applications of lawesson’s reagent in organic and ... · 6 synthesis of organometallic compounds...

REVIEW 1929

Applications of Lawesson’s Reagent in Organic and Organometallic SynthesesApplications of Lawesson’s Reagent in Organic and Organometallic SynthesesMartin Jesberger,* Thomas P. Davis, Leonie BarnerCentre for Advanced Macromolecular Design, School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, Sydney NSW 2052, AustraliaFax +61(2)93856250; E-mail: [email protected] 24 March 2003; revised 24 April 2003

SYNTHESIS 2003, No. 13, pp 1929–195818.09.2003Advanced online publication: 10.09.2003DOI: 10.1055/s-2003-41447; Art ID: E09203SS.pdf© Georg Thieme Verlag Stuttgart · New York

Abstract: This review, including 245 references, describes the ap-plication of Lawesson’s reagent [2,4-bis(p-methoxyphenyl)-1,3-dithiaphosphetane-2,4-disulfide] LR in organic and organometallicsyntheses. Thionations of carbonyl-containing compounds as wellas unexpected reactions are shown for different applications (e.g.cyclizations, rerrangements, syntheses of heterocyclic compoundsetc.). Syntheses of novel organometallic compounds by LR are alsodiscussed.

1 Introduction2 Mechanism of the Thionation Reaction Using Lawesson’s

Reagent 3 Reactions with Carbonyl-Containing Compounds3.1 Ketones3.2 Esters and Lactones3.3 Amides and Lactams3.4 Amino Acids, Peptides, Nucleosides, and Nucleotides3.5 Macrocycles and Polymers4 Cyclization Reactions to Thiophenes, Thiazoles and Other

Compounds5 Synthesis of Heteroatom-Containing Compounds5.1 Sulfur-Containing Heterocyclic Compounds5.2 Phosphorus- and Sulfur-Containing Heterocycles5.3 Other Phosphorus- and Sulfur-Containing Compounds6 Synthesis of Organometallic Compounds6.1 Transition Metals6.2 Main Group Metals7 Usage of Lawesson’s Reagent for Special Syntheses7.1 Glycosidations7.2 Transformation of Alcohols to Thiols7.3 Reduction of Sulfoxides7.4 Catalyst for Aldol Reactions7.5 Preparation of Other Compounds8 Unexpected Reactions Following the Usage of Lawesson’s

Reagent9 References

Key words: Lawesson’s reagent, thionation, organothiophos-phorus reagent, heterocycles, sulfur-compounds, organometalliccompounds

1 Introduction

Thionation reactions are widely applied in organic synthe-ses. Phosphorus pentasulfide has been used as a thionationreagent for transformations of carbonyl groups into thecorresponding thiocarbonyl groups. These reactions arenormally performed in boiling toluene, xylene, or pyri-

dine and require a large excess of reagent. Furthermore,long reaction times are needed; yields are usually low andvariable.1–4 Many research groups have investigated otheruseful thionation reagents as a substitute for phosphoruspentasulfide.5–9 In 1956, syntheses of varying aryl thiono-phosphine sulfides were described by Lecher et al.10,11 In1967, Hoffman and Schuhmacher reported on the trans-formation of benzophenone to thiobenzophenone via 2,4-bis(p-methoxyphenyl)-1,3-dithiadi-phosphetane-2,4-di-sulfide 1 (Figure 1).12 Starting in 1978, Lawesson and co-workers published systematic studies on using reagent 1,commenly named Lawesson’s reagent (LR). LR can beobtained readily by reaction of phosphorus pentasulfidewith anisole or reaction of red phosphorus, elemental sul-fur and anisole in moderate yields.13,14 LR is commercial-ly available and usually packed under argon. It is unstablein solution at temperatures above 110 °C, and decompos-es slowly.8,10

Figure 1 2,4-bis(p-methoxyphenyl)-1,3-dithiadiphosphetane-2,4-disulfide 1 or Lawesson’s reagent (LR)

Since 1978, increasing numbers of papers report on thion-ation indicating that LR is preferred over other, classicalthionation reagents. High yields, convenient handling,easy work up, availability, and usage for soft thionationreactions contribute to the popularity of LR. Cyclic pep-tides, like [Tyr-3-y-(CS-NH)-Ala-4; Tyr-6-y-(CSNH)-D-Ala-1]RA-VII 2,15 steroids, e.g. 17b-hydroxy-3-thioxo-4-aza-5-androstane 3,16 or nucleosides, as 1-(2,3,5-tri-O-benzoyl-b-D-ribofuranosyl)-4-thioxo thieno[3,2-d]pyrim-idin-2-one 4,17 shown in Figure 2, are examples of com-plex structures that can be thionated readily via LR.

