suzuki–miyaura reaction by heterogeneously supported pd in...

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REVIEW

Suzuki–Miyaura

aDepartment of Chemistry, University of Ka

[email protected]; Fax: +91 33bDepartment of Chemistry, University of Ka

[email protected]; Fax: +91 33 2582cDepartment of Chemistry, University of Kal

Cite this: RSC Adv., 2015, 5, 42193

Received 31st December 2014Accepted 7th April 2015

DOI: 10.1039/c4ra17308b

www.rsc.org/advances

This journal is © The Royal Society of C

reaction by heterogeneouslysupported Pd in water: recent studies

Susmita Paul,*a Md. Mominul Islamc and Sk. Manirul Islam*b

This review summarizes the progress made essentially in the last fifteen years in the Suzuki–Miyaura

coupling reaction by heterogeneous palladium catalysis in water as the sole solvent. The discussion

focuses on the heterogenization of the palladium catalyst, efficiency and reusability of the

heterogeneous catalysts as well as on the reaction conditions from a sustainable chemistry point of view.

Introduction

For the last few decades, palladium remains the most usefultransition metal catalyst in the array of transformations inorganic synthesis, in particular for carbon–carbon bondformations.1 The unique nature of the palladium catalyst forselective reactions, easy tuning of the catalyst reactivity andselectivity by ligands or additives, and the high turnovernumbers (TONs) and turnover frequencies (TOFs) usingextremely small amounts of palladium (ppm or ppb levels)under milder conditions, are the main reasons for attractingresearchers, and as a result of these facts a number of palladiumcatalysts are commercially available2 and employed in manyareas, including natural product syntheses.3 Among thedifferent types of palladium-catalyzed reactions, the Suzuki–Miyaura reaction, which is the reaction between aryl halidesand arylboronic acids, represents possibly the most importantand widely used one.4

Suzuki–Miyaura reaction

The Suzuki–Miyaura reaction is characterised by the cross-coupling of two aryl subunits, one from an aryl boronic acid orits derivative and the other from an organohalide or -triate, togive a biaryl motif.1,5 The relative reactivity order is as follows: R–I> R-OTf > R–Br [ R–Cl. This reaction has become one of themost adaptable methods for the expansion of the carbonframework in organic molecules since its discovery in 1979.6

Amongst its wide applicability, the Suzuki–Miyaura reaction isparticularly useful as a way of assembling conjugated diene andhigher polyene systems of high stereoisomeric purity, as well asbiaryl and related systems. Incredible progress has beenmade inthe development of Suzuki–Miyaura coupling reactions of

lyani, Nadia, West Bengal, India. E-mail:

2582 8282; Tel: +91 94 7419 7728

lyani, Nadia, West Bengal, India. E-mail:

8282; Tel: +91 33 2582 8750

yani, Nadia, West Bengal, India

hemistry 2015

unactivated alkyl halides, enabling C(sp2)–C(sp3) and evenC(sp3)–C(sp3) bond-forming processes.1h,i,7 The non-toxicity andsimplicity related to the preparation of organoboron compounds(e.g. aryl, vinyl, alkyl),5b,c their relative stability to air and water,combined with relatively mild reaction conditions as well as theformation of nontoxic by-products, makes the Suzuki–Miyaurareaction an importantmethod for enlarging the carbon skeleton.

The general and widely accepted mechanism of the Suzuki–Miyaura reaction is depicted in Fig. 1. The rst step is theoxidative addition of palladium 1 to halide 2 to form the orga-nopalladium species 3. Reaction of the organopalladiumspecies with a base gives intermediate 4, which via trans-metalation with boronate complex 6 forms the organo-palladium species 8. Reductive elimination of the desiredproduct 9 restores the original palladium catalyst 1.

Water – a green reaction medium

From the academic as well as industrial viewpoints, alternativereaction media are of considerable concern at present for

Fig. 1 Schematic representation of the general mechanism of theSuzuki–Miyaura coupling reaction.

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making this palladium catalyzed cross-coupling process“greener” by minimizing the use of organic solvents.8 Water isthe obvious foremost choice in conjunction with cost, envi-ronmental benets, and safety. The use of water in Pd-catalyzedcross coupling reactions dates back to the early development ofthe Suzuki–Miyaura coupling9 with the rst example beingreported by Calabrese and co-workers in 1990.10 Since then, alarge number of water-soluble Pd catalysts bearing hydrophilicligands have been reported, and several reviews have beendevoted to this subject.11 Attempts have been made for synthe-sizing water-soluble catalysts or water-soluble ligands,12 addingsurfactants or phase-transfer agents,13 using organic co-solventsor inorganic salts as a promoter,14 and utilizing microwaveheating or ultrasonic irradiation.15 The use of water as reactionmedium would be practically green and welcoming only whenany traces of organics or metals can be fully removed from thewater used for the reaction.16 For metal catalyzing reactions inwhich the catalysts are to some extent water soluble, heteroge-neous catalysis could solve this issue using a water insolublesupport or catalyst which can easily be removed from themedium by ltration. Moreover, for large scale processes,organic products can be separated by simple decantation.

Scheme 1 Use of NaB(Ph)4 as the phenylboronic acid substitute.

Scope and limitations ofheterogeneous catalysis

For the synthesis of symmetrical and nonsymmetrical biarylsthe palladium-catalyzed carbon–carbon coupling reactionremains an important method, and a broad variety of homo-geneous catalytic systems have been developed to achieve thistransformation,17 mainly because homogeneous catalystsdisplay high activity and are better dened and understood.Although homogeneous catalysts have many advantages, thecomplications regarding the separation and recovery of thecatalyst, and product contamination with traces of heavymetals, could not be ignored, which limit their applications inchemical and pharmaceutical industries,18 and become an issueof great economic and environmental concern especially forexpensive and/or toxic heavy metal complexes.19 These limita-tions of homogeneous catalysis have resulted in the progress ofnew strategies for transition-metal catalysis which facilitatecatalyst recovery and recycling.20 Recently, many recoverable,supported palladium catalysts have been reported to catalyzeSuzuki–Miyaura coupling reactions such as polymers, bioma-terials, porous silica, carbon nanotubes, polyurea, naturalphosphates etc.21 However, some supported catalysts which areknown as heterogeneous catalysts, oen resulted in a signi-cant loss of catalytic activity when reused and leaching oftransitionmetal during the reaction,22 and the nature of the truecatalyst is still unclear.

The problem of distinguishing homogeneous from hetero-geneous catalysis is an important question that arises more andmore oen, in particular when heterogeneous systems aredeveloped. Heterogeneous catalytic systems may partly dissolveto yield a homogeneous component whichmight be muchmorereactive than the parent metal surface. For this reason, in some

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cross-coupling reactions especially with facile substrates cata-lyzed by trace amounts of metal of various origins, checking isnecessary for the genuine source of catalytic metal. Carefulkinetic studies, ltration tests, selective poisons for catalysts insolid or soluble systems, and Rebek–Collman 3-phase tests arevery helpful and informative to solve the question ofheterogeneity.

Aim of the review

The aim of this review is to provide an overview of heteroge-neous palladium chemistry for the Suzuki–Miyaura cross-coupling reaction in water as the sole reaction medium orsolvent. Since water is genuinely useful for green chemistry as asolvent itself, only protocols carried out in water are covered.One review article23 in this regard by Felpin and co-authors isworth mentioning. The reactions which are not mentioned inthat article and the newer reports (up to September, 2014),including all the heterogeneous systems already mentioned byFelpin, are comprised in this review article. The reports con-sisting of semi-heterogeneous or quasi heterogeneous catalysts,reactions carried out by soluble supports and examples fromthe patent literature are discarded in this review.

Supports are mainly divided into three categories, viz. inor-ganic, organic and hybrid of inorganic–organic materials, andthe discussion is again subdivided according to necessity andfor lucidness.

Inorganic supports

Palladium supported on carbon. Bumagin and Bykov havereported the cross-coupling of water-soluble 3-bromobenzoicacid with tetraphenylborate in neat water using Pd(0)/C(Scheme 1).24 This report is the rst example of a Pd/C-catalyzed Suzuki–Miyaura reaction in neat water.

The coupling between iodophenols and boronic acids atroom temperature (Scheme 2) could be performed using K2CO3

as the base with a lower loading of Pd/C (0.3 mol%).25 Theobtained yields were excellent and fairly independent of thenature of the boronic acid. The reactivity order decreased fromiodophenol to bromophenol, and the successful reactionrequired higher temperatures. Aer completion of the reaction,the Pd/C catalyst was recovered by simple ltration and reusedve times with only a slight decrease in activity.

For non-water-soluble aryl halides, a number of reports usesurfactants as additives to increase the solubility. Arcadi and co-workers used cetyltrimethylammonium bromide (CTAB) for thispurpose, which was found to be quite effective when combinedwith K2CO3 with a catalyst loading (Pd/C) of 5 mol%

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Scheme 2 Cross-coupling of iodophenols with boronic acids.

Scheme 3 Suzuki–Miyaura reactions using CTAB as a surfactant.

Fig. 2 Proposed mechanism of the Pd/C-catalysed cross-coupling ofaryl bromide with sodium tetraarylborate by Xu and co-workers.

Scheme 5 Approach by Kohler and co-workers.

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(Scheme 3).26 Recycling of the ltered and used catalyst sufferedfrom gradually diminished activity which raises the question oftrue heterogeneity.

Xu and co-workers27 (Scheme 4) established that water-soluble bromoarenes react efficiently with sodium tetraphe-nylborate in reuxing water even in the presence of 0.0025mol% of Pd/C when the reaction time was prolonged from 1 to 7h. Comparison of the inorganic bases utilized showed thesuitability of sodium bases over potassium ones. The reusabilityof the catalyst showed capability over ve cycles but withgradual loss of reactivity (Fig. 2).

Coupling of aryl chlorides with aryl boronic acids usingligandless Pd/C in water has been described by Kohler andLysen (Scheme 5).28 All reactions were performed under anambient atmosphere to reduce homocoupling. Activated arylchlorides reacted at lower palladium concentrations (0.2–0.5mol%) while deactivated chloroarenes required higher catalystconcentrations (2.0 mol%) and longer reaction times (sixhours). Addition of TBAB was found to be essential, and NaOHwas found to be the superior base among the several basestested. Aryl iodides and bromides could also be completelyconverted to the corresponding biaryls by a small variation ofthe reaction conditions. Recovery of the catalyst was performedby simple ltration through celite or by centrifugation. Not onlyboronic acids but also boronate esters and potassium tri-uoroborate salts were effective under the reaction conditionsdeveloped by these authors. The reactivation using iodine as theoxidizing agent [Pd(0) to Pd(II)] was necessary to improve therecycling ability of the catalyst, showing consistent activity overthree cycles.

