde wasdramatically increased under ultrasonication.Furthermore, product selectivity for thehydrodimeric product (D1) was also increasedby ultrasonication, and the effects could berationalized experimentally and theoretically as

307

being due to the promotion of the masstransport of the substrate molecule to theelectrode surface from the electrolytic solutionby ultrasonic cavitation [83,84]. The productselectivity in the electrochemical reduction ofdimethyl maleate (Eq. 6.27) and benzoic acid(Eq. 6.28) was also greatly influenced byultrasonication, as shown in Table 6.6.

(6.26)

(6.27)

(6.28)

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Table 6.6 Electroreduction ofp-methylbenzaldehyde, dimethyl maleate andbenzoic acid

Starting compound Stirringmode

Currentefficiencyfor [D1]+ [M1],[D2] +[M2] or[M3] +[M4]/%

Selectivity[D1]/[M1],[D2]/[M2] or[M3]/[M4]

p-methylbenzaldehydea Still 35 0.0

p-methylbenzaldehydea Mechanicalb 74 1.0

p-methylbenzaldehydea Ultrasonicc 89 24

Dimethyl maleated Still 66 0.0

Dimethyl maleated Mechanicalb 93 0.3

Dimethyl maleated Ultrasonicc 96 0.4

Benzoic acide Still 1 0.0

Benzoic acide Mechanicalb 17 0.2

Benzoic acide Ultrasonicc 54 1.5aElectrolyzed by passing 0.5 F mol−1 at 20 mA cm−2 on a leadcathode in a 0.25 M H2SO4/50% MeOH solution.

bStirred by a rotating magnet bar.

309

Starting compound Stirringmode

Currentefficiencyfor [D1]+ [M1],[D2] +[M2] or[M3] +[M4]/%

Selectivity[D1]/[M1],[D2]/[M2] or[M3]/[M4]

cThe cathode was positioned 1.7 cm apart from the top of anultrasonic horn (0.6 cm diameter, 12 W, 20 kHz).dElectrolyzed at a lead cathode in a 0.025 M KH2PO4/0.025M Na2HPO4/0.5 M NaCl solution.

eElectrolyzed at a lead cathode in a 0.05 M H2SO4/0.2 Mcitric acid solution.

As mentioned in section 5.8, conductingpolymers exhibit not only electroconductivitybut also unique optical and chemical properties[85]. The diversity of properties exhibited byconducting polymers offers these materials tobe used in numerous technological applications.Generally, the properties of polymers originatefrom their chemical (molecular) and physical(morphological) structures. It therefore followsthat the structures of conducting polymersshould be able to be controlled in order to tailorthem to the purposes of their utilization. Theirchemical structures can be controlled bychanging the molecular structures of thecorresponding monomers and by selectingconditions and procedures for polymerization

310

[85]. On the other hand, methods forcontrolling their physical structures have beenrelatively limited, but recently many studieshave focused on applying ultrasound topolymerization processes, particularlyelectropolymerization, for this purpose.

Osawa et al. found that the quality ofpolythiophene films electropolymerized on ananode can be enhanced by ultrasound. Byconventional methodology the films becomebrittle, but by using ultrasound from a 45-kHzcleaning bath, flexible and tough films (tensilemodulus 3.2 GPa and strength 90 MPa) can beobtained [86].

The work of Atobe and co-workers wasprobably the first ‘modern’ study investigatingelectropolymerization under sonication in acomplete series of papers at low frequencies.Starting from electro-organic reactions underultrasonic fields [87], polymerization of anilinewas studied both in electrochemical [88] andchemical routes [89,90] as well as synthesis ofnanoparticle synthesis [91,92].

The behaviour of polypyrrole filmselectropolymerized under ultrasonication wasalso investigated, and unique properties in thedoping–undoping processes were highlighted.The authors attributed their results to theelaboration of highly dense film undersonication, but also deplored the degradation ofthe film due to high cavitation at 20 kHz

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(Figure 6.32) [93]. The Besançon group alsostudied the use of high-frequency ultrasound(500 kHz, 25 W) for electropolymerization of3,4-ethylenedioxythiophene (EDOT) orpolypyrrole in aqueous medium in order toinvestigate its effects on conducting polymerproperties. They showed that (i) mass transferenhancement induced by sonication improveselectropolymerization and (ii) the mass transfereffect is not the only phenomenon induced byultrasound during electrodeposition [94,95].

Figure 6.32 SEM images of polyaniline filmsprepared (a) without and (b) withultrasonication

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6.12 Electrosynthesis UsingSpecific Electrode Materials

6.12.1 Electrochemical Synthesis UsingHydrophobic Electrodes

6.12.1.1 Hydrophobic Composite-PlatedElectrodes

Most electrodes are hydrophilic, buthydrophobic electrodes can be prepared bycomposite-plating electrodes like Ni, Zn, Pb etc.in the corresponding metalsalt-poly(tetrafluoroethylene) (PTFE)dispersion plating bath. Graphite fluoride andfluorinated pitch other than PTFE are also usedas hydrophobic composite materials. Duringplating, the fine hydrophobic particles areincorporated into the plated layer. Theelectrodes thus prepared have goodelectroconductivity and their plated layersurface shows water-repellent properties as wellas mechanical and chemical stability. Ingeneral, hydrogen and oxygen evolution occurcompetitively in electrolytic reactions of organiccompounds in an aqueous solution. Inparticular, hydrogen evolution readily occurs inan acidic aqueous solution while oxygenevolution easily takes place in an alkalineaqueous solution. Accordingly, it is not easy tocathodically reduce carbonyl compounds inaqueous acidic solution and anodically oxidizealcohols in aqueous alkaline solution. High

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hydrogen-overpotential cathodes andoxygen-overpotential anodes therefore have tobe used for electrochemical reactions inaqueous acidic and alkaline solutions,respectively. However, nickel/PTFEcomposite-plated Ni electrodes are used for thereduction and oxidation, as mentioned above,to provide the corresponding alcohols andcarbonyl compounds, respectively, with muchhigher current efficiency compared to unplatedNi electrodes [96,97]. This is not because of thehigher hydrogen- and oxygen-overpotentials ofthe composite-plated electrode suppressinghydrogen and oxygen evolution but because ofthe substrate-collecting effect as a result ofstrong hydrophobic interaction between thehydrophobic electrode surface and hydrophobicorganic substrates [96,97].

(6.29)

6.12.1.2 PTFE-Fibre-Coated Electrodes

PTFE-fibre-coated electrodes, which areprepared by wrapping the electrode in PTFEstrings, are hydrophobic. It has beendemonstrated that anodic oxidation ofhydroquinones at this electrode in the presence

314

of dienes in NaClO4/MeNO2 generatedquinones, which underwent Diels–Alderreaction with the dienes to provide cyclicproducts in excellent yields [98]. Use of anuncoated glassy carbon anode also generatesquinones from hydroquinones, but thecoexisting dienes are easily oxidized anddecompose prior to reaction with the quinones.However, when a PTFE-fibre-coated glassycarbon electrode is used, the easily oxidizablediene is maintained on the PTFE-fibres andonly the polar hydroquinones can reach theelectrode through the hydrophobic fibres, asshown in Figure 6.33. Next, the anodicallygenerated hydrophobic quinones immediatelyreact with the dienes adsorbed on the PTFEfibres, and the resulting hydrophobic productsare also maintained on the fibres to avoidfurther oxidation of the products. This novelmethod is useful for the synthesis of variousterpenes [99].

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Figure 6.33 Diels–Alder reaction of anodicallygenerated quinone derivatives with diene on theelectrode surface modified with PTFE fibres

6.12.2 Electrolytic Reactions UsingDiamond Electrodes

Diamond is a solid crystal consisting of sp3

carbons forming covalent bonds. It hasexcellent transparency and thermoconductivityas well as mechanical and chemical stability.Although pure diamond is non-conductive, itbecomes electrically conductive by doping withan impurity such as boron. Boron-dopeddiamond films can be used as electrodes, whichexhibit excellent and superior electrochemicalproperties compared to known metal andcarbon electrode materials. Accordingly,boron-doped diamond (BDD) electrodes arepromising novel electrode materials [100].

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6.12.2.1 Electrochemical Features andApplication to Highly SensitiveElectroanalysis

Wide Potential Window in AqueousSolution [101,102]: BDD electrodes have ahydrogen overpotential at about −1 V vs. Ag/AgCl, which is comparable to mercury, and anoxygen overpotential at about +2.2 V vs. Ag/AgCl, which is higher than other commonanode materials. Its potential window istherefore as wide as 3.2 V and this is apronounced feature of the BDD electrode. Evenin aprotic organic solvents like MeCN, thecathodic window is about 500 mV wider thanthat of a Pt electrode. Accordingly, such uniqueproperties could result in replacement of eithercostly noble metals or toxic heavy metals.