In 1985, Cava and Levinson reviewed thionation reactionsusing LR 1.18 Cherkasov et al. included LR in their reviewabout organothiophosphorus reagents.19 Starting fromthese and other reviews20–27 we re-examine the develop-ments in the application of this powerful reagent and pro-vide an overview about novel reactions and uses ofLawesson’s reagent.

Our research interest28 is centered on living free radicalpolymerization, especially reversible-addition-fragmen-tation chain transfer polymerization (RAFT) invented byCSIRO.29 RAFT uses dithioesters as chain transfer agents.

MeO PS

S

S

P OMe

S

1 (LR)

1930 M. Jesberger et al. REVIEW

Synthesis 2003, No. 13, 1929–1958 © Thieme Stuttgart · New York

We became interested in using Lawesson’s reagent for thesynthesis of novel RAFT agents.

2 Mechanism of the Thionation Reaction Using Lawesson’s Reagent

Two similar mechanisms for thionation reactions usingLawesson’s reagent are described in the literature.18,19 LRcan be in equilibrium with a highly reactive dithiophos-phine ylide (5, 6). Both mesomeric structures 5 and 6, de-picted in Scheme 1, can react with carbonyl-containingcompounds to form thiaoxaphosphetane 7, which decom-poses in a Wittig-analogous reaction to the correspondingthioketone.18,19,30–38

The mechanism in Scheme 1 is similar to the proposedO,S-exchange reactions of phosphorus dithioacids. The

Martin Jesberger, born inLudwigshafen/Rh. (Germa-ny) in 1970, studied chemis-try at the Universities ofSiegen and Kaiserslautern.In 1998, he received hisM.Sc. in industrial chemis-try from the University ofKaiserslautern. In the sameyear he joined the group of

Prof. A. Kirschning at theTechnical University ofClausthal and received hisPh.D. in 2002 by working inthe field of polymer-assistedsynthesis of deoxyglyco-conjugates. In March 2002,he joined the group of Prof.T. P. Davis (Sydney, Aus-tralia) as a postdoctoral fel-

low and is presentlyworking in the field of con-trolled living polymeriza-tions, especially RAFT-polymerizations for combi-natorial chemistry applica-tions, and syntheses ofRAFT agents, which con-tain e.g. thiocarbonylthio-or trithiocarbonate function-alities.

Thomas P. Davis, receivedhis B.Sc. (1983) and Ph.D.(1987) from Salford Univer-sity in England. From 1987to 1989, he worked as apostdoctoral fellow withProfessor Ken O’Driscoll atWaterloo University in Can-ada. His postdoctoral workinvolved the application ofpulsed-laser polymeriza-tion for determining propa-gation rate coefficients.From 1989 to 1992, he

worked as a team leader atICI Acrylics Research &Technology in Runcorn,England, where he workedon thermosets, reaction in-jection molding, and com-posites. In 1993, he movedto Sydney, Australia to takeup a senior lectureship at theUniversity of New SouthWales (UNSW). In 1999, hewas promoted to a profes-sorship and was appointedResearch Director of the

School of Chemical Engi-neering & Industrial Chem-istry and Director of theCentre for Advanced Mac-romolecular Design. In2002 he was awarded anAustralian Professorial Fel-lowship, and in 2003 he waspromoted to Scientia Pro-fessor at UNSW. He haspublished more than 170refereed articles, patents,and book chapters.

Leonie Barner, born inStuttgart (Germany) in1969, studied chemistry atthe Universities of Kasseland Göttingen. In Göttingenshe joined the group of Prof.M. Buback and received herM.Sc. (1994) and Ph.D.(1998) in Macromolecularand Physical Chemistry.From 1998 to 2001, she held

a senior research position atSartorius AG, Göttingen(Germany) developing mi-crofiltration membranes forbiotechnology applications.In 2001, she joined the Cen-tre for Advanced Macromo-lecular Design at theUniversity of New SouthWales, Sydney/Australia,where she currently holds a

senior research associateposition. Her prime researchinterests are controlled/liv-ing radical polymerizationmethods (RAFT and ATRP)and the development of nov-el polymeric surfaces forbiotechnology and combi-natorial chemistry applica-tions.

Biographical Sketches

Figure 2 Cyclic peptide 2, steroid 3 and nucleoside 4

BzOO

OBzOBz

HN

NO

S

S

NH

Me

Me

S

OH

H

O

NH S

OMe

HN

O

Me

NH

S

NMe

Me

HNMe

O

NMe

O

O

OMe

2 3

4

REVIEW Applications of Lawesson’s Reagent in Organic and Organometallic Syntheses 1931


phosphorus atom coordination increases in dithiophos-phates from 4 to 5 and in thionophosphines from 3 to 5 atintermediate stages of thionation.