Scheme 4 Suzuki–Miyaura reactions by Xu and co-workers.


The benecial effect of microwave heating was explored byFreundlich and Landis (Scheme 6)29 for the coupling of boronicacids with bromophenols. A variety of boronic acids werecoupled at 120 �C in aqueous potassium hydroxide for shortreaction times (15 min). The reactions remained unsuccessfulwith the chloro substituent. The coupling of substituted bro-mophenol with potassium phenyl triuoroborate salt was lessefficient than with phenyl boronic acid under the optimizedreaction conditions.

Arvela and Leadbeater reported a combined TBAB andmicrowave activation procedure for the cross-coupling of arylchlorides with boronic acids (Scheme 7).30 For substratesbearing electron-withdrawing groups, the effects of

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Scheme 6 Suzuki–Miyaura reactions using microwave irradiation byFreundlich and Landis.

Scheme 7 Catalytic system developed by Leadbeater et al.

Fig. 3 Schematic representation for the preparation of MWCNT/Pd–DMAP NP composites.

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simultaneous cooling on the product yield were not signicant,attributed to the fact that the coupling reaction is faster thanthe decomposition of the chloride substrate, but with substratesbearing electron-neutral or electron-donating substituents,simultaneous cooling signicantly increased the product yield.Very fast reaction rates were observed in only ten minutes andthe method was found to be efficient for aryl chlorides con-taining electron-withdrawing groups.

The coupling of bromoarenes with tetraphenylborate(Scheme 8) has been described by Bai using a similar catalyticsystem.31 Excellent yields were obtained in less than twentyminutes at 120 �C in the presence of K2CO3 as the base with acomparatively higher palladium loading (5 mol% Pd). Thecatalyst showed excellent recycling ability over more than vecycles.

Modied multi-walled carbon nanotubes have beenproposed as a support for palladium nanoparticles for the cross-coupling in neat water.32 4-Dimethylaminopyridine (DMAP)stabilized palladium nanoparticles were prepared by mixingsolutions of Na2PdCl4 and DMAP followed by the reduction withNaBH4. Thiol-modied multi-walled carbon nanotubes(MWCNTs), prepared by a carbon arc discharge method, werefunctionalized via an amide coupling reaction followed by

Scheme 8 Coupling of bromoarenes with tetraphenylborate by Bai.

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sequential treatment with HNO3, KMnO4, HClO4, citric acid,DMAP and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimidehydrochloride (EDAC) and 2-mercaptoethylamine hydrochlo-ride (Fig. 3). Multi-walled carbon nanotube/DMAP-stabilized Pdnanoparticle composites (MWCNT/Pd–DMAP NP composites,catalyst 14) were prepared by the addition of a known amount ofa DMAP-stabilized palladium nanoparticle dispersion to thiol-modied multi-walled carbon nanotubes under sonication.The 4-iodo-substituent gave 87% conversion within 10 min with0.004 mol% of catalyst 14, while the bromo-substituent gave53% conversion and the chloro-analogue gave 25% conversionwith 0.024 mol% of catalyst 14 when reuxing for 6 h. Thecatalyst was recovered by ltration through a polycarbonatelter, and experimental studies showed good recyclability oversix runs with leaching of the Pd species below the detectionlimit of AAS (Scheme 9).

Graphene modied with palladium nanoparticles byreducing palladium acetate [Pd(OAc)2] in the presence ofsodium dodecyl sulfate (SDS) was reported by Zhang and co-workers (Scheme 10)33 (SDS is used as both surfactant and thereducing agent). The palladium nanoparticle–graphene hybrids(Pd–graphene hybrids, catalyst 15) were characterized by spec-trometric methods, and HRTEM showed that the mean size ofthe Pd nanoparticles dispersed on the graphene sheets is about4 nm. Catalyst 15 acted as an efficient catalyst for the Suzuki–Miyaura reaction under aqueous and aerobic conditions, withthe reaction reaching completion within 5 min. Bromobenzeneand allyl iodides were also employed in this coupling reaction

Scheme 9 Suzuki–Miyaura reaction of interest, using the MWCNT/Pd–DMAP NP composites, between phenylboronic acid and 4-hal-obenzoic acids.


Fig. 4 Preparation of the nanocrystalline MgO-stabilised palladiumcatalyst 17.

Scheme 10 Work by Zhang and co-workers.

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but produced poor yields. The catalysts were recovered bysimple centrifugation and reused successfully for ten consecu-tive runs.

Palladium supported on metal oxides. A sepiolite-supportedpalladium(II) catalyst (catalyst 16) has been prepared by mixingsepiolite with an aqueous solution of [Pd(NH3)4]Cl2 at 298 K for48 h, followed by centrifuging and washing with deionizedwater, and subsequent drying in vacuo at the same temperature.The prepared catalyst was successfully used for the cross-coupling of 4-bromophenol with phenylboronic acid or tetra-phenylborate at room temperature (Scheme 11)34 with a palla-dium loading of 0.1 mol%. The Pd(II)/sepiolite catalyst could bereused three times without any apparent deactivation.Successful reaction could be achieved by decreasing the catalystloading to 0.0001 mol% but with higher temperatures.

Highly basic nanocrystalline magnesium oxide (NAP-MgO)as a palladium nanoparticle support has been exploited forthe Suzuki–Miyaura coupling reaction (Scheme 12).35 Fig. 4shows the schematic presentation for the preparation of Pd–NAP-MgO (catalyst 17). The cross-couplings of iodo- and bro-moarenes with arylboronic acids were efficiently carried out inwater, but the reactions involving chloroarenes were performedin DMA with catalyst 17. The cross-couplings were completed in

Scheme 11 Example of a cross-coupling with an ultra low catalystloading.

Scheme 12 Examples of cross couplings with NAP-MgO–Pd(0) as thecatalyst.


only 5 to 6 h at room temperature at a quite low loading (0.5mol%); an even lower loading as low as 0.01 mol% is alsoeffective with longer reaction times (40 h). It is supposed thatthe high activity of the catalyst is due to the nanostructuredMgO material that possesses a high surface area (�600 m2 g�1)and a strong basicity. The catalyst was recyclable for all reac-tions up to ve cycles with almost consistent activity.

Artok and co-workers prepared a highly active catalyst(catalyst 18) by loading NaY zeolite with Pd(NH3)4Cl2 (Scheme13)36 (NaY zeolite (SiO2/Al2O3 molar ratio: 5.1)) by ion exchange,which gave good yields for the corresponding biphenylcompounds by cross-coupling soluble and insoluble arylbromides with benzeneboronic acid with catalyst loadings of(0.01–0.001) mol% in water. Electron-rich bromoarenes werefound to be much less reactive and required the use of surfac-tants such as TBAB or CTAB for better results. A possibleinstability of the catalytic system under localized heating wasestablished by performing the reaction under microwaveheating.

The Pd/ZrO2 nanocatalyst formed by electrochemicalimpregnation of nanostructured tetragonal ZrO2 with palla-dium nanoparticles (PdNPs/ZrO2, catalyst 19) was demon-strated to be a very efficient catalyst in Suzuki–Miyaurareactions of aryl halides in water, by Nicola Cioffi and co-workers (Scheme 14).37 The catalyst efficiency was attributedto the stabilization of Pd nanophases provided by tetra alkylammonium hydroxide, which behaved both as a base as well asa PTC (phase transfer catalyst) agent. The Suzuki–Miyauracross-coupling reactions were carried out in water at 90 �C using

Scheme 13 Work by Artok and co-workers.

Scheme 14 Suzuki–Miyaura reactions catalyzed by Pd-NPs/ZrO2

(catalyst 19) in water.

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Scheme 16 Suzuki–Miyaura reactions using the Pd–HAP catalyst 21.

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aryl bromides and iodides as substrates and phenyl boronicacids. The supported catalyst could be recycled up to ten timeswithout any appreciable loss of activity which was supported byan average yield of 83% for a number of aryl bromides, exceptfor the less reactive electron-rich 4-bromoanisole.

Palladium supported on hydroxyapatite. Hydroxyapatite-supported palladium(0) (Pd/HAP, catalyst 20) was prepared bystirring a mixture of hydroxyapatite and Pd(OAc)2 in ethanolfollowed by the dropwise addition of hydrazine hydrate (80%)under continuous stirring, and conditioning of the catalyst byreuxing for 6 h in each ethanol, toluene and acetonitrile. Theconditioned catalyst was quite stable and could be used forseveral days. The TEM micrograph showed an average palla-dium particle diameter of about 20 nm on hydroxyapatite.Suzuki–Miyaura cross-couplings of bromoarenes with arylbor-onic acids were carried out with this catalyst in the presence ofTBAB as a surfactant and K2CO3 as the base (Scheme 15).38 Pauland co-workers obtained excellent yields for biphenylcompounds from facile substrates using the prepared catalyst20 (0.33 mol% Pd/HAP). The stability of the hydroxyapatitesupported palladium catalyst was demonstrated by the recy-cling ability studied for the coupling of 4-bromoacetophenonewith benzeneboronic acid over ve cycles with no apparentdeactivation of the reused catalyst.

Two types of these supported palladium catalyst, one byimmobilization of [Pd(COD)Cl2] (COD ¼ 1,5-cyclooctadiene) onhydroxyapatite (catalyst 21) and another catalyst by subsequentreduction of the previous catalyst with sodium borohydride(catalyst 22), were prepared (Fig. 5) for the Suzuki–Miyauracoupling reaction in water.39 The catalyst with Pd2+, was foundto be almost ve times more active than the reduced catalystunder similar reaction conditions. The best catalytic activitieswere observed in the presence of potassium carbonate as thebase and tetrabutylammonium bromide as a promoter usingthe non-reduced catalyst and water as the solvent under aerobicconditions (Scheme 16). This catalyst system has been tested fordifferent electronically neutral, electron-rich, electron-poor and

Scheme 15 Suzuki–Miyaura reactions using Pd/HAP as the catalyst.

Fig. 5 Synthetic outline for the synthesis of catalysts 21 and 21a.

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sterically hindered aryl boronic acids, and several different arylhalides including aryl chlorides. More than one thousandturnovers and high selectivities toward the hetero-coupledproducts have been observed in most cases. A negligible dropin activity was observed over ten cycles.

Palladium supported on mesoporous silica. Ordered meso-porous MCM-41 material has been used as a suitable supportfor uniformly sized palladium nanoparticles (Fig. 6, catalyst 22)by Sayari and Das, and has been explored in Suzuki–Miyaura

Fig. 6 Schematic outline of the synthesis of catalyst 22 from pore-expanded MCM-41 and supported monodispersed Pd nanoparticles.


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reactions (Scheme 17).40 Although the reactions were carried outin water, the recycling experiments and mechanistic consider-ations were evaluated in EtOH solvent. A truly heterogeneousmechanism was proven in EtOH.