Small Background Current (SmallResidual Current Density) [101,102]: Thecapacitance of the BDD electrode surface is afew mF cm−2, which is two orders smallercompared to glassy carbon, and its residualcurrent density is as small as a few hundred nAcm−2. This is attributable to the fact thatneither dissolved gas nor ions of supporting saltadsorb on the BDD surface, making redoxreactions difficult.

Large Overpotential of OxygenReduction [103]: Because of the difficultcathodic reduction of dissolved oxygen, the

317

desired reduction current can be observed evenwithout removal of dissolved oxygen.

Physical and Chemical Stability andExcellent Durability [100,104]: BDDelectrodes are basically degradation-free andcan be used even in a corrosive solution.Electrolysis can be conducted at high currentdensity (10 A cm−2) and can also be carried outat temperatures up to 600°C. The electrodesurface is stable and hardly contaminated byadsorption of impurities.

Fast Electron-Transfer Rate betweenRedox Species and BDD Electrode[101,104,105]: Outer-sphere one-electron redoxsystems like Fe(CN)6

4−/3−, IrCl62−/3− and

Ru(NH3)62+/3+ show reversible waves in cyclic

voltammograms and their electron transfer atthe electrode interface is fast.

Unique Electrochemical Selectivity [106]:Oxygen-terminated BDD electrodes preparedby treatment with oxygen plasma or anodicoxidation show selective responses to specificchemical species.

Since BDD electrodes have a wide potentialwindow and extremely small backgroundcurrent in addition to the above features, highlysensitive sensors for detection of biologicalmaterials have been developed using BDDelectrodes. For example, when anoxygen-terminated BDD electrode is used, a

318

trace amount of dopamine and uric acid can bedetected and quantitatively analyzed even in thepresence of a large amount of ascorbic acid[106]. Moreover, since the BDD electrodesurface is resistant to contamination withadsorbed impurities, it can be used as anelectroanalytical detector [107].

Quite recently BDD microelectrodes weredeveloped, which enable in vivo pH monitoringfor stomach disorder diagnoses as well as invivo assessment of cancerous tumours [108].

6.12.2.2 Application to OrganicElectrosynthesis

Various anodic substitutions like fluorination,methoxylation, acetoxylation and cyanation ofaromatic compounds, heterocycles and sulfideshave been achieved so far.

Kolbe electrolysis was also investigated using aBDD anode [109]. Quite recently it wasdemonstrated that a BDD anode is highlyeffective for regioselective homocoupling ofsubstituted phenols to provide biphenols, asshown in Eq. 6.30 [110]. BDD electrodes arealso superior anodes for the generation ofalkoxy radicals from alcohols. Accordingly,anodic 1,2-dimethoxylation of isoeugenol hasbeen realized under microflow conditions [111].Inagi and co-workers have achieved parallelelectrochemical reactions of an alternatingcopolymer of 9-fluorenol and

319

9,9-dioctylfluorenone on a BDD bipolarelectrode in Et4NOTs/iPrOH, giving amulti-coloured gradient film [112]. This is anexcellent application of the wide potentialwindow of BDD electrodes.

(6.30)

6.12.2.3 Application to InorganicElectrosynthesis

The BDD electrode is a superior anode for thegeneration of ozone and hydrogen peroxidefrom water compared to platinum and iridiumoxide anodes [113]. Chlorine gas is readilygenerated by BDD electrodes and the efficiencyfor hypochloric acid formation is high, thereforeapplications in disinfection treatments areexpected. The preparation of persulfuric acidfrom sulfuric acid can be performed using BDDanodes with 70% current efficiency, while theproduction of ammonia from nitric acid ispossible using BDD cathodes [114,115].Graphite reacts with fluorine gas and graphite

320

fluoride is formed in the bulk of the graphite,resulting in suppression of the Faradaiccurrent. In sharp contrast, only the surface ofthe BDD electrode is fluorinated, therefore itmaintains its electroconductivity [116]. Since itis stable to hydrogen fluoride, this electrode isexpected to be useful for the preparation offluorine gas.

6.13 Photoelectrolysis andPhotocatalysis

6.13.1 PhotoelectrolysisWhen a semiconductor electrode is irradiated,charge separation occurs to generate electron(e)–hole (h+) pairs. The generated electronworks as a reductant, while the hole works as anoxidant. Honda and Fujishima demonstratedfor the first time the possibility of suchphotosensitized electrolysis using the n-typesemiconductor TiO2 and Pt electrodes, asshown in Figure 6.34 [117]. When the surface ofthe TiO2 electrode is irradiated with awavelength shorter than 410 nm, current flowsto generate oxygen and hydrogen at thesurfaces of the TiO2 and Pt electrodes,respectively. TiO2 is excited under irradiationto generate electron–hole pairs, and the holesin the valence band move to the surface,resulting in oxidation of water, while theelectrons in the conduction band move into the

321

bulk and then further move through theexternal circuit to the counter Pt electrode toreduce protons. This suggests that water isdecomposed by visible light into oxygen andhydrogen, without the application of anyexternal voltage, according to Eq. 6.31. This isan innovative breakthrough and is referred toas the Honda–Fujishima effect [117].

(6.31)

Photoelectrochemical cells have the advantageof producing oxygen and hydrogen in additionto electrical energy. However, there have beenfew examples of applications of suchphotoelectrolysis to organic reactions to date.

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Figure 6.34 Photochemical cell with TiO2electrode

6.13.2 PhotocatalystsWhen the circuit between TiO2 and Ptelectrodes is shortened, Pt is eventuallydeposited on the surface of TiO2. Accordingly, aparticle of TiO2 with some Pt deposited on it(Figure 6.35) can be considered as a shortcircuit photoelectrochemical cell. In the case ofsuch a photocatalyst, oxidation and reductionoccur simultaneously at two sites on the smallparticle. Since the oxidation and reduction sitesare very close to each other, various uniqueorganic synthetic reactions have been

323

developed [118]. For instance, anodic oxidationof a primary amine generally forms an aldehydevia an imine intermediate, while the sameoxidation using a TiO2 catalyst with Ptdeposited on it affords a secondary amine, asshown in Figure 6.35 [119]. In this case, theprimary amine is oxidized with the holegenerated by photo-excitation to form analdehyde, which reacts with the unreactedstarting amine to form an imine intermediate.The imine is immediately reduced at a Pt site,resulting in the formation of the secondaryamine. This is due to both oxidation andreduction occurring at active sites close to eachother. By using this principle, the synthesis ofoptically active pipecolinic acid from opticallyactive lysine was achieved by photocatalyticreaction using platinized semiconductor TiO2powder particles, as shown in Eq. 6.32 [120].Thus, photocatalytic reactions can beconsidered to be wireless electrolytic reactions.

(6.32)

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Figure 6.35 Example of photocatalyticreactions

6.14 Electrochemical PolymerReactionsAs described in Chapter 5, conducting polymersare redox active, especially in the film state, onthe electrode surface. In a number ofapplications, the doping and dedoping processof conducting polymers needs to be reversibleto avoid over-oxidation and over-reduction,which may cause undesirable reactions in thepolymer structure. Because of this, a conductingpolymer in its doped state can be regarded as areactive species capable of undergoingsubsequent reactions. The design of specificelectrogenerated species generated fromspecific polymers and the subsequent chemicalreactions represents a powerful means of

325

introducing versatile functionalities intopolymers. This electrochemical polymerreaction is a kind of post-polymerizationfunctionalization (or simplypost-functionalization) (Figure 6.36) [121]. Thereaction ratio (or the degree offunctionalization) of the repeating reaction sitesinfluences various properties of the resultingconducting polymer, including its absorption,photoluminescence and electrochemistry. Asanodic reactions, oxidative halogenations,cyanations and pyridinations of conductingpolymers are known to proceed when using thecorresponding nucleophilic dopants. Cathodicreaction such as reductive hydrogenation of the9-fluorenone moiety in a conducting polymer isalso possible, so that paired electrochemicalpolymer reactions in a single cell can bedesigned and give good current efficiency.

Figure 6.36 Electrochemical polymerreactions

The main advantage of electro-organicsynthesis is the ability to rapidly switch theapplication of electrical potential to the workingelectrode either on or off. By applying thisadvantage to electrochemical polymer

326

reactions, the reaction ratio can be finely tunedby precisely controlling the amount of chargepassed between the anode and cathode of theelectrode. As a result, the physical properties ofthe conjugated polymer can be tailored byadjusting the reaction ratio.

To progress this solid-phase reaction, thechoice of electrolyte is critical. In theelectrochemical fluorination of a polyfluorenederivative, ionic liquid hydrogen fluoride saltsplay the roles of supporting salt, reactionmedium and fluorine source, preventing filmdetachment during electrolysis (Eq. 6.33) [122].