Another important factor is the thermodynamic stabilityof the resulting product. P–O-bonds are much strongerthan P–S-bonds and the formation of thionated products issupported by exchanging sulfur for oxygen.39 The mecha-nism in Scheme 1 has been confirmed for thioketones byLawesson,40 Fluck and Binder,31 Rauchfuss and Zank,41

and Yoshifuji et al.42 The structure and geometry of LR it-self was examined via 31P NMR-spectroscopy by Nakaiand McDowell.43

Lawesson and co-workers30 also found that the trimer p-methoxyphenylmetathiophosphonate 8 is formed in mildreaction conditions following thionation of a suitable re-actant (Scheme 2). Some limitations of the reactivity ofLR have also been shown (Figure 3). Ester containingether-functionalities do not undergo thionation easily. Thecrown ether-diester 9 in Figure 3 could not be thionatedby LR, however the more reactive monothionated furanocrown ether-diester 10 thionated as expected in goodyields forming the dithiono crown ether-diester 11.44 Ad-ditionally, Baxter and Bradshaw found that electron-with-drawing substituents conjugated with an ester carbonyl(e.g. methyl p-nitrobenzoate) could not be thionated byLR, while conjugated electron-donating groups like meth-yl p-methoxybenzoate or methyl furoate increased the rateof the reaction.44

3 Reactions with Carbonyl-Containing Com-pounds

3.1 Ketones

Aliphatic and aromatic ketones react readily with LR inanhydrous toluene under reflux.18,19

Scheme 3 Thionation of ketones with LR

In general thioketones 12 are the only reaction productsunder these reaction conditions, even if there are otherfunctional groups in the starting material (Scheme 3).18,19

Varma and Kumar45 reported on a rapid and solvent-freesynthesis of thioketones, thioflavones, and thioisofla-vones by mixing the substrates with Lawesson’s reagentwith subsequent exposure to microwave irradiation. Theyachieved high yields within 3–4 minutes (Table 1).

Weiß et al.49 described the regioselective thionation of 3-oxo-1,4-diene steroid systems. 3-Oxo-1,4-diene steroidsare dissolved in anhydrous THF, LR (0.6 equiv) is addedunder argon and the mixture is stirred for 1 hour(Scheme 4). Yields of 32–96% are achieved . Products14b–f are air and moisture sensitive (Table 2).

Dithiosquarylium dyes (DTSQ) have been synthesized bythe reaction of squarylium dyes (SQ) with LR.50 SQ dyes15a–c (2.5 mmol) and LR (2.5 mmol) are suspended in amixture of xylene and hexamethylphosphoramide (HM-PA). The suspension is refluxed for 5 hours achievingyields of 32–46% (Scheme 5).

Scheme 1 Mechanism of the thionation reaction using Lawesson’s reagent 1

MeO PS

S

S

P OMeS

P OMeS

SP OMe

S

S

O

R1

R2P OMe

S

S

O

R1

R2 P OMe

S

S

O

R1

R2 P OMe

S

O

S

R1

R2

+

2 2

+ LR

1 5 6

7

Scheme 2 Formation of p-methoxyphenylmetathiophosphonate 8

O

PO

P

OP

OMe

S

MeO

OMe

S

S

P OMe

S

O

3

8

Figure 3 Limitations of reactivity of Lawesson’s reagent

O

O

O

OO

O

OO

O

O

O

OO

O

SX

10 X=O11 X=S

9



Scheme 5 Thionation of squarylium dyes 15a–c with LR50

Strehlow et al.51 used LR to synthesize 2,4-bis(diphenyl-methylene)-3-thioxo-1-cyclobutanone (17) and 2,4-bis(diphenylmethylene)-1,3-cyclobutane-di-thione (18)(Figure 4).

Figure 4 2,4-Bis(diphenylmethylene)-3-thioxo-1-cyclobutanone 17 and 2,4-bis-(diphenylmethylene)-1,3-cyclobutane-dithione 1851

Horner and Lindel52 found that LR also converts phospho-nates and phosphates into the corresponding phosphono-thioates and phosphorothioates. Several research groupsreported on the thionation of the phosphoryl group P=Owith LR.53–56 Chiral phosphorus compounds led predomi-nantly to retention of configuration with LR.57

3.2 Esters and Lactones

Esters and lactones react readily with LR forming the cor-responding thiono, dithio esters, and thiolactones.18,19 Es-ters react at 140 ºC; while lactones react smoothly at 80 ºCwith LR (Scheme 6).