Palladium supported on hydrotalcite. Ruiz and co-workersopted for the use of a Mg/Al hydrotalcite-supported palladiu-m(II) (catalyst 23) for a single example of the Suzuki–Miyauracross-coupling reaction involving bromobenzene and phenyl-boronic acid at room temperature (Scheme 18).41 The supportedcatalyst was prepared by mixing appropriate amounts of palla-dium acetate, pyridine and hydrotalcite at 80 �C for 1 h, aerwhich the solid was ltered off and washed with toluene. Thecatalyst thus obtained was named HT-Pd(AcO2)Py2 (catalyst 23).Only 52% conversion was obtained. Optimization studiesshowed that the use of sodium dodecyl sulphate as a surfactantwas crucial for an acceptable conversion among those tested forthis purpose, which included anionic, cationic and neutralsurfactants.

Palladium supported on porous glass. The catalytic activityof Pd supported on porous glass (catalyst 24) in the Suzuki–Miyaura reaction was studied by Ondruschka and co-workersunder aerobic conditions (Scheme 19).42 The catalyst wasprepared by dissolving Pd(OAc)2 (20 mg, 0.09 mmol) indichloromethane containing the porous glass support (1 g;TRISOPOR) followed by removal of the solvent in vacuo andcalcination of the catalyst precursor for 2 h at 300 �C in a mufflefurnace to obtain the catalyst with a Pd loading of 1 wt%. Forvarying catalyst loadings, different amounts of Pd were dis-solved in dichloromethane (e.g. 10 mg for 0.5 wt% and 5 mg for0.25 wt%). The reactions were carried out in water under

Scheme 17 Examples of Pd/MCM-41-catalyzed Suzuki–Miyaurareaction.

Scheme 18 Suzuki–Miyaura cross-coupling by Ruiz and co-workers.

Scheme 19 Suzuki–Miyaura reactions by Ondruschka and co-workers.


microwave irradiation. The effects of the catalyst preparationprocess (calcination time and temperature), as well as the base,substrate, and boron compound used in the coupling reactionwere investigated in relation to the reusability of the catalyst.Among the bases used to recalcinate the catalyst, HNEt2 andNEt3 were successful. Substitutions in the ortho, meta, or parapositions of the aryl halide showed a negligible inuence on theyield of the desired coupling product. Except for the phenol-typesubstrates, all other substrates required the addition of thephase-transfer catalyst tetra-n-butylammonium bromide (TBAB)to enhance their solubility in the solvent (deionized water). Theclassical order of reactivity of aryl iodides > bromides > chlo-rides was conrmed.

Palladium supported on natural phosphate. F. Aziz and co-workers have reported a convenient method for the prepara-tion of a recyclable and heterogeneous natural phosphate-supported palladium catalyst (catalyst 25) by treatment ofnatural phosphate (NP) with PdCl2(PhCN)2 in acetone and itsapplication for the synthesis of biaryls via Suzuki–Miyauracouplings using water as solvent (Scheme 20).43 Aryl bromidesand heteroaryl bromides efficiently reacted with arylboronicacids providing a useful way for the synthesis of aryl-substitutednitrogen heterocycles. However, the coupling reaction of 2-bromothiophene and phenylboronic acid did not occur evenwith increased catalyst loading or reaction time. A considerablesteric effect was observed when the reaction was carried outwith sterically hindered 2-bromo-m-xylene and phenylboronicacid which led to the desired product in poor yields. Underidentical conditions, the aryl chlorides bearing electron-withdrawing groups reacted in good yields, but no conversionwas obtained in the coupling of aryl chlorides bearing electron-donating groups. Catalyst 25 was recovered by simple ltrationand the product yields for the 2nd and 3rd cycle were nearly thesame (93%) but reduced during the 4th cycle (88%). No leachingof the catalyst to the organic layer was reported.

Organic support

Palladium supported on polystyrene. Incorporation ofnanosized Pd particles into a hyper-crosslinked polystyrenematrix by reduction was developed, and used for the Suzuki–Miyaura coupling reaction in water.44 Catalyst 26 was preparedby mixing an acidic solution of PdCl2 (PdCl2 (83 mg), 2 ml ofH2O and 0.2 ml of concentrated HCl)) with a pre-washed anddried Macronet MN100 resin (950 mg of the MN100 resin in 10ml ask). The resin was allowed to swell for 10 min in the

Scheme 20 Heterogeneous Suzuki–Miyaura couplings of arylbromides and aryl chlorides with phenylboronic acid using PdNP.

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mixture followed by the addition of sodium formate andsodium hydroxide. The resulting mixture was then heated for 10min at 80 �C. The obtained grey beads were washed with waterandMeOH, and dried in vacuo (1 mmHg) under heating (90 �C).The average palladium content in the resin was found to be3.75%. Electron microscopy analysis of the milled Pd catalystshowed an average size of the Pd nanoparticles almost equal to12.5 nm. The catalyst was employed in the coupling of arylbromides and chlorides (with greater amounts of the catalyst,viz. double of the amount used for the bromo substrates) withphenylboronic acid, with water being the preferred solvent(Scheme 21). The reuse of the polymer-supported Pd resin wasperformed analogously to the Suzuki–Miyaura procedure.

Palladium nanoparticles stabilized onto linear polystyrene(catalyst 27) by thermal decomposition of Pd(OAc)2 was examinedin the Suzuki–Miyaura reaction in 1.5 M aqueous KOH solution(Scheme 22).45 A fairly uniform particle size of 2.3 � 0.3 nm wasobtained and ICP-AES revealed that the catalyst contained anaverage of 2.5 mmol g�1 of Pd. The immobilization degree ofpalladiumwas dependent on themolecular weight of polystyrene,while the size of the nanoparticles was not. The cross-couplingreaction of bromobenzene with p-methylphenylboronic acidproceeded efficiently to give 4-methylbiphenyl in 99% yield. Bothelectron-rich and electron-decient aryl bromides were reactiveunder these reaction conditions, affording the desired couplingproducts in high yields. The catalyst could be recovered by simpleltration. The average yield of 4-methylbiphenyl from the 1st

through to the 10th recovered catalysts was 99%. No leaching ofpalladium into the solution during the reaction was observed byICP-AES. Worth mentioning is that the reaction proceeded wellwith aryl chloride and the catalyst could even be recycled.

Scheme 21 Suzuki–Miyaura reactions of phenylboronic acid witharylbromides and an arylchloride.

Scheme 22 Suzuki–Miyaura reactions using polystyrene stabilized Pd.

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In a similar study, linear polystyrene-stabilized PdO nano-particles (PS–PdONPs, catalyst 28) were prepared in water bythermal decomposition of Pd(OAc)2 in the presence of poly-styrene, and the Pd nanoparticles (PS–PdNPs) were alsoprepared using NaBH4 and phenylboronic acid as reductants.46

The catalytic activity of PS–PdONPs was found slightly higherthan that of PS–PdNPs for the Suzuki–Miyaura coupling reac-tion in water probably due to the presence of oxygen and thesize effect. The TEM image of catalyst 28 showed a uniformparticle size of 2.3 � 0.3 nm. Under optimized conditions, theSuzuki–Miyaura coupling reaction of bromobenzene with 4-methylphenylboronic acid in 1.5 M KOH aqueous solution at80 �C for 1 h proceeded efficiently to give 4-methylbiphenyl in99% yield (Scheme 23). Both electron-rich and electron-decient aryl bromides were reactive, affording the desiredcoupling products in high yields. However, the reaction ofchlorobenzene gave a lower yield. The catalyst was recovered byltration and was recycled for 10 times without any loss ofactivity. No leaching of palladium into the reaction occurredduring the reaction, as conrmed by ICP-AES.

Polypyrrole–palladium nanocomposite-coated cross-linkedpolystyrene latex particles (PS/PPy–Pd, catalyst 29) have beenapplied with an excellent catalytic activity to the Suzuki–Miyaura coupling reaction in water (Scheme 24).47 The catalystwas prepared by adding an aqueous solution of PdCl2 and NaClto a premixed aqueous dispersion of pyrrole and polystyrene.The polymerization was allowed to proceed for 7 days at 200rpm. The PS/PPy–Pd particles were subsequently puried byrepeated centrifugation–redispersion cycles followed by freeze-drying overnight. The potency of the PS/PPy–Pd particles as acatalyst with lowmetal loading (0.03 mol% of Pd) was examinedin the Suzuki–Miyaura coupling reaction of various aryl halideswith arylboronic acids in 1.5 mol L�1 aqueous potassiumcarbonate solution as test reactions. Steric hindrance did notmatter as was observed from the high yield of 2,4-o-

Scheme 23 Suzuki–Miyaura reactions using PS–PdONPS.

Scheme 24 Suzuki–Miyaura reactions using PS/PPy–Pd.


Scheme 26 Schematic diagram for the synthesis of the resin-sup-ported triarylphosphine–palladium complex.

Scheme 27 Suzuki–Miyaura reactions by Uozumi et al.

Scheme 28 Synthetic approach for catalyst 35.

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dimethylbiphenyl from the coupling reaction of 2-bromoto-luene with 4-methylphenylboronic acid. ICP-AES analysesconrmed that both the aqueous phase and the organic phasecontained barely detectable levels of palladium and that the Pdloading in the particles did not change even aer the h run,which indicated that no/little Pd nanoparticles detached fromthe PS/PPy–Pd particles.

Palladium immobilized by polymer-supported phosphineligands. Wang and co-workers reported an exceedinglystraightforward and competent catalytic system for the couplingof aryl bromides with sodium tetraphenylborate in water underfocused microwave conditions.48 The coupling reaction wascompleted within 15 to 20 min under the applied reactionconditions involving 1 mol% of the catalyst. The heterogeneouspalladium catalyst, consisting of a complex of PdCl2 bonded to apolystyrene–diphenylphosphine ligand, is fairly stable for yearsat room temperature under aerobic conditions. Potassiumcarbonate was the choice as the base, and TBAB as the phase-transfer catalyst. Various aryl and heteroaryl bromides weresuccessfully coupled to NaBPh4 under microwave heating(Scheme 25). The heterogeneous palladium catalyst could beeasily recovered by ltration and recycled at least ten times withunfailing activity.

An amphiphilic resin-supported triarylphosphine–palla-dium complex bound to a polyethylene glycol–polystyrene gracopolymer (PEG–PS resin) has been described for the cross-coupling of aryl iodides with boronic acids using KOH as thebase.49 The PEG–PS resin-supported palladium–mono-phosphine complex Pd–PEP (32) was readily prepared by treat-ment of the resin-supported phosphine (31) with an excessamount of di(m-chloro)bis(h-allyl)dipalladium(II) ([PdCl(h3-C3H5)]2) (Pd/P > 1/1) followed by the removal of not immobilized[PdCl-(h3-C3H5)]2 by washing with chloroform (Scheme 26).