(6.33)

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87. Atobe, M. and Nonaka, T. (1998) NewDevelopments in Sonoelectrochemistry, NipponKagaku Kaishi, 219–230.

88. Atobe, M., Kaburagi, T. and Nonaka, T.(1999) Ultrasonic Effects on ElectroorganicProcesses. Part 13. A Role of UltrasonicCavitation in Electrooxidative Polymerization ofAniline, Electrochemistry, 67, 1114–1116.

89. Atobe, M., Chowdhury, A.N., Fuchigami, T.and Nonaka, T. (2003) Preparation ofConducting Polyaniline Colloids underUltrasonication, Ultrason. Sonochem., 10,77–80.

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90. Chowdhury, A.N., Atobe, M. and Nonaka,T. (2004) Studies on Solution andSolution-Cast Film of Polyaniline ColloidsPrepared in the Absence and Presence ofUltrasonic Irradiation, Ultrason. Sonochem.,11, 77–82.

91. Park, J.E., Atobe, M. and Fuchigami, T.(2005) Sonochemical Synthesis of ConductingPolymer-Metal Nanoparticles Nanocomposite,Electrochim. Acta, 51, 849–854.

92. Park, J.E., Atobe, M. and Fuchigami, T.(2005) Sonochemical Synthesis ofInorganic-organic Hybride NanocompositeBased on Gold Nanoparticles and Polypyrrole,Chem. Lett., 34, 96–97.

93. Atobe, M., Tsuji, H., Asami, R. andFuchigami, T. (2006) A Study onDoping-undoping Properties of PolypyrroleFilms Electropolymerized underUltrasonication, J. Electrochem. Soc., 153,D10–D13.

94. Taouil, A.E., Lallemand, F., Hihn, J.Y. andBlondeau-Patissier, V. (2011) Electrosynthesisand characterization of conducting polypyrroleelaborated under high frequency ultrasoundirradiation, Ultrason. Sonochem., 18, 907–910.

95. Taouil, A.E., Lallemand, F., Hihn, J.Y.,Melot, J.M., Blondeau-Patissier, V. and Lakard,B. (2011) Doping properties of PEDOT films

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6New Methodology of Organic ElectrochemicalSynthesis

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Shinsuke Inagi and Toshio Fuchigami

The world is facing severe problems such asenvironmental problems, exhaustion ofresources and energy-relevant problems.Organic electrochemistry, includingelectrosynthesis, has the potential to solve theseproblems. This chapter describes thecontribution of organic electrochemistry toorganic electronics, the reuse of biomass,C1-chemistry and environmental clean-up.

7.1 Application in OrganicElectronic DevicesOrganic molecules and polymers with uniqueelectronic properties have many deviceapplications in luminescent materials,conductive materials and energy storage. Theseorganic-based materials are superior toinorganic materials in terms of lightness,flexibility and cost. Furthermore, the millions ofmolecular designs for organic materials make iteasy to tailor their optoelectronic properties.The basic principles of organic electronicsdevices are described below from the point ofview of organic electrochemistry.

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7.1.1 Organic Electroluminescence [1,2]Organic photoluminescent materials generallyemit through the relaxation process of oncephotoexcited electrons to their ground state(Figure 7.1). The excited state can also beformed electrochemically. When enoughvoltage is applied to the anode and cathodesandwiching the organic emitting layer (thinfilm), an injection of holes at the anode and aninjection of electrons at the cathode occur. Thecharges migrate inside the layer and recombineto emit light (Figure 7.2). In order to promotesmooth electron injection from the electrodesurface to the LUMO of the emission layer, anadditional thin layer, i.e. the electron-transportlayer, is introduced between the interfaces. In asimilar manner, the hole-transport layer is usedfor hole injection from the electrode to theHOMO of the emission layer. Although thesetransport layers are important for injectingcharge carriers into the emission layer, theyshould not interrupt the emission of theemission layer. Each organic layer is formed bychemical vapour desorption (CVD) ofcrystalline molecules or a wet-process ofpolymer solution.

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Figure 7.1 Mechanism of photoluminescence

Figure 7.2 Mechanism of electroluminescence

7.1.2 Organic Photovoltaic Cells [3,4]The principle of photoelectric conversion in anorganic photovoltaic cell (OPVC) is the inverseprocess of organic electroluminescence (OEL).

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OPVC generates electric energy through thecharge separation state obtained byphoto-excitation, whereas electric energycauses electron injection and hole injection, andthe subsequent charge recombination results inemission in the OEL process.

To induce a charge separation state,electron-donating and electron-accepting layersare necessary in the thin film of the OPVC(Figure 7.3). When the photo-excited state(exciton) is generated in the donor layer bysolar energy at the near interface of the donorlayer and the acceptor layer, the excitedelectron may migrate spontaneously to theLUMO of the acceptor layer, resulting in theformation of a charge-separated state. Thecharge carriers then migrate by hoppingthrough each layer to the electrode terminalsand this process drives the photovoltaic cell.

Figure 7.3 Photo-electronic conversionprocess

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The design of the junction of the layers isimportant to induce charge separationefficiently before deactivation of the exciton.The construction of a path to the electrodesurface of the generated charge carriers is alsoimportant for high photo-electron conversionefficiency.

7.1.3 Dye-sensitized Solar Cells [5,6]Semi-conductor electrode like metal oxide cangenerate electric energy based on the principleof photocatalysts. A photo-excited electron in asemi-conductor electrode reaches the electriccircuit and the remaining hole receiveselectrons from a redox system in the electrolyte,followed by electron transfer from the counterelectrode to the redox system. Because of therelatively wide band gap of semi-conductorelectrodes, UV light is usually employed todrive this type of photovoltaic cell.Dye-sensitized solar cells (DSSCs) work in thevisible region of solar light using organic dye ona semi-conductor electrode as thephoto-sensitizer. The organic dye should have ahigher LUMO level than the semi-conductorelectrode and a lower HOMO level than theredox system in the electrolyte. A typical DSSCsystem, known as the Grätzel cell, is composedof a transparent anode, titanium oxide (TiO2)

coated with ruthenium complex, an I−/I3−

redox system in electrolyte and a Pt cathode(Figure 7.4). The voltage generated in the DSSC

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corresponds to the difference in energy levelsbetween the Fermi-level of TiO2 and the

standard electrode potential of the I−/I3− redox

system.

Figure 7.4 Structure of Grätzel cell

7.1.4 Organic Transistors [7,8]Organic transistors, in which organic materialis used for the semi-conductor layer, have beenwidely explored. A model of an organic fieldeffect transistor (OFET) is shown in Figure 7.5.The mechanism of the OFET is as follows: agate voltage induces charge carriers at theinterface of the organic and insulating layers,resulting in decreasing resistance between thesource electrode and the drain electrode.

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Figure 7.5 Illustration of top-contact OFET

Polycyclic aromatic hydrocarbons and otherconjugated molecules are suitable for thispurpose. Intermolecular carrier mobilitythrough the organic layer is an importantfactor, therefore orientation control of themolecules and the design of the junctionbetween the organic layer and the electrodesare important. A number of polymer-basedmaterials (conjugated polymers) for OFET havebeen developed. A wet-process is available andis useful for fabricating large area devices at lowcost, although carrier mobility is generallylower than for highly oriented small molecules.

7.1.5 Electrochromic Devices [9]Electrochemical reaction of organic moleculessometimes accompanies drastic colour change.Reversible redox behaviour with colour changeis applicable to electrochromic devices. Thiscolour developing and reducing property, whichis different to luminescence phenomena, issuitable for electronic paper applications. Once

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coloured by the electrochemical reaction, thegiven image and information remain even aftera power cut.

Figure 7.6 shows a typical electrochromicdevice consisting of a sandwiched structure ofelectrolyte containing a chromic compoundwith anode and cathode plates. To visualizecolour changes, a transparent electrode shouldbe used for the electrochemical reaction. Thechromic compounds used need to havereversible redox properties, i.e. their radicalanion or radical cation state should be stable.They may be fixed on a transparent electrode toavoid colour reducing once a coloured state isgenerated at a counter electrode. As describedin section 5.8.6, the use of a stable doping stateand the reversible colour changes of conductingpolymers for electrochromic applications is veryconvenient. In addition, film-forming propertyof electropolymerized conducting polymers isadvantageous. Multicoloured electrochromicdevices are obtained by stacking each cell,showing their individual colours.

Figure 7.6 Mechanism of electrochromicdevice

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7.1.6 Conducting Polymer-basedCapacitorsConventional aluminium electrolytic capacitorsare composed of an aluminium anode,aluminium oxide film, electrolytic solution andan aluminium cathode (Figure 7.7). Aluminiumsolid capacitors replace the electrolytic solutionwith solid conductive materials such asβ-MnO2, organic conductors and conductingpolymers. In contrast to the ionic-conductingmechanism of electrolytic solutions, solidcapacitors are driven by an electron-conductingmechanism, and thus conductivity is greatlyimproved. Furthermore, the thermal propertiesof solid capacitors are superior to those ofconventional ones.