Thionoesters and thiolactones as described in Table 3 canalso be synthesized with LR by the microwave-accelerat-

Table 1 Solvent-Free Synthesis of Thioketones, Thioflavones, and Thioisoflavones

Product Time (min) Yield (%) Ref.

3 97 45,13

3 96 45,13

3 86 45,32

4 96 45,32

3 95 45,46

3 92 45,47

3 94 45,48

S

S

Br

S

O

S

O

S

O

S

O

S

O O

N

X

Me

Me

N

X

Me

Me

2+

15 a X=H b X=CH3

c X=C2H5

LR, xylene/HMPA,reflux, 5 h

2+

16 a (46 %) b (39 %) c (32 %)

S S

N

X

Me

Me

N

X

Me

Me

S

O

S

S

17 18

Scheme 6 Thionation of esters and lactones using LR

R

CO O

R1

R

CS O

R1

R

CS O

R1

R

CS S

R1

+ LR

19 20

+ LR

20 21

Scheme 4 Regioselective thionation of 3-oxo-1,4-diene 13 steroidsystems

R3R1

R2

Me

O

R3R1

R2

Me

S

LR/THF, r.t., 1h

13 14

Table 2 Synthesis of Cross-Conjugated 3-Thioxo-Steroids 14a–f 49

13, 14 R1 R2 R3 Yield (%)

a =O H 96

b =CHCO2Et H 40

c CH(Me)CO2OAc H H 44

d CH(Me)CO2Me H H 32

e OAc H H 60

f C≡CH OAc H 54



ed solvent-free method reported by Varma and Kumar.45

For the synthesis of thiolactones by this method only 0.5equivalents of LR are required. In the case of esters, 0.8equivalents of LR are needed.

A thiocoumarine dye was successfully prepared by react-ing the corresponding coumarine dye with LR.58 Lawes-son’s reagent was also used for the synthesis ofdihydrocoumarin.59

Lee et al.60 reported that various thionolactones 22 reactwith methyl bromozincacetate leading to the correspond-ing vinylogous carbonates 23 in good yields under mildconditions (Scheme 7) .

Scheme 7 Synthesis of vinylogous carbonates 23 from thionolac-tones 2260

Macrodithionolactones have been shown to be good start-ing materials for the construction of bicyclic systems.61

These can be synthesized by reaction of LR with macro-diolides followed by bridging across the macrocyclic ringby exposure to sodium naphthalenide, leading to stable bi-cyclic systems upon addition of methyl dioxide.

Schupp and co-workers62 as well as Bringmann and co-workers63 reported the preparation of thionolactone-bridged axially progesterogenic biaryls 24, depicted inFigure 5, by reacting the corresponding oxolactonebridged biaryl with LR.

LR can also be used to synthesize derivatives of sugars.Firstly, sugars have to be transformed into the correspond-ing lactones. They can then be transformed into thioestersby reaction with LR. Kahne et al.64 synthesized a hemithioortho ester 25 in two steps by reacting lactone 26 with 4equivalents of LR and crushed molecular sieves (3 Å) intoluene at 110 ºC for 4 hours (Scheme 8). The thionolac-tone 27 was subsequently converted to the correspondinghemithio ortho ester by refluxing in methyl iodide con-taining 10 equivalents of methanol and 2 equivalents of2,6-di-tert-butyl-4-methylpyridine (DBMP) for 12 h.

Scheme 8 Synthesis of hemithio ortho ester 25. Reagents and Con-ditions: a) LR, MS, toluene, 110 °C, 4 h; b) MeI, MeOH, 40 °C, 12 h,DMBP

3.3 Amides and Lactams

Thionation of aromatic, aliphatic, and unsaturated carbox-amides and lactams by LR into the corresponding thio de-rivates is a well-established process (Scheme 9).18,19 Theyields of thiocarboxamides reacted in HMPA at 80–100ºC are almost quantitative.

Scheme 9 Thionation of carboxamindes by LR

Olsson et al.65 developed a method for rapid parallel syn-thesis of a thioamide library by extending the microwave-assisted solvent-free procedure published by Varma andKumar.45

The intermediate amide library was synthesized by react-ing a diamine 28 with an acyl chloride 29 in ether. Afterremoving the solvent by filtration, LR was added to the

Table 3 Synthesis of Thionoesters and Thiolactones under Solvent-Free Conditions

Product Time (min) Yield (%) Ref.