This catalytic system was found to be more active than thecomparable usual homogeneous palladium–phosphinecomplexes under the same reaction conditions (Scheme 27). Noexamples with chloroarenes were reported but good yields wereobtained with aryl iodides and bromides under mild conditions(25 �C).

In another report, another resin-supported palladium cata-lyst (PS–PEG-adppp) for the Suzuki–Miyaura coupling reactionin water has been described by Uozumi and co-workers bymerely changing the ligand.50 The catalyst 35 was prepared bytreatment of PSPEG–NH2 with diphenylphosphinomethanol(Scheme 28) in toluene–MeOH at 25 �C for 3 h to give PS–PEG-adppp with a quantitative loading value of 0.32 mmol g�1. The

Scheme 25 Suzuki–Miyaura reactions by Wang et al.


palladium complex of the bisphosphine ligand PS–PEG-adpppwas prepared by mixing [PdCl(h3-C3H5)]2 in toluene at 25 �Cfor 15 min to give [PS–PEG-adppp-Pd-(h3-C3H5)]Cl (catalyst 35)in a quantitative yield. Altogether, ninety six combinations ofeight aromatic halides and twelve different boronic acids werereported from which a clear idea about the extremely efficientand stable heterogeneous catalyst 35 could be obtained. Thereactions were carried out in aqueous K2CO3 at 85 �C (Scheme29). Catalyst 35 can be recovered by simple ltration and reusedwithout any loss of activity.

Scheme 29 Suzuki–Miyaura reactions by Uozumi et al.

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Scheme 32 Suzuki–Miyaura reactions with the network catalyst 40 byIkegami et al.

RSC Advances Review

A similar strategy was followed by the same author tointroduce the asymmetric Suzuki–Miyaura cross-couplingreaction,51 and achieved by anchoring chiral imidazoindole-phosphine to an amphiphilic polystyrene–polyethylene glycolcopolymer (PS–PEG) resin (Scheme 30). Excellent yields withvery good enantioselectivities (88–99% ee) were obtained atroom temperature in water with the aid of a large excess of TBAF(10 equiv.) and boronic acid (5 equiv.). A large amount (10mol%) of the palladium catalyst was required, but it could bereused aer simple ltration with consistent results.

Ikegami and co-workers designed a self-assembled complexof palladium and a non-crosslinked amphiphilic polymer sup-ported through phosphines (Scheme 31).52 The catalyst supportwas prepared by random polymerization of 4-diphenylstyryl-phosphine (37) with 12 equiv. of N-isopropylacrylamide (38) inthe presence of 4 mol% AIBN, which gave 39 in 89% yield.Catalyst 40 was prepared by self-assembly of 39 and (NH4)2-PdCl4. The catalytic activity of this palladium-network catalyst40 has been investigated for Suzuki–Miyaura reaction inreuxing water where Na2CO3 was the ultimate choice as a base(Scheme 32). The protocol allowed the reaction of aryl bromidesand aryl iodides as substrates, but aryl triates remainedunaffected.

Only trace amounts of the highly active palladium-networkcomplex (50–500 ppm) were required for a successful reaction.The catalyst was found to achieve the coupling of unusualalkenyl halides and alkenylboronic acids at a low catalystconcentration (500 ppm). The recyclability of catalyst 40 was

Scheme 30 Selected examples of the asymmetric Suzuki–Miyaurareaction by Uozumi et al.

Scheme 31 Preparation of the self-assembled catalyst 40.

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examined for the preparation of biphenyl for up to tenconsecutive cycles with a consistent activity.

Uozumi and co-workers further developed the idea of a novelpalladium complex embedded in a three-dimensional networkcomplex (Scheme 33).53 A novel 3D palladium-network complexcatalyst 45 was obtained by self-assembly of PdCl2 and C3-trisphosphine 44, which was prepared from the commerciallyavailable 2,4,6-tris-(bromomethyl)mesitylene (41) in four steps.

Scheme 33 Preparation of the palladium network complex catalyst45.


Scheme 36 Selected examples using the catalytic system developedby Kirchning et al.

Review RSC Advances

Catalyst 45 showed a high catalytic efficiency at a low loading of0.05 mol% palladium, and twenty seven examples of theSuzuki–Miyaura reaction in reuxing water with a variety ofbromo- and iodoarenes were reported (Scheme 34). Finally, thecatalytic complex displayed reusability properties over foursuccessive cycles.

Polymer-supported oxime-based ligands. The group ofKirschning reported in 2004 an insoluble pyridine–aldoximepalladium catalyst active under microwave heating in Suzuki–Miyaura reactions in water (Scheme 35).54 Although the exactstructure of the catalyst was not elucidated, the absence ofpalladium–carbon bonds excluded any palladacycle-type struc-ture. Experiments showed that under microwave activationwater as the solvent turned out to be superior to toluene underthe catalytic conditions. In the subsequent studies, in order toimprove its lifetime the authors covered catalyst 46 (1 mol%)with an Irori Kan™. Under optimized conditions, catalyst 46with K2CO3 as a base and TBAB as a phase-transfer agentshowed good activity for the coupling of various substitutedboronic acids with p-chloro-, p-bromo-, p-iodo- and p-tri-uoromethylsulfonylacetophenone (Scheme 35). Catalyst 46was reused for the coupling of 4-bromoacetophenone withbenzeneboronic acid, and 93% conversion was observed aerthe 14th run.

Following their earlier studies, Kirschning et al. consideredthe aqueous Suzuki–Miyaura reactions of another closelyrelated catalytic system prepared from a 2-pyridine aldoxime-based Pd(II) complex covalently anchored onto a glass–poly-mer composite material (catalyst 47).55 Aryl and heteroarylbromides were efficiently coupled with boronic acids at fairlylow palladium loadings (0.7 mol%) with the aid of TBAB as thesurfactant and KOH as the base, under both thermal (100 �C) ormicrowave heating (160 �C) conditions (Scheme 36). For chlor-oarenes, only the coupling of 4-chloroacetophenone has beenreported. The catalyst could be reused at least seven times withconsistent activity regardless of the source of heating.

Scheme 34 Suzuki–Miyaura reactions with the network complex 45by Uozumi et al.

Scheme 35 Selected examples of cross-couplings using the catalyst46.


In another report following the previous one, Kirschning andco-workers explored an alternative approach for immobilizationof an oxime carbapalladacycle (48) onto polyvinylpyridine as thesupport (Scheme 37).56 The polymeric phase was prepared froma heated solution (70 �C) of the monomers vinylpyridine anddivinylbenzene with AIBN in a nonpolar solvent. While arylchlorides were more efficiently coupled in water, aryl bromideswere preferentially reacted with boronic acids in toluene underthese catalytic conditions (Scheme 38). The protocol is associ-ated with the use of TBAB (0.5 equiv.) andmicrowave activation.The protocol was equally applicable as a thermal, microwave orcontinuous ow method.

Taking advantage of a rich experience in oxime carbapalla-dacycle catalysts for cross-coupling reactions in organic andaqueous media,57 Najera and co-workers prepared the palla-dated Kaiser oxime resin catalyst 50 as an active precatalyst fordifferent types of the Suzuki–Miyaura reaction (Scheme 39).58

Scheme 37 Immobilization of the oxime carbapalladacycle ontopolyvinylpyridine.

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Scheme 38 Cross-coupling reactions developed by Kirschning et al.

Scheme 39 Selected examples from the work of Najera and co-workers.

Scheme 40 Synthesis of dipyridylmethylamine-based palladiumcomplex 51.

Scheme 41 Cross-coupling reactions according to the work of Najeraand co-worker.

RSC Advances Review

Several examples were investigated involving the cross-couplingof aryl bromides, and allyl and benzyl chlorides with aryl-, alkyl-and alkenylboronic acids in neat water. Aryl bromides wereefficiently cross-coupled with benzeneboronic acid, but arylchlorides were poorly reactive. Alkylboronic acid, trivinylbor-oxine and trimethylboroxine reacted with aryl bromides in thepresence of TBAB as an additive. Analysis of the solutionshowedmoderate metal leaching which allowed the reuse of therecovered catalyst 50 with a gradually decreased catalyticactivity.

Polymer-supported pyridine ligands. Based on earlierstudies59 on dipyridyl based ligands for palladium complexa-tion, showing good catalytic activity for C–C bond-formingreactions, Najera and co-workers explored a dipyridyl–palla-dium complex anchored to a styrene–maleic anhydride co-polymer (Scheme 40).60 Only three haloarenes were examinedfor the coupling with benzeneboronic acid in the presence ofK2CO3 as a base and the supported palladium catalyst 51(Scheme 41). Although the activated 4-chloroacetophenone is

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reactive, compared to the bromides it requires a much higherpalladium loading (4.5 mol% vs. 0.1 mol%) and TBAB as aphase-transfer agent. Microwave irradiation was found to bedisadvantageous for the reaction. Recycling studies showedgood yields for up to three or four cycles.


Review RSC Advances

Palladium nanoparticles xed in the layer of core–shellpoly(styrene-co-4-vinylpyridine) microspheres (catalyst 52) werefound to be catalytically active for Suzuki–Miyaura cross-couplings in water (Scheme 42).61 The supported catalyst wasprepared by adding an aqueous solution of PdCl2 into thecolloidal dispersion of the core–shell PS-co-P4VP microspheresat room temperature followed by the dropwise addition ofexcess NaBH4 aqueous solution. The resultant colloidaldispersion was puried by dialyzing against water at roomtemperature for 4 days. Transmission electron microscopy(TEM) analyses evidenced that the palladium nanoparticleswere uniformly distributed with an average size of 4.4 nm on thepolyvinylpyridine shell. Optimization studies revealed thathydrophobic reagents were best coupled with Et3N as a base,while hydrophilic substrates preferably required K2CO3. Thecatalytic system was examined for the coupling of benzene-boronic acid with a range of unchallenging bromo- andiodoarenes. Chloroarenes, however, were almost unreactiveunder the same reaction conditions. Recycling studies carriedout for the coupling of 4-bromoacetophenone with benzene-boronic acid showed 99% yield of the targeted biaryl compoundthroughout ve consecutive runs. The average size of thepalladium nanoparticles remained the same during recycling.

A novel heterogeneous transition-metal catalyst comprisinga polymer-supported terpyridine palladium(II) complex (catalyst53) was prepared (Scheme 43) and found to promote theSuzuki–Miyaura reaction in water under aerobic conditions

Scheme 42 Cross-coupling reactions developed by Zhang et al.