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Figure 7.7 Aluminium electrolytic capacitor

7.2 Electrochemical Conversionof Biomass to Valuable MaterialsBiomass is a renewable organic resource andtherefore it does not cause a depletion problem,unlike fossil resources such as petroleum andcoal. In the 21st century, the social demands ofthe effective utilization of biomass, which canbe transformed to high-performance chemicalsand fuels, have rapidly increased. Chemicaltransformation of biomass has to be performedusing energy conservation technology and withlow emissions, and accordingly various trials toachieve such goals have been carried out usingcatalytic and enzymatic reactions. Asmentioned earlier, electrochemical reactionshave the advantage of both energy conservationand low emissions. Intensive study of theelectrochemical processes applied to thechemical transformation of biomass startedmore than 30 years ago, and the transformationof biomass to high-performance chemicals wasinvestigated as a national project in the USA[10]. Today, the study of biomasstransformation is increasingly important.

The main components of plant-based biomassare cellulose and lignin, which are polymers.Since neither of these are suitable forelectrolysis because of their insolubility insolvents like water, biomass transformation to

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useful chemicals has been attempted througheither hydrolysis followed by electrolysis orindirect electrolysis with appropriatemediators.

Since water-soluble glucose is available byhydrolysis of cellulose, its transformation togluconic acid and sorbitol has been achievedwith 90% and 50% current efficiency,respectively, by electrochemical oxidation andreduction of glucose using an undivided cell, asshown in Figure 7.8. Each of theseelectrochemical processes has been practicedseparately on a commercial scale.

Figure 7.8 Paired electrosynthesis of gluconicacid and sorbitol from glucose

Baizer and his co-workers realized the pairedelectrosynthesis of gluconic acid and sorbitolfrom glucose using both electrode reactions[11].

Synthesis of dialdehyde starch was achieved byoxidation of starch with IO3

− mediator usingthe Ex-cell indirect electrolysis method (100%current efficiency). Other various

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electrochemical transformation processes ofbiomass have also been developed, for examplepaired electrosynthesis of furfuryl alcohol and2-furancarboxylic acid from furfural derivedfrom hemicellulose, and electrosynthesis ofacetone, 2-butene and methylethyl ketone from2,3-butanediol, which is readily availablethrough fermentation of glucose and xylose[12].

Furthermore, anodic oxidative conversion oflignin to small molecules has been attempted,and aromatic compounds like vanillin obtained[13]. Waldvogel and BASF developed a processfor the electrochemical degradation of lignin ata boron-doped diamond (BDD) anode in anaqueous solution to producehydroxybenzaldehyde derivatives like vanillinand/or phenol derivatives in higher than 5%yield by weight [14]. Quite recently, oxidation oflignosulfonic acid at silver or nickel anode in anaqueous alkaline solution using a flow-type cellhas been investigated in order to increase theselectivity and yields of useful compounds likevanillin, vanillic acid and guaiacol (Schmitt andWaldvogel, unpublished).

Efficient use of biomass should be increasinglyimportant in the world, and electrolysis isexpected to be a powerful tool to achieve thisbeneficial utilization of natural resources.

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7.3 Application to C1 ChemistryThe concept of C1 chemistry is the synthesis ofvarious useful organic compounds through theformation of carbon–carbon bonds using C1compounds as a starting substrate or by theintroduction of various atoms to C1 compounds.In the 1980s, related studies were carried out asa national project, and recently C1 chemistryhas again attracted much attention in relationto green sustainable chemistry.

A typical example of C1 chemistry, the fixationof CO2, is a highly important subject. Since CO2is a final product of the combustion processes,the reverse conversion and use of CO2 arehighly important research subjects. However,the reduction potential of CO2 is very negative,and hence the cathodic evolution of hydrogentakes place as a competitive reaction, reducingthe current efficiency and product yield. Thecathodic reduction has therefore beenintensively studied using various cathodematerials. Among these materials, copper wasfound to be most effective for the reduction ofCO2 [15]. In order to increase efficiency,cathodic reduction of CO2 was carried outunder high pressure. Cathodic reduction of CO2provides formic acid, oxalic acid, methane andso on (Eq. 7.1).

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(7.1)

Quite recently, Nakata, Einaga and co-workersachieved the electrochemical reduction of CO2in seawater using a BDD electrode underambient conditions to provide formaldehydeselectively with high current efficiency (74%), asshown in Eq. 7.2 [16]. The high currentefficiency is attributable to the wide potentialwindow and the sp3-bonded carbon of the BDD.

(7.2)

Cathodic reduction of CO has also been carriedout to form squaric (quadratic) acid, as shownin Eq. 7.3 [17].

(7.3)

7.4 Environmental CleanupElectrochemical treatment involving anodicdecomposition is a highly promising method forthe reduction of toxic pollutants dissolved inwaste water. It is important to select the proper

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anode materials to optimize this techniquebecause the electrolytic products stronglydepend on these materials as well as theoperating conditions such as the currentdensity and temperature [18]. Various materialshave been developed to date as anodes. Theyare classified as follows: carbon (amorphouscarbon, graphite), noble metal or metal oxides(Pt, IrO2, RuO2) and non-noble metal oxides(PbO2, SnO2, TiOx). Since early times,electrolysis has been used for the treatment ofdye waste water by anodic oxidation of saline togenerate sodium hypochlorite solution, which isused for decolouration of dye waste water.Effluent water treatment has also beenperformed using ozone generated by oxidationof water with PbO2. In recent years, BDDelectrodes have been developed, and they haveproved to be effective for detoxifying treatmentof waste water containing dye, organic acids,phenols, soluble polymers and so on [19–21]. Ithas also been shown that diamond-like carbon,is effective for the degradation of persistentorganic fluoro compounds [22]. The efficientdegradation is attributable to hydroxyl radicalselectrogenerated at the BDD anode, resulting incomplete degradation to CO2 [18].Interestingly, this decomposition mechanism isquite similar to that with TiO2 photocatalyst.BDD electrodes are also effective in thedegradation of organic additives in plating

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baths, and hence a possibility of recycle use ofplating bath is also demonstrated.

It has also been demonstrated thatdecomposition of harmful organic chlorides likeDDT can be achieved by electrolysis usingcobalt complex mediator like hydrophobicvitamin B12 under photo irradiation [23].

Furthermore, a novel flow and circulatingsystem using a Pd tube as a cathode has beendeveloped for electrocatalytic hydrogenation,and this system was shown to be highlyeffective for the dechlorination of polychloroaromatics, as shown in Figure 7.9 [24,25].

Figure 7.9 Electrocatalytic dehalogenationsystem

Conversion of chlorofluorocarbon (CFC) touseful substances using gas diffusion electrodessuch as Au, Cu, Pd and In has been attempted,as shown in Eq. 7.4.

(7.4)

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14. (a) Griesbach, U., Fischer, A., Stecker, F.,Botzem, J., Pelzer, R., Emmeluth, M. andWaldvogel, S.R. (2009) Method forelectrochemically cleaving lignin on a diamondelectrode. WO 2009138368 A1, filed May 11,2009 and issued Nov. 19, 2009. (b) Griesbach,U., Fischer, A., Stecker, F., Botzem, J., Pelzer,R., Emmeluth, M. and Waldvogel, S.R. (2011)Process for the electrochemical cleavage oflignin at a diamond electrode. US Patent20110089046 A1, filed May 11, 2009 and issuedApr. 21, 2011. (c) Stecker, F., Fischer, A., Kirste,A., Waldvogel, S.R., Regenbrecht, C., Schmitt,D., (2014) Process for the preparation ofvanillin. US Patent 20140034508 A1, filed July3, 2013 and issued Feb. 6, 2014. (d) Stecker, F.,Fischer, A., Kirste, A., Voitl, A., Wong, C.H.,Waldvogel, S.R., Regenbrecht, C., Schmitt, D.,Hartmer, M.F. (2014) Process for producingvanillin from vanillin-comprising compositions.

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7Related Fields of Organic Electrochemistry

365

Toshio Fuchigami

Organic electrosynthesis can be performedunder mild conditions such as roomtemperature and normal pressure, and does notrequire hazardous oxidants or reductants, suchas heavy metals. Moreover, it produces lesswaste compared to ordinary chemical synthesis.In spite of such advantages, commercializedorganic electrode processes are limited, incontrast to inorganic industrial processes [1–3].