3 85 45,18

3 96 45,18

3 98 45,18

3 94 45

3 92 45

S

OCH3

S

OCH2CH3

O S

O

H3CO

H3CO S

OS

OCH3

O

O

(CH2)n

R1

R2

S

BrZnCH2CO2Me

O

(CH2)n

R1

R2

CHCO2MeTHF/reflux/1 h

22 23

R1,R2=H, alkyl, aryl; n=1,2,3,4; yields from 63 to 78 %

Figure 5 Thionolactone-bridged axially progesterogenic biaryls 24

O

S

R

R

24a-c: a. R=Me, b. R=OMe, c. R=tBu



solid amide 30, mixed thoroughly and the mixture wasthen microwave irradiated for 8 minutes (Scheme 10). Af-ter solid phase extraction, thioamides (e.g. 31) were af-forded in adequate purities and yields.

Lawesson’s reagent is also used more and more in solidphase synthesis and combinatorial chemistry. Brookfiledand co-workers66 reported the synthesis of thiazoles 32 viatraceless cleavage of modified Rink resin bound sub-strates (Scheme 11). During the first step, a carboxylicacid 33 is attached to a Rink amine resin 34 to give a resinbound amide 35. The resin bound amide is then treatedwith 3 equivalents of LR in refluxing THF for 4 hours

yielding the corresponding thioamides 36. The reactioncan be monitored by IR analysis of the resin beads. Theresin bound thioamide is reacted with 0.6 equivalents ofan a-bromoketone 37 in THF at reflux for 16 hours, caus-ing cleavage of the resin and formation of a thiazole 32.

Lawesson’s reagent together with solid supported synthe-sis afforded functionalized 1,2,4-triazin-6-ones 38(Scheme 12).67 Blass et al. used commercially availableMerrifield resin bound, Boc-protected amino acids 39 andconverted them to the corresponding amide 40 by depro-tection and condensation with an appropriate acid chlo-ride. The resin bound amide was reacted with LR intoluene at 75 ºC to give the thioamide 41. Resin cleavageand cyclization with a 2% solution of hydrazine in 2-pro-panol at 75 ºC for 24 hours affords 1,2,4-triazin-6-ones 38in excellent yields and purities.

3.4 Amino Acids, Peptides, Nucleosides, and Nu-cleotides

During the last two decades, the importance of LR as athionation reagent for amino acids and peptides has grownsignificantly. The replacement of amide bonds in physio-logically active peptides with thioamide bonds is one ofseveral backbone modifications used frequently in thesearch for more potent and/or selective compounds thanthe parent structures. LR is a promising thionation reagentfor peptides and amino acids due to the possibility of se-lective transformations based upon reaction temperature.LR reacts with lactones and lactams at 80 ºC, with ketonesand urethanes at 110 ºC, and with acylic esters at 130ºC.18,68 The temperature-dependent reactivity of LR en-ables the synthesis of thioamides from amides in the pres-ence of urethanes or esters, which are often found inamino acids and peptides. LR can react with nucleophilessuch as amines, alcohols, phenols, and thiols above cer-tain temperatures; therefore peptides that contain thesefunctionalities have to be protected adequately prior tothionation reactions. However, the size of peptide sub-strates for thionation by LR is usually limited to two ami-no-acid residues, owing to the problem of regioselectivityand solubility with larger peptides.69

Wang and Phanstiel70 presented a vivid example for thetemperature-dependent reactivity of LR. N-(2-phenyl-ethyl)-N-(benzoyloxy)acetamide 42 reacts with LR in

Scheme 10 Microwave-assisted solvent-free parallel synthesis ofthioamides.65 Reagents and Conditions: a) Et2O, r.t., 15 min; b) filtra-tion; c) LR, microwave irradiation, 8 min; d) solid-phase extraction

Scheme 12 Solid-phase synthesis of 1,2,4-triazin-6-ones.67 Reagents and Conditions: a)TFA, CH2Cl2, r.t., 1 h; b) Et3N, CH2Cl2, r.t., 1 h, thenR2COCl, i-Pr2NEt, CH2Cl2, r.t., 24 h; c) LR, toluene, 75 °C, 5 h; d) 2% hydrazine in i-PrOH, 75 °C, 24 h; Purified yields 23–75%

Scheme 11 Thiazole formation via traceless cleavage of Rink resin.Reagents and Conditions: a) DIC, DMAP; b) LR, THF, reflux, 4 h;c) THF, reflux, 37, 16 h



anhydrous THF at room temperature to give N-(2-phenyl-ethyl)-N-(benzoyloxy)thioacetamide 43 (Scheme 13). LRconverts amides to thioamides at room temperature butdoes not usually react with esters under these conditions.

Steric hindrance in the amide bond region can also influ-ence the reactivity of LR. Jensen et al.71 showed thatthionation of 44a with LR proceeded smoothly at roomtemperature to give 45a. The peptide 44b could not betransformed under these conditions due to the steric hin-drance caused by the gem-methyl groups of the amidebond region. However, 44b reacted with LR in toluene at100 ºC to give 45b (Scheme 14).