Scheme 43 Preparation of the PS–PEG resin bound terpyridinepalladium complex.


with high to excellent yields.62,63 The Suzuki–Miyaura cross-coupling reaction of iodobenzene with phenylboronic acidwas carried out with K2CO3 (2 equiv.) in the presence of poly-meric catalyst 53 (5 mol% Pd) in water to give biphenyl in 93%yield (Scheme 44). A variety of boronic acids and halo areneswith different types of substitution at different positionsshowed almost excellent yields, thereby proving that thesubstrate or reactant structures do not affect the reaction yield.The catalyst was recovered by simple ltration and directlyreused several times without loss of catalytic activity (ICP-AESanalysis (detection limit of Pd: <3 mg L�1) from aqueous ororganic ltrates).63

Polymer-supported salen ligands. A new polystyreneanchored Pd(II) azo complex (catalyst 54) has been synthesized(Scheme 45) and characterized by S. M. Islam et al., which actedas a efficient heterogeneous catalyst in the Suzuki–Miyauracoupling reaction in a water medium.64 Aryl halides werecoupled with phenylboronic acids smoothly to afford the cor-responding cross coupling products in excellent yields (83–100%) under phosphine-free reaction conditions. Varioussubstituted aryl iodides and bromides with deactivated(electron-rich) and activated (electron-poor) groups were effi-ciently converted to the desired products in good to excellentyields. The less reactive chlorobenzene showed moderateconversion. The sensitive heteroaryl halides bearing pyridyl or

Scheme 44 Suzuki–Miyaura coupling reactions using polymericcatalyst 53 in water.

Scheme 45 Synthesis of the polystyrene anchored Pd(II) azo complex54 by S. M. Islam et al.

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RSC Advances Review

sterically hindered moieties reacted to the correspondingproducts in good yields. This polymer-supported Pd(II) catalystcould be easily recovered by simple ltration of the reactionmixture and reused for more than six consecutive trials withouta signicant loss of its catalytic activity.

A set of three new polymer-anchored palladium(II) Schiffbase catalysts have been synthesized (Scheme 46), characterizedand their catalytic activity was investigated in the Suzuki–Miyaura cross-coupling reaction between aryl halides and aryl-boronic acids in the presence of Cs2CO3 as the base.65 Theyshowed excellent catalytic activity in the coupling of arylbromides or aryl iodides with phenylboronic acid under theoptimized reaction conditions in water (Scheme 47). Thepolymer-anchored Pd(II) complexes provided turnover frequen-cies of 29 700 and 58 200 h�1 in the Suzuki–Miyaura couplingreactions of phenylboronic acid with p-bromo acetophenoneand p-iodobenzene, respectively, which are the highest valuesever reported for the Suzuki–Miyaura coupling reaction in wateras the sole solvent. The highest conversion reached was up to45% in the presence of Cs2CO3 within 30 min in water at 100 �C,and longer reaction times did not yield any further conversion

Scheme 46 Polymer-anchored palladium(II) Schiff base catalysts.

Scheme 47 Suzuki–Miyaura coupling reactions using the polymer-anchored palladium(II) Schiff base catalysts.

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for aryl chlorides and phenyl boronic acids. Catalyst 55 main-tained 97% of its initial catalytic activity at the end of the eencycle.

Polymer-supported triazole ligands. The triazole-functionalized polystyrene resin-supported Pd(II) [PS-tazo-Pd(II)] complex (catalyst 58) was prepared by stirring a suspen-sion of PS-tazo in a solution of PdCl2(PhCN)2 in reuxing EtOH(Scheme 48).66 The amount of palladium incorporated into thepolymer was 0.28 mmol g�1 of the heterogenized catalyst. Thecatalyst was air-stable, easily available and found to be an effi-cient catalyst in the palladium-catalyzed Suzuki–MiyauraMiyaura coupling reactions of aryl iodides and bromides(Scheme 49). Under the appropriate reaction conditions, all thereactions gave the desired products in moderate to excellentyields. The supported palladium catalyst was easily separated byltration, and could be reused for several times without asignicant loss of catalytic activity.

Polymer-supported N-heterocyclic carbene ligands. N-Heterocyclic carbenes have strong donor properties, and bindtransition metals more strongly than phosphines. An amphi-philic polymer-supported N-heterocyclic carbene palladiumcomplex (catalyst 61) has been prepared by anchoring ahydrophilic polyethylene glycol chain (PEG-200, PEG-600) to apolystyrene core, covalently bound to an N-heterocyclic carbene(Scheme 50) for Suzuki–Miyaura reactions in water with Cs2CO3

as the base at 50 �C (Scheme 51).67 The methodology could bemainly applied to the cross-coupling of aryl iodides and to alesser extent to aryl bromides. The catalytic system was reusedwith gradually decreasing activity up to ve recycling runs.

Oxime-thiosemicarbazone ligand. A palladium complex, 1-phenyl-1,2-propanedione-2-oxime thiosemicarbazone function-alized polystyrene resin-supported Pd(II) (catalyst 62), was foundto be a highly active catalyst for the Suzuki–Miyaura reaction ofphenylboronic acid with aryl iodides and bromides, givingexcellent yields (Scheme 52).68 The reactions were performed

Scheme 48 Triazole-functionalized polystyrene resin-supportedPd(II) [PS-tazo-Pd(II)] complex (catalyst 58).

Scheme 49 Suzuki–Miyaura reaction of aryl halides with phenyl-boronic acid using PS-tazo-Pd(II) complex 58.


Scheme 51 Coupling reactions with the heterogeneous carbenecomplex catalyst 61 by Lee et al.

Scheme 52 Suzuki–Miyaura coupling reactions using the heteroge-neous catalyst PS-ppdot-Pd(II).

Scheme 53 Synthesis of PIC.

Scheme 50 Immobilization of palladium on the PS–PEG–NHCprecursor resin.

Scheme 54 Selected examples of the Suzuki–Miyaura coupling usingPd/PIC.

Scheme 55 Suzuki–Miyaura coupling reaction using the Pd–PluronicF68 triblock copolymer catalyst 63.

Review RSC Advances

under phosphine-free conditions in an air atmosphere. Thereaction was effectively carried out in the presence of a widevariety of functional groups on the aryl iodides and arylbromides, giving good to excellent conversions to the corre-sponding products. 4-Nitro-iodobenzene and 3-nitro-iodobenzene were found to be the most reactive among thearyl iodides studied. The less reactive bromobenzene showed alower yield. However, the activated aryl bromides, e.g. 4-bro-mobenzonitrile and 4-nitrobromobenzene, gave the corre-sponding products in excellent yields. The palladium catalystwas easily separated by centrifugation aer completion of thereaction, and could be reused for several times, but the productyield decreased slightly over four recycling runs.

Palladium supported on an ionic copolymer. Jun Huang andco-workers69 demonstrated a facile one-step synthetic strategyfor a catalyst system by immerging a porous ionic copolymer


(PIC) (Scheme 53) into a Pd(OAc)2 acetone solution. Acetone wasthen removed by evaporation to give the Pd catalyst Pd(OAc)2/PIC. A high yield for 4-methoxybiphenyl was obtained in waterunder air or argon, and the addition of the phase-transfer agentTBAB enhanced the yield signicantly (Scheme 54). Even with10 ppm (0.001 mol%) loading of Pd, a high yield (95%) wasobtained, which showed the extremely high activity of the Pdcatalyst system. The coupling reaction of electron-poor arylchlorides afforded the corresponding biphenyl compounds inexcellent yields at 120 �C in 10 hours with 0.01 to 1% Pd loading.The deactivated aryl chloride, 4-chloroanisole, was also coupledwith phenylboronic acid in a good yield but with higher Pdloading. Pd/PIC could be separated easily by ltration aer onecycle of the reaction, and the Pd/PIC nanocatalyst was recyclablewithout loss of efficiency.

Pluronic F68 triblock copolymer. Palladium nanoparticlesstabilized by the Pluronic F68 triblock copolymer were preparedby reduction of Na2PdCl4 in the presence of Pluronic F68(catalyst 63).70 TEM data showed that the catalyst containedmainly cubo-octahedral Pd NPs with an average diameter of 5.4nm and with relatively low dispersion. The electron diffractionpattern indicated the crystalline nature of Pd NPs. Suzuki–Miyaura reactions were performed with phenylboronic acid andwater-soluble aryl iodides and aryl bromides containing anelectron-withdrawing or electron-donating substituent, in water

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RSC Advances Review

in the presence of potassium hydroxide as the base (Scheme 55).The catalyst showed a high efficiency, and the reaction occurredat room temperature in the presence of an optimal amount ofKOH (5 equiv.), and a palladium concentration of 1 mol%, andthe reaction with m-iodobenzoic acid was almost completewithin 0.5 h. The catalytic activity was found to depend on thesize of the palladium nanoparticles and their morphology. Inparticular, the most active under the examined conditions werethe trigonal Pd NPs.

Entrapped palladium with functionalized polymers.Entrapping of palladium by the immobilization of palladiumacetate on a TentaGel resin has been reported by Bradley and co-workers for the Suzuki–Miyaura reaction.71 The cross-linkedresin-captured palladium (XL-RC Pd, catalyst 64) was preparedby treating a mixture of aminomethylated TentaGel resin withpalladium acetate (10 wt% of resin) in toluene at 80 �C for 10min followed by stirring at room temperature for 2 h to affordbrown colored resin-captured palladium acetate. The resin wasltrated and treated with 10% hydrazine hydrate in methanol toget Pd(0). The resulting resin was cross-linked with succinylchloride to x the captured palladium. TEM images showedPd(0) nanoparticles with an average size of 7.4 nm. The catalyticsystem based on the loading of the complex catalyst 64 (10mol%) and K2CO3 as a base in water at 80 �C, showed goodactivity for the reaction with aryl bromides, while aryl chloridesproved to be poorly reactive under these reaction conditions(Scheme 56). The coupling reaction required a short durationunder microwave activation compared to that under thermalheating. The “three-phase test” showed the high stability of thecatalyst. Simple ltration was sufficient for the recovery of thecatalyst, which could be recycled at least six times without anydecrease in activity.

Poly(vinylpyrrolidone) support. In the absence of any ligand,a Suzuki–Miyaura reaction of aryl iodides and bromides withpotassium aryltriuoroborate catalyzed by recoverable

Scheme 56 Active, entrapped complex catalyst 64 in Suzuki–Miyaurareactions.

Scheme 57 Poly(vinylpyrrolidone)-supported palladium catalyst inSuzuki–Miyaura reactions.