8.1 Avenue to IndustrializationOrganic synthesis usually requires heating. Thestarting materials and products are easilydecomposed to decrease yields and selectivity athigh reaction temperatures. In contrast, organicelectrosynthesis is a synthetic chemical processthat can be operated under mild conditionssuch as room temperature and normalpressure, which avoids heat deterioration oforganic compounds. If optimum electrolyticconditions are established, organicelectrosynthesis could be a superior syntheticprocedure with excellent yield and selectivity.In order to decrease activation energy, variouscatalysts are commonly employed in chemicalreactions, but many catalysts are expensivehence recovery and recycling of catalysts is veryimportant.

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In electrolytic reactions, electrodes areelectroconductive interfaces where electrons aretransferred to and from substrates, as well asacting as the catalyst (electrocatalyst), thereforea costly catalyst is not necessary. Furthermore,the applied potential and current for theelectrochemical reaction can be inexpensivelyand precisely controlled. Thus, organicelectrosyntheses have many technicaladvantages over ordinary organic syntheticprocesses.

However, organic electrosynthesis also hassome disadvantages. Ordinary chemicalreactions are homogeneous, while the reactionfield of electrolysis is a heterogeneous interface,therefore electrosynthesis has a productivedrawback. Moreover, quite differently frominorganic electrode processes like saltelectrolysis, in organic electrosynthesis theworking electrode providing products is usuallyeither the anode or the cathode. The counterelectrode is usually not used for the formationof valuable products except for the pairedsynthesis developed by BASF, as explainedbelow. Although the main role of the counterelectrode is for current flow to the workingelectrode, the maintenance of the electrode isnecessary, resulting in an increase in runningcost. When the cathode is a working electrode,the lifetime of the anode as a counter electrodebecomes a barrier to industrialization. Ingeneral, the anode is readily corrosive and

367

therefore expensive electrode materials have tobe used for it. As a result the maintenance costof the counter anode is often much higher thanthat of the working cathode. These problemsmake industrialization of organicelectrosynthesis difficult and, as mentionedearlier, it must be recognized that organicelectrosynthesis has severe limitations. In spiteof this, cathodic hydrocoupling of acrylonitrilehas been used for more than 40 yearsworldwide. This is because the product,adiponitrile, is profitable because of itsversatility and in high demand because of itsexcellent physical properties. The cathodichydrocoupling method is also superior to othersynthetic procedures from an energy costaspect. Initially, the process was operated usinga divided cell, but later electrolysis without adiaphragm was developed in order to decreasethe electric power cost of the process.

8.2 Examples

8.2.1 Electrosynthesis of Adiponitrile6,6-Nylon is produced by catalytichydrogenation of adiponitrile to formhexamethylenediamine, followed bydehydration-condensation with adipic acid, asshown in Eq. 8.1. Since 6,6-nylon has excellentstrength and durability, it is widely used in tyrecord, synthetic clothes, engineering plastics etc.Hexamethylenediisocyanate as a starting

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material for polyurethane is also produced fromhexamethylenediamine. Because of the benefitsof nylon and polyurethane, cathodichydrocoupling of acrylonitrile is an essentialtechnology for daily life.

(8.1)

Cathodic hydrocoupling of acrylonitrile hasbeen known since the 1940s, but the yield ofadiponitrile was not high initially. In 1960,Baizer at Monsanto improved both the yieldand current efficiency of the hydrocoupling byusing quaternary ammonium salts as thesupporting electrolyte [2]. In 1963 and 1965,Monsanto developed and commercialized,respectively, the electrohydrodimerization ofacrylonitrile, which was produced at low cost ona large scale using the SOHIO methodestablished at that time (Eq. 8.2). An aqueoussolution containing a large amount ofquaternary ammonium salts (about 40%) wasused to dissolve hardly-soluble acrylonitrile athigh concentration. Thus, the Monsantoprocess adopted a homogeneous electrolyticsystem.

369

(8.2)

On the other hand, the Asahi ChemicalCompany (now the Asahi Kasei Corporation) inJapan developed emulsion cathodichydrocoupling of acrylonitrile using thethermomorphicity of acrylonitrile and watercontaining 10% quaternary ammonium salt,and started operation of the process in 1971 [4].Both processes were operated in a divided cellequipped with a separator-like cation exchangemembrane at the early stage of the processes.

Later on, both companies improved theirprocesses, and in the mid-1980s theyestablished processes with undivided cells inorder to reduce the higher cell voltage causedby separator resistance. To develop theundivided cell process, they focused on (i) highselectivity as well as less corrosion, (ii)establishment of electrolytic conditions withstable long-term operation and systems forrecovery of quaternary ammonium salt/purification of products, and (iii) developmentof a facility in which the explosive gas mixtureof acrylonitrile and oxygen can be treatedsafely.

The cathodes used in this process are Hg, Pb orCd, which have higher hydrogen overpotentialbecause hydrogen generation as a side reaction

370

decreases the current efficiency for thehydrocoupling. On the other hand, Fe, Ni or Pbis used for the anode, and these have loweroxygen overpotentials to prevent the anodicoxidation of organic compounds. Thesupporting electrolyte is a mixture ofquaternary ammonium salts and inorganic salt.Quaternary ammonium salts such asethyltributylammonium salt are effective notonly for increasing the selectivity of adiponitrileformation but also for protection of the cathodefrom corrosion. Inorganic salts such aspotassium phosphate and alkali metal borateincrease the conductivity of the electrolyte andprevent corrosion of the anode.

The detailed operation conditions andperformances of this coupling process havebeen reported by several workers [1–4] and anexample of operation conditions is as follows:

Cathode: Pb

Anode: Fe (Ni 9%)

Supporting electrolyte: (EtBu3N)2HPO4 +K2HPO4 + K2B4O7

Current density: 2 kA m−2

Temperature: 55 °C

Current efficiency: 90.7%

At the present time more than 300,000 tonnesper year of adiponitrile is manufactured bycathodic hydrocoupling of acrylonitrile

371

worldwide, and in 2010 24% of this adiponitrilewas manufactured by cathodic hydrocouplingby whole production.

8.2.2 Electrosynthesis of AromaticAldehydesThe electrochemical production of acetals ofaromatic aldehydes like anisaldehyde andp-tolualdehyde has been running for more than30 years at BASF, as shown in Eq. 8.3 [2].Oxidation of the methyl group on aromaticrings provides aromatic aldehydes, but ordinarychemical methods using oxidants often causeoveroxidation, resulting in the formation ofcarboxylic acids as a by-product. Theelectrochemical oxidation of toluene derivativesin methanol provides anodically stabledimethylacetals, which are readily hydrolyzedto form aldehydes and methanol. The recoveredmethanol can be recycled for anodicacetalization. This process is operated in thecapillary gap cell developed by BASF, as shownin Figure 6.26 [5].

(8.3)

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8.2.3 Paired Electrosynthesis ofPhthalide and t-ButylbenzaldehydeRecently, BASF commercialized the pairedelectrosynthesis of phthalide andt-butylbenzaldehyde dimethylacetal, as shownin Eq. 8.4 [2]. The products are useful as plantprotectors, additives for plating baths,ultraviolet absorbers, aroma chemicals,fungicides and so on. Phthalide has beenprepared by classical catalytic hydrogenation ofphthalic acid anhydride, but this methodrequires pure hydrogen gas and high pressure.On the other hand, during anodicdimethoxylation of t-butyltoluene, protons arepredominantly reduced at the cathode togenerate hydrogen gas. BASF made great effortsto find a compatible reduction process that canbe used instead of proton reduction. Eventually,the combination of anodic dimethoxylation oft-butyltoluene and cathodic reduction ofphthalic acid diester was found to establish thepaired electrosynthesis, as shown in Eq. 8.4.Cathodic reduction of phthalic aciddimethylester produces phthalide andmethanol, while the resulting methanol is usedfor anodic dimethoxylation of t-butyltoluene toprovide dimethylacetal in good yield. In thispaired electrosynthesis, the balance ofmethanol, proton and electron is remarkable.

373

(8.4)

This process runs in an undivided capillary cellwith a stack of bipolar round graphiteelectrodes (Figure 6.26) [5]. The electrodeshave a centre hole, and are separated by spacersand connected in series. Since quaternaryammonium salt is used as the supportingelectrolyte, cathodic reduction of methanol issuppressed and reduction of o-phthalic aciddimethyl ester proceeds highly efficientlywithout formation of by-product. Sinceefficiency at both electrode reactions is high,the energy efficiency of the paired synthesis ismuch better than that of conventionalprocesses.