Scheme 13 Selective thionation of N-(2-phenylethyl)-N-(ben-zoyl)acetamide 42

Scheme 14 Thionation of peptides with LR

As mentioned before, thionation of peptides with LR isusually limited to smaller peptides. However, Itokawa andco-workers72,73 proved that even relatively large cyclicpeptides can be thionated by LR. They reacted the astinsA, B, and C with 2 equivalents of LR in anhydrous diox-ane at 50 °C for 12 hours to give the thioastins A, B, andC in moderate yields. In each case, thionation with LRgave a single major product; only the amide carbonylgroup of Serine was thionated. Morita et al.74 also per-formed thionation reactions on the cyclic peptides segeta-lins A and B with 3 equivalents of LR in anhydrousdioxane at 50 °C. These reaction conditions afforded twodithionated segetalins A, thiosegetalins A1 and A2, andtwo dithionated segetalins B, thiosegetalins B1 and B2.Seebach et al.69 successfully reacted LR with cyclosporinA, a neutral, cyclic undecapeptide containing only lipo-philic amino acids, seven of which are N-methylated.Cyclosporin A was treated with 3–5 equivalents of LR

in 3,4,5,6-tetrahydro-1,3-dimethylpyrimidin-2(1H)-one,DMPU, at room temperature for 2–4 days; four majorproducts were formed.

Guziec and Mayer Wasmund75,76 encountered some diffi-culties due to the insolubility of LR during the synthesisof thiopeptides. They saw the need to develop milder andmore selective conditions for the conversion of peptides tothiopeptides. Instead, they used the thionating reagentphenyl phosphoro tetrathioate (Japanese reagent) 46(Figure 6).77

Figure 6 Phenyl phosphorotetrathioate (Japanese reagent) 46

Guziec and Mayer Wasmund observed that thiopeptidescould readily be prepared from their corresponding pep-tides using 46 in THF at room temperature. Table 4 showsthat yields obtained using 46 were significantly higherthan those obtained using LR. They also found selectivityincreased by using reagent 46.

Thioamino acids can also readily be prepared by reactionof the corresponding amino acids with LR. Larsen et al.78

transformed 5-oxo-L-proline 47 into 5-thioxo-L-proline48 by thionation of the lactam function with LR in1,2-dimethoxyethane (DME) at room temperature(Scheme 15). Protection of the carboxy group of 5-oxo-L-prolin is not necessary, as nucleophilic attack by the car-boxy group on LR does not occur at room temperature.

Scheme 15 Synthesis of 5-thioxo-L-proline 48

H3C

O

N

O O

H3C

S

N

O OLR, THF, r.t.

42 43 (76 %)

Table 4 Comparison of Peptide Thionating Reagents

Thiopeptide Yield (%)

Lawesson’s Reagent

Reagent 46

Z-Gly-≡(CSNH)-Gly-OMe 69 75

Z-Gly-≡(CSNH)-Gly-OEt 69 75

Z-Gly-≡(CSNH)-Phe-OEt 72 94

Boc-Ala-≡(CSNH)-Gly-OMe 35 61

Boc-Ala-≡(CSNH)-Phe-OMe 29 62

Boc-Gly-≡(CSNH)-Gly-OMe 24 93

Boc-Leu-≡(CSNH)-Gly-OMe 60 66

Boc-Gly-≡(CSNH)-Val-OMe 49 59

S PSS

SP

S

S

46

HNO

OH

HNS

OHLR, DME, r.t.

47 48 (63 %)

O O



Lawesson’s reagent can also be used as a racemization-free coupling reagent in peptide synthesis.79–81 Scheme 16shows a selected example of this new coupling reaction.

Z-S-Pro-S-Val-S-Pro-OtBu, Z-S-Pro-R-Val-S-Pro-OtBu,Z-S-Leu-S-Phe-S-Val-OtBu, and Z-S-Leu-R-Phe-S-Val-OtBu were prepared by [2+1] segment coupling reactionsfrom the appropriate peptides with LR. The stereomericproducts are separated by HPLC; for Z-S-Pro-S-Val-S-Pro-OtBu and Z-S-Leu-S-Phe-S-Val-OtBu, only a smallamount of epimerization was detected, 0.5 and <0.1%,respectively.

LR is also used to prepare nucleoside and nucleotide de-rivatives. Robins and co-workers82 synthesized a 5-thio-carboxamide derivate of pyrazofurin 49 by treatment ofthe triacetyl derivate of 50 with LR in boiling dioxane(Scheme 17).