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palladium(0) on poly(vinylpyrrolidone) (PVP) was developed(Scheme 57).72 The optimized reaction conditions involved theuse of Pd/PVP (106 mg of 1% Pd on PVP, 0.01 mmol of metal),K2CO3 (272 mg, 2.0 mmol), aryl halide (1.0 mmol), potassiumorganotriuoroborate (1.0 mmol) and water (5 ml) reuxing at100 �C. Bromoarenes and boronic acids were also reactive underthe reaction conditions but produced the products in signi-cantly lower yields. Palladium on PVP could be recycled at leasteight times without loss of activity via a simple decantationprocedure. No clues on the heterogeneous or homogeneouspathway for the catalysis were reported in this study.

QuadraPure support. Cross-linked resin-captured palladium(XL-QPPd) was prepared73 by treating a mixture of solid supportQuadraPure with palladium acetate in toluene at 80 �C for 10min. The resulting resin was then cross-linked with succinylchloride and triethylamine in dry DMF solvent followed by thetreatment with hydrazine hydrate in methanol (10%) at roomtemperature to get black resin palladium catalyst (1.5 mmol ofPd per g). TEM image and X-ray diffraction analysis revealedthat the palladium nanoparticles were well dispersed withdiameters ranging from 4–10 nm. The catalyst showed goodcatalytic activity in Suzuki–Miyaura cross-coupling reactionswith various aryl halides and phenylboronic acid in the pres-ence of the resin (5 mol% based on Pd content) for 10 min inwater under aerobic and microwave conditions (120 �C, 100 W)(Scheme 58). Although the catalytic system performed well foraryl bromides and aryl iodides, the resin gave poor yields witharyl chlorides under the same reaction conditions. The resincould be recovered by simple ltration and showed consistentcatalytic activity aer 10 runs with no signicant loss of activity.The leaching of Pd analyzed using ICP-MS was only 0.21 ppm.

Palladium supported by polyaniline. Diaconescu and co-workers74 reported the preparation of biphenyls by polyanilinenanober-supported palladium nanoparticle-catalyzed crosscoupling reactions of chloroarenes and boronic acids. Using alow loading of Pd/PANI (0.05 mol%) and NaOH as the base,activated and deactivated chloroarenes were coupled withboronic acids in the absence of an additive in reuxing water(Scheme 59). These results were reinforced by the good reus-ability of the catalyst for more than ten cycles.

Another report of polyaniline-supported palladium was byM. L. Kantam and co-workers.75 Four types of catalyst wereprepared from different palladium precursors and all the cata-lysts were tested in Suzuki–Miyaura couplings of bromo- andchloroarenes in water (Scheme 60), and the catalyst preparedfrom the PdCl2 precursor was found to be the most effective

Scheme 58 Microwave-promoted Suzuki–Miyaura couplings of arylhalides with phenylboronic acid catalyzed by XL-QPPd nano.


Scheme 59 Selected examples of the Suzuki–Miyaura reaction byDiaconescu et al.

Scheme 60 Suzuki–Miyaura couplings using PANI–Pd by L. Kantamet al.

Scheme 61 Schematic representation of the synthesis process of thePd@poly(NIPA-co-PMA) hydrogel catalyst.

Scheme 62 Suzuki–Miyaura cross-coupling reactions of aryl halideswith arylboronic acids in water.

Scheme 63 (a) Preparation of the PNIPAM-co-4-VP co-polymers,and (b) preparation of Pd nanoparticles on the PNIPAM-co-4-VP co-polymer.

Review RSC Advances

catalyst. Cs2CO3 was found to be the best one though otherinorganic bases like K3PO4 and KF were also found to be fairlyactive under these conditions in comparison to organic baseslike Et3N and Bu3N. Bromoarenes with electron-withdrawingfunctionalities reacted at a faster rate than the bromoarenesbearing electron-donating functionalities. Arylboronic acidswith an electron-withdrawing group required longer times thanthose with an electron-donating group. Sterically hinderedarylbromides as well as arylboronic acids reacted sluggishly.The catalyst loading could be reduced to 0.5 and 0.1 mol% butlonger reaction times were required for excellent yields. Thecatalyst was used for ve consecutive cycles and the differencein palladium content between the fresh and used catalyst wasca. 2%.

Palladium supported by a hydrogel. Ligand-free, palladium-supporting poly(N-isopropylacrylamide-co-potassium methac-rylate) [poly(NIPA-co-PMA)] hydrogel nanocomposites withdifferent co-monomer ratios were synthesized (Scheme 61)76

and examined in Suzuki–Miyaura cross-coupling reactions ofaryl halides with arylboronic acids in an aqueous medium(Scheme 62). The hydrogel with a co-monomer ratio of 8.8 : 1.6mmol of NIPA : PMA exhibited optimum catalytic activity, andcould effectively be reused 5–6 times without loss of catalyticactivity. The study revealed that the hydrophilicity of the catalystand its swelling in the water medium played a decisive role indetermining the catalytic performance.


Palladium nanoparticles on a hydrogel (catalyst 66) preparedby Rhee and co-workers (Scheme 63)77 showed high activity inSuzuki–Miyaura coupling reactions of arylboronic acids andaryl bromides with a wide range of functional groups, con-taining electron-donating and electron-withdrawing groups, inwater and under mild reaction conditions (80 �C) (Scheme 64).

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Scheme 64 Suzuki–Miyaura coupling reactions of arylboronic acidsand aryl bromides using catalyst 66.

Scheme 67 Preparation of the CS-g-mTEG or -mPEG Pd(0) catalyst.

Scheme 68 Suzuki–Miyaura coupling using the CS-g-mTEG or-mPEG Pd(0) catalyst.

RSC Advances Review

The catalyst showed excellent activity for the rst ve runs,resulting in high yields above 90–95% in 40 min. Aer each run,the amount of Pd leaching was estimated by performing ICPmeasurements on the supernatant solutions. No signicant Pdleaching (<0.5 ppm) was observed.

Natural source. Chitosan, a biopolymer composed of D-glucosamine and N-acetylglucosamine, has been considered asa suitable water-compatible solid support for palladium(Scheme 65) in the Suzuki–Miyaura reaction (Scheme 66).78 Theoptimized reaction conditions involved TBAB as a phase-transfer agent and K3PO4 as a base in the presence of 0.5mol% of the palladium catalyst (Pd/chitosan), which showedgood activity under microwave activation for iodo- and bro-moarenes while chloroarenes were much less reactive. Theheterogeneous catalyst was reused for the reaction of 4-iodoa-cethophenone with phenylboronic acid with consistent activi-ties over ve cycles.

Following the previous report, a chitosan-g-mTEG (methoxytriethylene glycol)- or -mPEG (methoxy polyethylene glycol)-supported palladium(0) catalyst (Scheme 67) for the Suzuki–Miyaura cross-coupling reaction in water has been demon-strated by the same author (Scheme 68).79 The catalyst showedexcellent catalytic activity in the Suzuki–Miyaura cross-couplingreaction without additional phase-transfer reagents due to theenhanced solubility of the organic substrate by PEG graing. Inaddition, the catalyst could be reused up to ve times with the

Scheme 65 Preparation of the chitosan-supported palladium(0)catalyst.

Scheme 66 Palladium supported on chitosan as a catalyst for Suzuki–Miyaura reactions.

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catalytic activity being recovered easily aer simplemanipulations.

The activity of the g-mTEG or -mPEG Pd(0) catalyst wassimilar to the one of the CS–Pd(0) catalyst when TBAB was usedas a phase-transfer reagent, but without TBAB, the activity of theCS-g-mPEG Pd(0) catalyst was superior to that of the CS–Pd(0)catalyst which demonstrated that graed mTEG or mPEGworked as an effective phase-transfer reagent in the Suzuki–Miyaura cross-coupling reaction in water. Aryl iodides andbromides gave viable product yields while aryl chlorides gavecomparatively lower yields. CS-g-mPEG Pd(0) showed bettercatalytic activity than the non-graed CS–Pd(0) catalyst and wasfound to be effective for the coupling of aryl chloride in water.The CS-g-mPEGPd(0) catalyst was reused up to ve times withgradually decreasing activity which might be due to the aggre-gation of palladium nanoparticles inside the chitosan beads,according to the authors.

Another report on chitosan-supported Palladium catalysts(Scheme 69) is by Pombeiro et al.80 Pd-Chit 70 and Pd-Chit 71(Fig. 7) have been exploited for model microwave-assistedSuzuki–Miyaura cross-coupling reactions in water to giveexcellent yields. The effects of catalyst loading, temperature,time, the phase-transfer agent tetrabutylammonium bromide(TBAB) and base were investigated. The catalytic reactions of thesupported material proceeds heterogeneously as proven byactivity studies and ICP-AES analyses on leaching. Although thepresence of the phase-transfer agent TBAB favoured the reac-tion, its effect was less pronounced with the increase intemperature and become negligible at 160 �C. The presence of abase was fundamental and potassium carbonate was the bestone among those screened. The chitosan-supported


Scheme 69 Synthesis of the fused bicyclic oxadiazoline A and ketoi-mine B palladium(II) complexes.

Fig. 7 Palladium catalysts by Pombeiro et al.

Fig. 8 Chemical structures of alginate (A) and gellan (G).

Scheme 70 Synthesis of Pdnp/A–G.

Scheme 71 Reaction of arenediazonium tetrafluoroborates withpotassium aryltrifluoroborates catalyzed by Pdnp/A–G.

Fig. 9 Wool-Pd catalyst 72.

Review RSC Advances

heterogeneous catalyst could be successfully recovered andreused up to seven times though with a gradual loss of catalyticactivity.

The use of palladium nanoparticles stabilized by naturalbeads made of an alginate/gellan mixture (Fig. 8, Scheme 70) inthe Suzuki–Miyaura cross-coupling reaction of arenediazoniumtetrauoroborates with potassium aryltriuoroborates (1 : 1molar ratio) with a catalyst loading as low as 0.01–0.002 mol%under aerobic, and phosphine- and base-free conditions inwater is described (Scheme 71).81 Good to high yields wereobtained with arenediazonium tetrauoroborates and potas-sium aryltriuoroborates containing both electron-donatingand electron-withdrawing substituents. However, a moresevere steric congestion leads to trace amounts of the desiredproduct. SEM analysis, at higher magnication, displayedaggregated nanoparticles and TEM measurements indicatedthat the catalyst system before use contained spheroidal palla-dium nanoparticles, whose average size was 3.4 � 1.4 nm. The


catalyst system could be reused several times without signi-cant loss of activity. The material recovered aer eight runsshowed nanoparticles of about 3.6 � 1.7 nm in diameter.

Biopolymer. A heterogeneous biopolymer complex wool–Pdcatalyst (Fig. 9, catalyst 72) has been applied in water-mediatedcoupling reactions of aryl iodides and bromides with arylbor-onic acids (Scheme 72).82 The reaction could be conducted

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Scheme 72 Suzuki–Miyaura coupling reactions using wool-Pd.