374

8.2.4 Electrochemical PerfluorinationElectrochemical perfluorination is a process inwhich all the hydrogen atoms in a startingorganic molecule are substituted with fluorineatoms without elementary fluorine generationduring electrolysis. J.H. Simons at 3M achievedthe electrochemical perfluorination of organiccompounds in anhydrous liquid HF usingnickel electrodes to provide perfluorinatedproducts for the first time in 1941 [6,7]. He is apioneer of electrochemical perfluorination andthis method is called Simons' process. Theprocess uses an undivided cell at lowtemperature to keep HF as a liquid (the boilingpoint of HF is 19.5 °C). Electrochemicalperfluorination of organic compoundscontaining oxygen, nitrogen or sulfur atomsforms salts with anhydrous HF, which providegood conductivity for the anhydrous liquid HFsolution. On the other hand, in the case ofhydrocarbon, salts such as KF and NaF must beadded to impart conductivity and allow theelectrochemical perfluorination process. Fromcarboxylic acids (its chlorides), sulfonic acids(its chlorides) and trialkylamines, theperfluorinated products are obtained in goodyields, as shown in Eqs. 8.5–8.7 [8]. However,in many cases the yield for electrochemicalperfluorination is rather low because ofcarbon–carbon bond cleavage duringelectrolysis [8]. The products, perfluoroalkyl

375

carboxylic acids and sulfonic acids, are useful asdetergents and lubricants.

(8.5)

(8.6)

(8.7)

The reaction mechanism has been discussed formany years [8–11]. One mechanism involvesanodically generated fluorine radical as the vitalreagent for perfluorination and anotherinvolves electrogenerated highly oxidized nickelfluorides such as Ni2F5, NiF3 and NiF4 on anickel anode surface. These act as fluorinatingreagents and are electrochemically regeneratedat the nickel anode. During the electrolysis,partially fluorinated products have polarity andthey stay in the electrolyte to be subjected tofurther electrolysis.

The final perfluorinated products are non-polarand their specific density is very high, thereforethey precipitate from liquid HF onto the cellbottom as a liquid.

The electrochemical perfluorination processnow runs on a large scale at 3M in the USA andat the Central Glass Company and theMitsubishi Materials Electronic Chemicals

376

Company in Japan. Mitsui Chemicals in Japanalso produces NF3 by electrochemicalperfluorination of NH3 [12]. NF3 is used as anetchant and cleaning gas for apparatus used inthe chemical vapour deposition (CVD)technique.

8.2.5 Other Examples

8.2.5.1 3,6-Dichloropicolic Acid

The Daw Chemical Company havecommercialized the electrochemical reductionof 3,4,5,6-tetrachloropicolic acid in aqueoussolution to 3,4-dichloropicolic acid, which isuseful as a precursor to agrochemicals (Eq. 8.8)[13]. As explained in Chapter 4 (Fig. 4.5), thehigh regioselectivity is attributed to thecontrolled orientation of the substrate at thecathode surface owing to the dipole moment ofthe molecule.

(8.8)

8.2.5.2 β-Lactam Derivative

The Otsuka Chemical Company havecommercialized the electrosynthesis of theantibiotic substance β-lactam derivative(GCLE), as shown in Eq. 8.9. The electrolysis is

377

operated in a two-phase system ofdichloromethane and water, and the reactionproceeds through anodic oxidation of chlorideions.

(8.9)

8.2.5.3 Cysteine

Electrochemical reduction of cystin to cysteineis used in many countries, including the USA,Japan, China and some European countries(Eq. 8.10). Since separation of cysteine fromcystin is quite difficult, the electrolysis has to becontinued until all the starting cysteine hasbeen completely consumed. This means that atthe final stage of the electrolysis electricity isconsumed mainly for the reduction of protonsto evolve hydrogen gas, resulting in a decreasein current efficiency, which is a problem thathas still to be overcome.

(8.10)

8.2.5.4 Tetramethylammonium Hydroxide

Electrochemical dialysis oftetramethylammonium chloride uses a cationicion exchange membrane to produce

378

chlorine-free and highly puretetramethylammonium hydroxide in Japan (Eq.8.11). In order to avoid contamination ofchlorine in the product, anodically generatedchlorine gas is removed completely from thedialysis system.

(8.11)

8.2.5.5 Other Examples

BASF manufactures2,5-dimethoxy-2,5-dihydrofuran anddihydrophthalic acid from furan and o-phthalicacid, respectively (Eq. 8.12). They also produceacetoin by anodic oxidation of cyclohexanonewith an iodine mediator.

(8.12)

Furthermore, SNPF in France producesanti-inflammatory phenoprofen by the cathodicreduction of m-(α-chloroethyl)phenylphenylether in the presence of carbon dioxide(Eq. 8.13) [14].

379

(8.13)

Electrosynthesis of the artificial sweetenermaltol and m-hydroxybenzyl alcohol used to becommercialized in Japan, but this process hadto be stopped because of its high cost comparedto alternative chemical processes and the lowerprice of products made in other countries.

As explained above, organic electrochemicalprocesses often compete with chemical onestherefore the development of electrochemicalprocesses that are superior to chemical ones isnecessary. It should be emphasized that organicelectrosynthetic processes that address the needfor low emissions are highly promising [2].

References1. Macdonald, D.D. and Schmuki, P. (eds)(2007) Electrochemical Engineering,Encyclopedia of Electrochemistry, Vol. 5,Wiley-VCH Verlag GmbH.

2. Pütter, H. (2001) Organic Electrochemistry,4th edn (eds H. Lund and O. Hammerich),Marcel Dekker, Chapter 31.

3. Genders, J.D. and Pletcher, D. (1990)Electrosynthesis. From Laboratory, To Pilot,

380

To Production, The Electrosynthesis Co. Inc.,New York.

4. Shimizu, A. (1998) Catalysts & Catalysis, 5,269.

5. Beck, F. and Guthke, H. (1969)Chem.-Ing.-Tech., 41, 943–950.

6. Simons, J.H. (1949) J. Electrochem. Soc.,95, 47–52.

7. Simons, J.H. (1986) J. Fluor. Chem., 32,7–24.

8. Suriyanarayanan, N. and Noel, M. (2008) J.Solid State Electrochem., 12, 1453–1460.

9. Hollitzer, E. and Satori, P. (1986)Chem-Ing-Tech., 58, 31–38.

10. Dimitrov, A., Rüdiger, S., Ignatyev, N.V.and Datcenko, S. (1990) J. Fluor. Chem., 50,197–205.

11. Sartori, P., Ignat'ev, N., Jünger, C., Jüschke,C. and Rieland, P. (1998) J. Solid StateElectrochem., 2, 110–116.

12. Tasaka, A., Kawagoe, T., Takuwa, A.,Yamanaka, M., Tojo, T. and Aritsuka, M. (1998)J. Electrochem. Soc., 145, 1160–1164.

13. Edamura, F., Kyriyacou, D. and Love, J.(1980) US Patent 4217185; Chem. Abstr. (1981),94, 22193.

381

14. Chausaard, J., Troupel, M. and Robin, Y.(1984) J. Appl. Electrochem., 19, 345–348.

382

8Examples of Commercialized Organic ElectrodeProcesses

383

A.1 Electrochemical FluorinationAnodic fluorination and anodic methoxylationof ethyl phenylthioacetate [1,2]

Anodic partial fluorination of organiccompounds usually uses a poly(hydrogenfluoride) complex of amine or ammonium saltas the fluorine source and supportingelectrolyte. In an undivided cell, hydrogenevolution by cathodic reduction of proton takesplace as well as the desired anodic fluorinationof a substrate. In an acetonitrile solution,fluorination proceeds via an anodicallygenerated cation intermediate, whereasmethoxylation occurs selectively in a methanolsolution (Eq. A.1). Optimal conditions foranodic fluorination depend on the substratesused. It is necessary to optimize thecombination of reaction media and supportingelectrolytes.

Anodic fluorination of ethyl phenylthioacetateproceeds via oxidation of the sulfur atomfollowed by deprotonation of its α-position. Thedeprotonation step is promoted by the effect ofan electron-withdrawing group substituted atthe α-carbon to the phenylthio group. Themonofluorinated product at the α-position ofthe ester group has to be relatively stable if it isto be used as a building block for furthertransformation.

384

(A.1)

Figure A.1 shows a typical electrolytic cell foranodic fluorination, equipped with a platinumplate anode and cathode (2 cm × 2 cm). The cellis filled with an electrolytic solution containingethyl phenylthioacetate (196 mg, 1 mmol),Et3N-3HF (1.6 ml, 10 mmol) and acetonitrile(8.4 ml). Constant current electrolysis (20 mA,current density 5 mA cm−2) for 2 h 41 min (2 Fmol−1) is conducted with stirring. The solutionis then neutralized by saturated NaHCO3 andextracted with ethyl acetate (30 ml × 3). Theorganic phase is washed with brine (100 ml)and dried over Na2SO4. The solvent is removedunder reduced pressure and the crude productis purified by silica gel column chromatographyto give the desired fluorinated product in50–70% yield. If methanol is used instead ofacetonitrile, the methoxylated product isobtained selectively.