Scheme 17 Synthesis of pyrazofurin derivates

This reaction gave the unstable thioamide 51, which with-out isolation was deacetylated with MeOH/NaOMe toyield a mixture of 4-(benzyloxy)-3-β-D-ribofuranosyl-pyazole-5-thiocarboxamide 49 and the corresponding ni-trile derivate 52.

Rios-Ruiz and co-workers83 reported the direct thionationof 7-theophylline nucleosides. They prepared 6-thio-threophylline nucleosides from the corresponding threo-phylline nucleosides by treatment wit LR in refluxing tol-uene. Only 6-thio-threophylline nucleosides are formed as

the oxo groups in C-2 have shown to be less reactive thanthose in C-6. Regioselective thionation of other nucleo-sides by LR have also been reported by Nielsen and co-workers84 and Felczak et al.85

3.5 Macrocycles and Polymers

Macrocycles

Thioxo derivates of triolide and pentolide 53 can be syn-thesized by reaction of triolide and pentolide 54 with LR,respectively (Scheme 18).86 Thionation with LR yields amixture of mono-, di- and trithiotriolide, and thiopen-tolide derivatives. The ester group is transformed into athionoester functionality.

Scheme 18 Synthesis of thioxo derivates of triolide and pentolide86

Thiotriolides and thiopentolides can be used to synthesizea variety of compounds. Reduction of thiotriolides gives12-membered rings containing up to three ether groups(chiral crown ethers). They also react spontaneously withammonia, certain primary amines, and hydroxylamine togive imine and oxime derivates with 12-membered ringbackbones. Clyne and Weiler87 published a detailed studyof the synthesis of 14-membered ring monoethers(Scheme 19). The preparation of these macrocyclic ethersinvolved the Baeyer–Villinger ring expansion of a cyclicketone to a lactone. The lactone carbonyl was removed byconversion to an intermediate thionolactone, by reactionwith LR and reduction with tri(n-butyl)tin hydride.

Scheme 19 Synthesis of 14-membered macrocyclic ethers.Reagents and Conditions: a) UHP, TFAA, Na2HPO4, CH2Cl2, 0 ºC,96%; b) LR, toluene, reflux, 73%; c) LiEt3BH, THF, –78 ºC, thenMeI, 91%, d) MeLi, THF, –78 ºC, then MeI, 90%; e) n-Bu3SnH,AIBN, toluene, reflux, 43% (59) or 63% (61)

Scheme 16 [2+1] segment coupling reaction of peptides with LR

P

S

S

OCH3

NHEt3

Z-AA1-AA2-OHi: NEt3

ii: 1/2 LRZ-AA1-AA2-O

i: HCl·H-AA3-Ot-Bu

ii: 2 NEt3Z-AA1-AA2-AA3-OtBu

AA1-AA2-AA3 = Pro-Val-Pro, Leu-Phe-Val

(23 - 39 %)

N

NH

BnO

H2N

O

OHO

OH OH

N

NH

BnO

H2N

S

OAcO

OAc OAc

N

NH

BnO

H2N

S

OHO

OH OH

N

NH

BnO

NC

OHO

OH OH

+

49

50 51

52

O

O

O

O

OO

MeMe

Me

O

X

X

O

XO

MeMe

Me

54 53 (2-42 %)

n-2

n = 3 (triolide), n = 5 (pentolide)

n-2

X = S or O

LR, o-xylene

O

O

O

O

R1

R2

a c or d

55 56, X = O57, X = S

58, R1 = H, R2 = SMe59, R1 = R2 = H60, R1 = Me, R2 = SMe61, R1 = Me, R2 = H

b e

e



Polymers

In polymer chemistry, Lawesson’s reagent is used to alterthe backbone of polymers or for the synthesis of cyclicpolymers.

Delêtre and Levesque88 transformed polyamides intopolythioamides by using LR in toluene at 100 ºC. Thismodification reaction needs finely divided polymer sam-ples to work efficiently. Wang and Zhang89–92 reported onthe intramolecular cyclization of 2,2¢-dibenzoyl-biphenylunits into phenanthrene groups using LR. The cyclizationreactions were carried out in 1,1,2,2-tetrachloroethane(TCE) at reflux for 2 hours.

Steliou et al.93 suggested a possible pathway, shown inScheme 20, for the enthalpically favored cyclization pro-cess. Firstly, a dithioketone 62 is formed that then rear-ranges to the 1,2-dithietane intermediate 63. Formation ofa carbon-carbon double bond upon extrusion of diatomicsulfur yields the poly(arylene ethers) 64.