Scheme 73 Suzuki–Miyaura coupling reaction presented by Ma et al.

Scheme 75 Suzuki–Miyaura coupling reaction presented by Xionget al.

RSC Advances Review

under atmospheric conditions without any specic protectionand phase-transfer agent. The experimental observationsshowed that the catalytic system was applicable to various aryliodides and bromides and tolerant to a broad range of func-tional groups, including H, NO2, NH2, OR, OH, COMe and CHO.The catalyst was reusable and easy to separate, and the productcould be obtained by simple ltration.

Hybrid organic–inorganic supports

Silica supported PEG. Cruden and co-workers reported ahighly active and stable silica-supported palladium complex forthe Suzuki–Miyaura reaction.83 A variety of iodoarenes andbromoarenes were coupled with boronic acids in high yieldswith only 0.1 mol% palladium catalyst 73 loading with K3PO4 asa base (Scheme 73). Leaching of palladium into the extractedsolvent was not observed and the aqueous suspension of thecatalyst might be reused at least ve times without noticeablydecreasing activity. The remarkable stability of this palladiumcomplex was demonstrated by the activity of the catalyst aer sixweeks of exposure to air.

A highly active heterogeneous catalyst was prepared by Xionget al. from coated mesoporous materials which contain a layerof readily available PEG (Scheme 74).84 Suzuki–Miyaura

Scheme 74 Synthesis of the mesoporous silica-supported catalyst 74.

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coupling reactions in water were performed with this catalyst at50 �C using K3PO4 as the base (Scheme 75). The presence ofelectron-donating or -withdrawing groups on aryl halides or arylboronic acids as well as steric hindrance did not have anyconsiderable effect on the product yield under these reactionconditions. In most cases, the reaction with 0.1 mol% catalystcould afford the products in fairly good yields; the catalystloading could even be reduced to 0.01 or 0.001 mol%. Thecoupling reaction was found to be effective with phenyl bromideand with the very unreactive phenyl chloride. The catalyst washighly stable and remained catalytically active aer exposure toair for up to 6 weeks. An aqueous suspension of the catalystcould be reused several times by simple extraction. No leachingof the catalyst to the organic layer was observed.

Silica-supported ionic liquids. An efficient and reusablecatalyst with PdEDTA immobilized on an ionic liquid brush(Scheme 76) and a green procedure have been developed for thecoupling of aryl iodides and bromides with phenylboronic acid(Scheme 77).85 For a smooth reaction, 1 mol% of the catalyst wassufficient, giving the desired coupling product in nearly

Scheme 76 Preparation of silica-immobilized ionic liquid brushcatalysts.

Scheme 77 Suzuki–Miyaura reactions with the complex catalyst byWei et al.


Scheme 78 Preparation of the Pd@PMO-IL catalyst.

Scheme 79 Suzuki–Miyaura cross-coupling of various aryl halideswith ArB(OH)2 in the presence of the Pd@PMO-IL catalyst in water.

Review RSC Advances

quantitative yield without any detectable unwanted self-coupling product. Even with an amount as small as 0.5 mol%,the catalyst exhibited good performance. Aryl iodides wereclearly more reactive and gave the coupling products in yieldsranging from 90% to almost 100% for the examples screened.The yield gradually decreased for aryl bromides to aryl chlorideswith longer reaction times. The catalyst also worked efficientlyfor heteroaryl halides. Electron-withdrawing and electron-donating substituents on either the aryl halide or phenyl-boronic acid did not have a detrimental effect on the ability ofthe catalyst. Couplings proceeded successfully to give thedesired products in high yields even in the presence of sensitivegroups such as MeCO, CO2H, NH2, CN, and OH, without anyprotection.

The palladium-in-brush catalyst could be recovered bysimple ltration. Ten consecutive preparations of 4-methoxy-biphenyl showed no signicant reduction in yield which wasagain supported by ICP-MS analyses showing low leaching ofless than 0.5% of the initial palladium loading).

The parent ordered mesoporous organosilica with ionicliquid framework (PMO-IL) was synthesized by hydrolysis andco-condensation of 1,3-bis-(3-trimethoxysilylpropyl)imidazo-lium chloride and tetramethoxysilane in the presence of plur-onic P123 as a template under acidic conditions. Thisnanostructured PMO-IL material was then reacted with a sub-stoichiometric amount of Pd(OAc)2 to afford the correspond-ing palladium-containing periodic mesoporous organosilica(Pd@PMO-IL) catalyst (Fig. 10) (Scheme 78).86 Pd@PMO-IL wasestablished as an efficient and reusable catalyst for the Suzuki–Miyaura coupling reaction of various types of aryl iodides,bromides, and even deactivated chlorides in water (Scheme 79).It was also found that, although the PMO-IL nanostructureacted as a reservoir for soluble Pd species, it could also operateas a nanoscaffold to recapture Pd nanoparticles into the mes-ochannels thus preventing extensive agglomeration of Pd.Among the various bases screened, K2CO3 provided the highestcross-coupling yield. Highly deactivated heteroaromatic

Fig. 10 Silica-immobilized PdEDTA2- ionic liquid.


substrates led to the corresponding bis(aryl) products in excel-lent yields. The catalyst was also capable of activating chloro-benzene to produce the desired products in signicant yields.Moreover, various functional groups, such as cyano, acyl, CHO,methyl and methoxy, were tolerated and the correspondingcoupled products were obtained in good to excellent isolatedyields. Surprisingly, hot ltration tests and selective catalystpoisons showed the presence of soluble Pd species during thereaction process but atomic spectroscopy and catalyst recoverystudies illustrated no signicant decrease in activity and metalcontent of recovered Pd@PMO-IL.

Silica-supported oxime-based ligands. Corma and co-workers inspired by the studies of Najera and co-workers87

described the use of an oxime palladacycle covalently anchoredonto silica-based inorganic supports (Scheme 80) in the Suzuki–Miyaura reaction in water.88 The oxime palladacycle wasanchored either on amorphous silica (Pd/SiO2, catalyst 76) or onMCM-41 (Pd/MCM-41), and the reactivities of the resultingcatalysts were evaluated in parallel experiments. Although bothcatalysts were catalytically active, the Pd/SiO2 catalyst 76 gavethe best results with bromoarenes and activated chloroarenes at5 mol% palladium loading with K2CO3 as the base (Scheme 81).

The use of an organic solvent such as 1,4-dioxane wasdetrimental for the successful reaction. Moreover, the sameoxime palladacycle anchored onto polymeric supports, i.e.polystyrene and polyethylene glycol bis(methacrylate), gave lowconversions for the coupling products. Recycling of the Pd/SiO2

catalyst 76 was evaluated using 4-chloroacetophenone andphenylboronic acid as the substrates and indicated that eightconsecutive reactions could be run without any decrease inactivity. Leaching studies using the “three-phase test” wereconclusive for a truly heterogeneous process with no detectablepalladium species being released into the medium.

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Scheme 81 Selected examples described by Corma et al.

Scheme 82 Synthesis of catalyst 77.

Scheme 83 Suzuki–Miyaura coupling reactions using the SBA-15palladium catalyst 78.

Scheme 80 Anchoring procedure of the oxime carbapalladacycleonto the mercaptopropyl-modified high surface silica.

Fig. 11 Structure of the fluorous silica-supported perfluoro-taggedphosphine palladium catalyst.

Scheme 84 Cross-coupling reactions according to the work of

RSC Advances Review

In a subsequent study, Corma and co-workers reported arelated oxime palladacycle anchored onto a periodic meso-porous organosilica (PMO) (Scheme 82).89 Although the resultsreported by Corma and co-workers were relevant for bromoar-enes, the Pd/PMO catalyst was essentially unreactive for arylchlorides. Moreover, the recycling of Pd/PMO was limited by asignicant decrease in catalytic activity upon reuse. As aconsequence, PMO did not convey any benets over the onlyamorphous silica.

Silica-supported mercaptopropyl ligands. Because of its highporosity and surface area, the mesoporous silicate SBA-15 wasalso exploited as a support with mercaptopropyl ligands byCrudden and co-workers.90Only three bromoarenes and a singlechloroarene were studied in cross-coupling reactions withphenylboronic acid in water. Excellent catalytic activity wasachieved for the mercaptopropyl-anchored SBA-15 palladiumcatalyst 78 (Scheme 83). Heterogeneity tests clearly indicatedthat the reaction occurred through a truly heterogeneouscatalysis. Although the mercaptopropyl ligands, anchored to an

42214 | RSC Adv., 2015, 5, 42193–42221

amorphous silica support (SiO2), showed a similar catalyticactivity, the recycling ability was hampered by a noticeable lossof activity aer the second cycle.

Silica-supported phosphine ligands. A study describing theuse of a uorous reversed-phase silica gel as a support for thenon-covalent immobilization of peruoro-tagged palladiumphosphine complexes has been described by the group ofBannwarth.91 Complex catalyst 80, immobilized on the uorous-reversed silica gel 79 (Fig. 11) by a simple dispersion technique,gave the best results in the cross-couplings of water-soluble arylbromides with arylboronic acids (Scheme 84), but less water-soluble substrates were essentially unreactive. The immobili-zation of triphenylphosphine–palladium complexes on normalsilica gel led to a supported-catalyst having similar activity,however, signicant palladium leaching was observed in thesolvent (7.4%). In comparison, the uorous complex appearedmuch more robust (0.8% leaching). The “three-phase test” wasconclusive for a homogeneous catalysis operating likely by a“release and capture” mechanism. The low leaching observedwith this catalytic system allowed the recycling of the complexfor ve runs with only limited and progressive deactivation.

Cai et al. also reported Suzuki–Miyaura cross-couplingreactions using peruoro-tagged palladium nanoparticles on auorous silica gel (FSG) as the catalyst (Scheme 85), K2CO3 asthe base and TBAB as an additive in H2O, affording the corre-sponding biphenyls in moderate to high yields (Scheme 86).92 Itwas obvious from the TEM image that the spherical palladium

Bannwarth and co-workers.


Scheme 85 Preparation of fluorous nanoparticle stabilizer and thefluorous silica gel-supported Pd catalyst.

Scheme 86 Pd–C/FSG-catalyzed Suzuki–Miyaura reactions.

Scheme 88 Water-medium Suzuki–Miyaura reactions on the Pd(II)–PMO(Ph)-3D-2 catalyst.

Scheme 89 Pd nanoparticles on an acetyl acetone modified silica gel(catalyst 81).

Review RSC Advances

nanoparticles were successfully dispersed in the silica matrixwith an average size of 2–3 nm. The catalyst could be recoveredby simple ltration and reused several times with a slightdecrease in activity.