385

Figure A.1 Illustration of set up for anodicfluorination by the constant current method

A.2 Electrosynthesis Using aHydrophobic ElectrodeSynthesis of chromane from terpene andelectrogenerated quinomethane [3]

A hetero Diels–Alder reaction of quinomethaneand terpene is regarded as a biogenetic pathwayfor euglobal derivatives. Electrochemicalgeneration of quinomethane and subsequentcycloaddition with terpene is possible to obtainchromane, a basic structure of euglobal (FigureA.2).

386

Figure A.2 Illustration of electrolytic set upwith a hydrophobic electrode and euglobalsynthesis

Sulfide 1 (0.1 mmol) and α-phellandrene 2 (0.4mmol) are dissolved in 1.0 M LiClO4/CH3NO2(15 ml). Constant potential electrolysis (1.2 Vvs. Ag/AgCl) is performed using a glassy carbonanode covered with poly(tetrafluoroethylene)(PTFE) and a platinum plate cathode. After thepassage of 1.2 F mol−1 of charge, the product isextracted with hexane (10 ml) several times.The organic layer is dried over Na2SO4 andproduct 3 is confirmed by TLC.

A.3 Natural Product SynthesisUsing Anodic OxidationSynthesis of Aniba neolignan by anodicoxidation of phenol derivatives [4]

387

Although anodic oxidation of phenol derivativesis very useful for obtaining alicyclic compoundsfrom aromatic compounds, its versatile reactionpaths make it difficult to produce the desiredproduct selectively. If reaction conditions areoptimized, anodic oxidation of phenols can be apowerful tool for obtaining the desiredproducts, which are difficult to prepare byconventional organic synthesis.

According to Eq. A.2, cation A, which isgenerated by anodic oxidation of phenol 1,undergoes cycloaddition with isosafrole to givea natural product, Aniba neolignan.

(A.2)

Phenol 1 (100 mg), isosafrole 2 (500 mg) andLiClO4 (200 mg) are dissolved in methanol/acetic acid (2:1, 30 ml) and used as theelectrolytic solution. In a glassy carbon beakeras anode, constant current electrolysis (10 mA,current density 0.19 mA cm−2) is carried out for130 min using a platinum wire cathode. The

388

solvent is removed and the product is purifiedby silica gel column chromatography.

A.4 Kolbe ElectrolysisDiester synthesis from monoester of adipicacid [5]

The Kolbe reaction, which involves one-electronoxidation of carboxylate and subsequentdecarboxylation to form its dimer, is veryeffective in producing higher alkanes. Forexample, the monoester of adipic acid can beconverted to its dimeric compound (Eq. A.3).

(A.3)

An electrolytic cell equipped with a platinumanode and a cathode (25 mm × 30 mm,distance 5 mm) is filled with a methanolsolution (250 ml) containing adipic acidmonomethyl ester (120 g, 0.75 mol), sodiummethoxide (4.1 g, 0.075 mol) and pyridine (10ml, 0.12 mol). Constant current electrolysis (1.1A, current density 0.13 mA cm−2) is performedfor 23 h. During electrolysis under theseconditions heat generates to induce reflux of thereaction mixture. After cooling to roomtemperature, acetic acid (20 ml) is added to themixture to acidify it. The solvent is removedunder reduced pressure and the residual solid is

389

dissolved in ether (500 ml). The filtrate iswashed with aqueous NaHCO3 and water, anddried over CaSO4. The product is obtained bydistillation under reduced pressure.

A.5 Indirect ElectrosynthesisUsing a MediatorKetone synthesis by indirect anodic oxidationof alcohol using iodine mediator [6]

The cation species of iodine is generated byanodic oxidation of iodide ions and is used forthe oxidation of alcohol to give a ketone productand recovery of iodide ions. The reaction can bemediated by a catalytic amount of iodidewithout metal oxide (Eq. A.4).

In an electrolytic cell, an aqueous solution (15ml) of potassium iodide (2.49 g, 0.015 mol) andalcohol (0.06 mol) is produced. Constantcurrent electrolysis using platinum or carbonanode is conducted for 4–15 F mol−1 ofelectricity. The organic layer is separated andthe water layer is extracted with diethyl ether.The combined organic phase is washed withaqueous Na2S2O3. The product is purified bydistillation.

390

(A.4)

A.6 Electrosynthesis ofConducting PolymersElectropolymerization of pyrrole

Electropolymerization of aromatic compoundsproceeds at the electrode surface and a thin filmis obtained. These conducting polymers areused for conductive materials, electrochromicdevices and catalysts. A typical mechanism ofoxidative polymerization of pyrrole is shown inEq. A.5.

(A.5)

In a beaker-type cell, platinum plate electrodes(1 cm × 1 cm) and a saturated calomel electrode(SCE) as a reference are set up. An aqueous

391

solution (50 ml) of pyrrole (0.1 M) and NaClO4(0.1 M) is added to the cell. A potential sweepfrom −0.6 V vs. SCE to +0.8 V vs. SCE at a scanrate of 100 mV s−1 involves polymerization onthe working electrode, which is monitored asshown in Figure A.3. The working electrodecovered with polypyrrole is purified by washingwith water.

Figure A.3 Cyclic voltammograms of pyrroleduring electropolymerization

392

References1. Fuchigami, T., Shimojo, M. and Konno, A.(1995) J. Org. Chem., 60, 3450–3464.

2. Fuchigami, T., Yano, H. and Konno, A. (1991)J. Org. Chem., 56, 6731–6733.

3. Chiba, K., Arakawa, T. and Tada, M. (1996)Chem. Commun., 1763–1764.

4. Shizuri, Y. and Yamamura, S. (1983)Tetrahedron Lett., 24, 5011–5012.

5. Haufe, J. and Beck, F. (1970) Chem. Ing.Tech., 42, 170–175.

6. Shono, T. Matsumura, Y., Hayashi, J. andMizoguchi, M. (1979) Tetrahedron Lett., 20,165–168.

393

Appendix AExamples of Organic Electrosynthesis

394

Table B.1 Potential window of organic solutionfor electrochemical reactions (Pt workingelectrode)

Solvent Supportingelectrolyte

Potential (V vs.SCE)

Cathodicside

Anodicside

AcOH AcONa −1.0 +2.0

Acetone n-Bu4NClO4 −1.0 +1.6

MeCN LiClO4 −3.0 +2.5

MeCN Et4NBF4 −1.8 +3.2

DMF n-BuN4ClO4 −2.8 +1.6

DMSO LiClO4 −3.8 +1.3

MeOH LiClO4 −1.0 +1.3

MeOH KOH −1.0 +0.6

CH2Cl2 n-BuN4ClO4 −1.7 +1.8

THF LiClO4 −3.2 +1.6

Sulfolane NaClO4 −4.0 +2.3

MeNO2 Mg(ClO4)2 −2.6 +2.2

Propylenecarbonate

Et4ClO4 −1.9 +1.7

Table B.2 Oxidation potentials of typicalorganic compounds

395

Compound Electrolyte/solvent

Potential(V)

Referenceelectrode

Aromatic compounds

BenzeneTolueneAnisoleBiphenylFluoreneNaphthalenePyridineThiophenePyrrole

NaClO4/MeCNNaClO4/MeCNPr4NClO4/MeCNNaClO4/MeCNNaClO4/MeCNNaClO4/MeCNNaClO4/MeCNNaClO4/MeCNNaClO4/MeCN

2.001.931.761.481.251.312.22.100.46

Ag/Ag+

Ag/Ag+

SCEAg/Ag+

Ag/Ag+

SCESCESCESCE

Olefins

EthyleneCyclohexeneStyrene

Bu4NBF4/MeCNNaClO4/MeCNNaClO4/MeCN

2.901.951.90

Ag/Ag+

Ag/Ag+

SCE

Nitrogen and sulfur compounds

AcetamideAnilineN-MethylanilineNitrobenzeneThiophenolDimethyl

sulfideDiphenyl

disulfide

Et4NClO4/MeCNBuffer/H2ONa2SO4/H2O0.1 M HCl/50%acetone/H2OCF3CO2H/CH2Cl20.1 M HCl/MeOHLiClO4/MeCN/CH2Cl2

2.001.040.700.581.650.861.75

SCESCESCENHEAg/Ag+

Ag/Ag+

Ag/Ag+

Alcohols

396


Potential(V)

Referenceelectrode

MethanolEthanolIsopropyl

alcoholt-Butyl alconolAlly alcoholBenzyl alcoholPhenol

Bu4NBF4/MeCNBu4NBF4/MeCNBu4NBF4/MeCNBu4NBF4/MeCNBu4NBF4/MeCNNaClO4/MeCNNaClO4/MeCN

2.732.612.502.602.65>2.001.04

Fc/Fc+

Fc/Fc+

Fc/Fc+

Fc/Fc+

Fc/Fc+

Ag/Ag+

Ag/Ag+

Table B.3 Reduction potentials of typicalorganic compounds


Potential(V)

Referenceelectrode

Halogen compounds

Chloromethanet-Butyl chloridet-Butyl bromidet-Butyl iodide

Et4NClO4/DMFEt4NClO4/DMFEt4NBr/DMFBu4NBF4/DMF

−2.76−2.60−2.19−1.91

SCESCESCESCE

Carbonyl compounds

AcetoneAcetophenoneFormaldehydeAcetaldehydeBenzaldehyde

Et4NBr/DMFLiOH/75%DioxanepH = 8pH = 9.1NH4Cl/40%EtOH

−2.84−1.26−1.22−1.51−1.32

SCESCENHENHESCE

397


Potential(V)

Referenceelectrode

Quinones

1,4-Benzoquinone1,4-Naphtoquinone9,10-Anthraquinone

50% EtOH50% EtOH95% EtOH

0.710.490.16

NHENHENHE

Olefins

Styrenetrans-Stilbenecis-Stilbene

Bu4NI/DMFBu4NI/DMFBu4NI/DMF

−2.45−2.30−2.07

SCESCESCE

Aromatic compounds

BenzeneNaphthaleneAnthracenePyreneBiphenylFurfural2,6-DimethylpyridineNicotinamide

Bu4NBr/Me2NHBu4NBr/Me2NHBu4NBr/Me2NHBu4NBr/Me2NHNaBPh4/THFBritton andRobinson/H2OBu4NI/DMFBritton andRobinson/MeOH

−3.42−2.53−2.04−2.29−2.68−1.04−2.85−1.34

Ag/Ag+

Ag/Ag+

Ag/Ag+

Ag/Ag+

Ag/Ag+

SCEAg/Ag+

Ag/Ag+

Nitrogen and sulfur compounds

398


Potential(V)

Referenceelectrode

NitromethaneNitrobenzeneDiphenyl disulfideMethyl phenyl

sulfoneBenzenesulfonic acid

pH = 7/H2OpH = 7/80%dioxaneBu4NI/DMFBu4NBr/DMFMe4NCl/dioxane

−0.88−0.62−2.75−2.41−1.50

SCESCEAg/Ag+

SCESCE

Table B.4 Physical properties of typicalsolvents

Compound Molecularweight

Boilingpoint(°C)

Meltingpoint(°C)

Densityd (g/cm3)(25 °C)

Viscosityηmilli-poise(25 °C)

Relativepermittivity

r (25 °C)

Dipolemomentμ ( ×10−30

C·m)

DonornumberDN

AcceptornumberAN

Self-ionizationconstant pK1

Acetic acid 60.05 117.8 16.64 1.049 2*1

12.2 *1 6.15 5.67 — 52.9 14.5

Acetone 58.08 56.2 −95.4 0.784 5 3.02 20.7 9.76 17.0 12.5 —

Acetonitrile (AN) 41.05 81.6 −45.7 0.776 6 3.39 35.95 13.06 14.1 18.9 28.5

Chloroform 119.38 61.27 −63.49 1.489 1*1 5.55*2 4.724 3.40 — 23.1 —

N,N-Dimethylformamide(DMF)

73.10 158 −61 0.944 3 7.96 36.71 12.86 26.6 16.0 —

Dimethyl sulfoxide(DMSO)

78.14 189.0(Decomp.)

18.55 1.096 19.6 46.6 — 29.8 19.3 ∼32

Ethanol 46.07 78.32 −114.15 0.785 1 10.78 24.3 5.63 20 37.1 18.9

399

Compound Molecularweight

Boilingpoint(°C)

Meltingpoint(°C)

Densityd (g/cm3)(25 °C)

Viscosityηmilli-poise(25 °C)

Relativepermittivity

r (25 °C)

Dipolemomentμ ( ×10−30

C·m)

DonornumberDN

AcceptornumberAN

Self-ionizationconstant pK1

Hexamethylphosphorictriamide (HMPA)

179.20 235 7.20 1.024*3 — 29.6 — 38.8 10.6 —

n-Hexane 86.18 68.7 −94.3 0.6594*1

3.258*1 1.90 0.00 — 0.0 —

Methanol 32.04 64.75 −97.68 0.786 6 5.42 32.6 5.63 19.0 41.3 16.7

Nitrobenzene (NB) 123.11 210.80 5.76 1.198 6 18.11 34.82 14.03 4.4 14.8 —

Nitromethane (NM) 61.04 101.2 −28.6 1.131 2 6.27 35.94 11.53 2.7 20.5 19.5

Propylene carbonate(PC)

102.09 241 −49 1.19 25.3 64.4 — 15.1 18.3 —

Pyridine (Py) 79.10 115 −41.5 0.977 9 8.824 12.01 7.17 33.1 14.2 —

Tetrahydrofuran (THF) 72.11 65.0 −108.5 0.880 4.6 7.39 5.67 20.0 — —

Water 18.01 100.0 0.00 0.997 1 8.903 78.54 6.47 18.0 54.8 14.0*120 °C*222.8 °C*330 °C

400

Figure B.1 Electrode potential regions forreduction of functional groups

Figure B.2 Electrode potential regions foroxidation of functional groups

401

Appendix BTables of Physical Data

402

Aacetoxylation

activation energy

actuator

adatom

adiponitrile

adsorption

Ag/Ag+ electrode

aminonitrene

anode

asymmetric synthesis

auxiliary electrode (counter electrode)

Bbeaker type cell

benzyne

biomass

bipolaron

boron-doped diamond (BDD)

Butler-Volmer equation

403

CC1 chemistry

capacitor

capillary gap cell

carbene

carboxylation

catalytic current

cathode

cation-flow method

cation pool method

cell voltage

chain polymerization

chemoselectivity

cogeneration

conducting polymer

constant current electrolysis

constant potential electrolysis

coulometer

coulometry

current density

current efficiency

cyanation

cyclic voltammetry (CV)

404

cylindrical cell

Ddecomposition potential

desorption

diamond-like carbon

diaphragm

diffusion coefficient

diffusion layer

dimerization

direct electrolysis

divided cell

dopant

doping

dye-sensitized solar cell

Eelectrical double layer

electric field

electricity

electroauxiliary

electrocatalysis

electrocatalytic hydrogenation

electrochromic device

405

electrode material

electrode potential

electrogenerated acid (EGA)

electrogenerated base (EGB)

electroluminescence

electrolytic cell

electron transfer rate

electro-oxidative polymerization

electropolymerization

electro-reductive polymerization

elimination

emulsion

environmental cleanup

ex-cell

exciton

FFaradic current

Fc+/Fc couple

Fermi-level

filter press type flow cell

fluorination

formal potential

406

fuel cell

GGrätzel cell

green sustainable chemistry (GSC)

Hhalf-peak potential

half-wave potential

hindered azobenzene

HOMO

H-type cell

hydrocoupling

hydrogen overpotential

hydrophobic electrode

hypervalent compound

Iin-cell

indirect electrolysis

inner-sphere electron transfer (bonded electrontransfer)

intermolecular coordination effect

intramolecular interaction effect

407

ion-exchange membrane

ionic liquid

iR drop

KKolbe electrolysis

LLangmuir-Blodgett technique

limiting current density

linear sweep voltammetry (LSV)

Luggin capillary

LUMO

Mmass transport

mediator

metalation

methoxylation

microflow system

modified electrode

molecular orbital interlaction

408

Nnitrene

nitrenium ion

non-steady-state polarization curve

Oonset potential

outer-sphere electron transfer (non-bondedelectron transfer)

overoxidation

overpotential

oxidation potential

oxygen overpotential

Ppaired electrosynthesis

parallel laminar flow

passivation

peak current

peak potential

perfluorination

phase-transfer catalyst

photocatalyst

photoelectrolysis

409

photovoltaic cell

polaron

poly(p-phenylene vinylene)

polymer reaction

polymerization

polypyridine

polypyrrole

polysilane

polythiophene

potential window

potentiostat

pulse electrolysis

Qquaternary ammonium salt

Rreactive electrode

reduction potential

reference electrode

regioselectivity

reversible process

room temperature molten salt

410

Ssacrificial anode

sacrificial electrode

salt bridge

saturated calomel electrode (SCE)

sensor

silver-silver chloride (Ag/AgCl) electrode

solid polymer electrolyte (SPE)

solid-supported acid

solid-supported base

standard electrode potential

standard hydrogen electrode (SHE)

steady-state polarization curve

stereoselectivity

supercritical carbon dioxide

supercritical fluid

supercritical fluoroform

superoxide ion

supporting electrolyte

suspension

Ttask-specific ionic liquid

411

template electrochemical polymerization

TEMPO

thin-layer flow cell

three-electrode system

transistor

Uultrasonic cavitation

ultrasonication

ultrasound

umpolung (polarity inversion)

undivided cell

Vvoltammetry

voltammogram

WWacker oxidation

Walden role

wave clipping

working electrode

412

Index

413

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