Aryl bromides bearing either electron-donating or electron-withdrawing substituents in the ortho- and para-positions,afforded the corresponding biphenyls in good to excellentyields. Aryl triuoromethanesulfonate and aryl per-uorooctanesulfonate were more active than bromobenzene interms of yield as well as the time. However, chlorobenzene wasnot active in the reaction and only a moderate yield wasobtained even when the catalyst was increased to up to 1 mol%.When activated aryl chloride was used, a relatively higher yieldwas obtained though the yield was still unsatisfactory. Recyclingstudies showed that the supported catalyst could be reusedseveral times with a slight decrease in activity, and Pd leachingwas less than 10 ppm.

A Pd(II) organometal catalyst with a three-dimensional (3D)cage-like Ia3d cubic mesoporous structure and a high surfacearea was prepared (Scheme 87).93 In comparison with the cor-responding catalyst with a two-dimensional (2D) P6mm hexag-onal mesoporous structure, the as-prepared catalyst exhibitedhigher activities in water-medium Suzuki–Miyaura coupling

Scheme 87 Illustration of preparing Pd(II)–PMO(Ph)-3D.


reactions owing to the diminished diffusion limit. It showedcomparable efficiencies with the Pd(PPh3)2Cl2 homogeneouscatalyst and could be easily recycled and reused for ve timeswithout signicant loss of activity (Scheme 88).

Acetyl acetonate ligands. Supported Pd nanoparticles on anacetyl acetone modied silica gel (catalyst 81) were prepared(Scheme 89), and the catalytic application in the Suzuki–Miyaura reaction of various aryl halides with phenylboronicacid was investigated (Scheme 90).94 The amount of Pd on thecatalyst was 0.04 mmol g�1 (4.26 mg g�1) determined using AAS.The TEM image indicated the presence of palladium nano-particles in the range of 6–12 nm.

Under optimized conditions, the reaction of 4-bromoaceto-phenone and phenylbronic acid using 10 mg (0.0004 mmol,0.04 mol% Pd) of the catalyst and 2 equiv. of the base gave thebest result. Water was the most efficient solvent and NaHCO3

was chosen as the best base for this reaction. The catalyst couldbe used for cross-coupling reactions of aryl iodides, bromidesand even less reactive aryl chlorides under the reux tempera-ture, which were transformed to the corresponding coupledproducts in good to excellent yields in short reaction times.However, aryl chlorides reacted more slowly in comparison to

Scheme 90 Suzuki–Miyaura coupling reaction using Pd nanoparticleson an acetyl acetone modified silica gel.

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Scheme 93 Preparation of the MFI-supported Pd(OAc)2–ionic liquid.

RSC Advances Review

the iodide and bromide derivatives. The most hindered arylhalides usually produced the corresponding biaryls with longerreaction times and lower yields. The catalyst was recycled fourtimes with essentially no loss of activity, even aer recycling itsix times, good product yields could still be obtained.

Amino pyridine linker. S. M. Islam et al.95 have reported thesynthesis and catalytic activity of a mesoporous silicananosphere-supported palladium(II)–2-aminopyridine complex(Pd–AMP-MSN) (Scheme 91). FTIR spectroscopic analysisconrmed the presence of 2-aminopyridine functionalitiesinside the mesopores of Pd–AMP-MSN. FESEM and HRTEMresults indeed showed the formation of nanospheres withmesoporous structures. This catalytic system exhibited excel-lent activity in Suzuki–Miyaura cross-coupling reactions of aryliodides, aryl bromides and also aryl chlorides with phenyl-boronic acids in a water medium with high yields (Scheme 92).This Pd–AMP-MSN catalyst could be quantitatively recovered bysimple ltration and was found to be highly active without anysignicant loss of catalytic activity aer eight consecutive runs.

The catalytic efficiency was not signicantly affected by thesubstituents on the aromatic ring of the halides. The Suzuki–Miyaura cross-coupling reaction of sterically hindered arylhalides could also proceed smoothly affording the desiredcoupled products in good yields. At the expense of time with upto 10 h, brominated heterocyclic derivatives afforded the prod-ucts in good yields when treated with phenylboronic acid underthe optimized conditions. The less reactive and less expensivephenyl chloride also showedmoderate reactivity (yield 82–83%).

Scheme 91 Schematic illustration of the synthesis of the Pd–AMP-MSN catalyst.

Scheme 92 Suzuki-Miyaura cross-coupling reactions of phenylbromide with ArB(OH)2 using the Pd–AMP-MSN catalyst.

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Zeolite-supported ionic liquids. Immobilization of palla-dium acetate with ionic liquids in the mesoporous channels of ahierarchically porous MFI zeolite (Scheme 93) has been envis-aged as an active catalyst for Suzuki–Miyaura reactions underaqueous conditions.96 The optimized catalytic system efficientlyuses the heterogeneous palladium catalyst 82, in the presenceof TBAB as the surfactant and K3PO4 as the base, for the prep-aration of various substituted biphenyls (Scheme 94). Morethan thirty examples showed that even very crowded bromoar-enes could react smoothly at low catalyst loading (0.3 mol% Pd).In addition to this excellent catalytic activity, the heterogeneouspalladium catalyst 82 was used in four consecutive runs withconserved efficiency (Fig. 12).

Metal oxide-supported ionic liquids. Jin and co-workersdescribed a very original approach based on a recoverablepalladium supported magnetite Fe3O4–ionic liquid catalyst.97

Scheme 94 Selected examples of the hindered biphenyl synthesis.


Fig. 12 Representation of catalyst 82.

Scheme 95 Schematic representation for the synthesis of catalyst 83.

Scheme 96 Few examples of challenging couplings using catalyst 83.

Scheme 97 Suzuki–Miyaura reactions of aromatic aryl halides andphenylboronic acid catalyzed by Pd–PVP/KIT-5.

Scheme 98 Suzuki–Miyaura coupling reactions using Pd–PHEMA/

Review RSC Advances

Due to the magnetic properties of nano-Fe3O4, the hybridcomplex catalyst 83 (Scheme 95) could be separated from thereaction mixture by an external magnet avoiding the usualltration or centrifugation process. The Pd–NHC/Fe3O4-ILcatalyst 83 was evaluated for the Suzuki–Miyaura cross-couplingof bromoarenes with a variety of arylboronic acids. The catalystsystem required K3PO4 as the base and TBAB as the phase-transfer agent in water at temperatures ranging from 40–85�C. Numerous examples were reported with 0.5 mol% Pd indi-cating that the protocol was particularly efficient even in thepresence of challenging substrates (Scheme 96). Recycling of 83proved to be successful along ve cycles. ICP analyses showed apalladium leaching aer the rst use of about 10 ppm whichnotably decreased upon subsequent reuses.

Poly-vinyl-pyrolidone/KIT. The composite poly(N-vinyl-2-pyrrolidone)/KIT-5 (PVP/KIT-5) was prepared by an in situpolymerization method and used as a support for palladiumnanoparticles obtained through the reduction of Pd(OAc)2 byhydrazine hydrate.98 The catalytic performance of this


heterogeneous catalyst was determined in Suzuki–Miyauracross-coupling reactions between aryl halides (X ¼ I, Br, Cl) andphenylboronic acid in the presence of water at room tempera-ture. The stability of the nanocomposite catalyst was excellent,and it could be reused eight times without much loss of activity.Aer completion of the reaction, the catalyst was recoveredfrom the reaction mixture by simple ltration (Scheme 97).

Following their previous work, a Pd nanoparticle–poly(2-hydroxyethyl methacrylate)/KIT-6 (Pd–PHEMA/KIT-6)composite was fabricated through an in situ polymerizationmethod and was evaluated as a novel heterogeneous catalyst inSuzuki–Miyaura cross coupling reactions of aryl chlorides,bromides and iodides and phenylboronic acid under aerobicconditions in water by the same group of workers (Scheme 98).99

The reactions were usually carried out at 40 �C for 1 h. Some-times a higher temperature of 95 �C as well as longer reactiontimes were required for the reaction depending on the nature ofthe reactants. The Pd content of the catalyst estimated by ICP-AES was 0.956 mmol g�1. The typical TEM micrographsshowed a cubic Ia3d pore array structure for the Pd–PHEMA/KIT-6 nanocomposite. This heterogeneous catalyst could bereused at least nine times without any decrease in activity.

Metal Organic Framework (MOF). Water-mediated couplingreactions of aryl chlorides over heterogeneous palladium

KIT-6.

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Scheme 99 Suzuki–Miyaura coupling reactions of aryl chlorides oversupported palladium catalysts.

RSC Advances Review

deposited on a zeolite-type MOF was reported.100 This workrepresents the rst example of an active catalyst, composed of aMOF as the support for metal NPs, for the coupling reactions ofaryl chlorides (Scheme 99). The TEM image showed that thepalladium NPs were highly dispersed, with a mean diameter of1.9–0.7 nm, as estimated by the size distribution. The crystallinestructure of the catalyst was mostly retained aer ve catalyticcycles. A very low amount of dissolved palladium (less than0.2% of the total palladium) was detected in the solution at theend of the reaction.

Conclusions

The Suzuki–Miyaura cross-coupling reaction is a method forcarbon–carbon bond formation, which is a highly useful andversatile method needed for the development of modern drugdiscovery and the synthesis of many natural products, polymersand other organic compounds of various importance. Tremen-dous effort has been put forward by chemists throughout theworld for improving the Suzuki–Miyaura coupling reaction,keeping in mind the current need for greener reactions. Thedevelopment of sustainable chemistry is a crucial area ofresearch for the future advancement of our society. To cope withthe growing demand for eco-friendly procedures, chemists haveimproved heterogeneous catalytic systems and moved to saferreaction media such as water. Although a considerable progresshas been achieved for the Suzuki–Miyaura coupling reactionusing palladium under heterogeneous conditions, the reactionpathway under heterogeneous conditions, the minimization ofleaching of palladium aer a few cycles, and the exploration ofother eco-friendly metal catalysts for this reaction remain still tobe investigated. Nevertheless, there are still many areas toexplore in the area of eco-compatible chemical synthesis, andno doubt supported organometallic chemistry will lead the wayalong the path of progress.

Acknowledgements

SP thanks UGC for awarding DS Kothari a Post DoctoralFellowship (no. F.4-2/2006 (BSR)/CH/13-14/0075). MMIacknowledges UGC, New Delhi, for awarding Maulana Azad aNational Fellowship (F1-17.1/2013-14/MANF-2013-14-MUS-WES-24492/(SAIII). SMI acknowledges the Department ofScience and Technology (DST) and Council of Scientic andIndustrial Research (CSIR) New Delhi, India, for funding. Wealso acknowledge DST & UGC, Govt of India, for providingsupport to the Department of Chemistry, University of Kalyani,under the PURSE, FIST and SAP program.

42218 | RSC Adv., 2015, 5, 42193–42221

Notes and references

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