Sato et al.94 reported on the intramolecular cyclization ofprecursor polyhydrazides with LR to conjugated poly-mers constituted of 1,3,4-thiadiazole and 2,5-dialkoxy-benzene units 65 (Scheme 21). The precursorpolyhydrazides were prepared from hydrazine and 2,5-di-alkoxyterephthalic acids 66 by direct polycondensation.The new conjugated polymer possesses an extended p-conjugated structure and suitable levels of band gap ener-gies.

Wynberg and co-workers synthesized polythiophenes 67from poly(1,4-diketones) 68 by thionation with subse-quent cyclization (Scheme 22).95 Hempenius et al. used asimilar approach to prepare a block copolymer with a longarray of 11 b-unsubstituted thiophene rings.96

4 Cyclization Reactions to Thiophenes, Thia-zoles, and Other Compounds

2,5-Disubstituted thiophenes are readily accessible via re-action with LR. The thiocyclization of 1,4-diketo com-pounds, depicted in Table 5, leads to the correspondingthiophenes.

Nishio postulated a mechanism for the cyclization of g-keto amides with LR to thiophenes (Scheme 23).99 Thion-ation of the two carbonyl groups is followed by ring clo-sure and subsequent elimination of H2S.

Ong et al. synthesized 2,5-disubstituted thiophenes 71 inhigh yields by reacting 1,6-dioxo-2,4-dienes 72 with LRand subsequent cyclization (Scheme 24).105

Scheme 20 Proposed cyclization pathway93

O C

O

C

O

O Ar

O C

S

C

S

O Ar

S S

O O

O O Ar

Ar

n

LR, TCE,

reflux, 2h

n

n n

- S2

62

63 64

Scheme 21 Synthesis of conjugated polymers composed of 1,3,4-thiadiazole and 2,5-dialkoxybenzene rings 65.94 Reagents and Condi-tions: a) NH2NH2·HCl, NMP, 4 h

Scheme 22 Thionation of poly(1,4-diketones) 6895

O O

S

LR

n n68 67



Scheme 23 Postulated mechanism by Nishio99

Higher substituted and annellated thiophenes preparedfrom substituted 1,4-diketones are depicted in Table 6.

Kang and Sun converted b,g-epoxycarbonyl compounds73 directly into their corresponding thiophenes 74 in thepresence of LR and a catalytic amount of p-toluene sul-fonic acid (Scheme 25).114,115

Scheme 25 Thiophenes 74 from b,g-epoxycarbonyl compounds 73.Reagents and Conditions: a) LR, p-TsOH (cat.), benzene, reflux114

Hörndler and Hansen used LR for the direct cyclization of8-isopropyl-11-methylheptaleno[1,2-c]furan-6-carbalde-hyde (75) to the annellated thiophene, 4-isopropyl-1-me-thyl-2H-thieno[4,2:5,6] heptaleno[1,2-c]furan (76), in61% yield (Scheme 26).116

Scheme 26 Treatment of carbaldehyde 75 with LR. Reagents andConditions: a) 2 equiv LR, toluene, 100 ºC, 30 min, 61%

Fang and co-workers described a cyclization of indole 77using LR. Treatment of 1,4-hydroxyketone yielded 3-(4-methoxyphenyl)thieno[3,4-b]indole (78) (Scheme 27).117

Scheme 27 Preparation of thieno[3,4-b]indole 78.117 Reagents andConditions: a) 1.33 equiv LR, 1,4-dioxane, reflux, 4 h, 63%

Özturk118,119 and Kaynak et al.120 reported on the treatmentof thiophenes 79 from 1,8-diketones 80. Desulfuration ofintermediates 1,4-dithiins 81 resulted in thiophenes 79(Scheme 28).

The preparation of trithiapentalenes 82, shown inScheme 29, by thionation of keto dienamines 83 has beenreported by Zhan and Henry.121 Closs et al. cyclized anunsaturated 1,3-diester 84 with LR (Scheme 30).122

Table 5 Syntheses of 2,5-Disubstituted Thiophenes

Conditions Ref.

LR 97–99

LR, toluene, 3 h stir, 1 h reflux, r.t.

100

1.2 equiv LR, MW irradiation, no solvent

99

0.6 equiv. LR, toluene, reflux, over night, 70%

101

0.6 equiv. LR, toluene, reflux, over night, 75%

101

2.1 equiv LR, toluene, reflux, 15–30 min, 57%

102

LR, toluene,reflux, 2 h,92%

103

0.6 equiv LR, toluene, reflux, 1.5 h,55%

104

O O

R1 R2 SR2R1

a

BrR1= R2= OC12H25

R1= R2= -CH2OBn

BrR1= R2= OC2H5

OR1= R2=

O SR1= R2=

R1= Ph R2= N(CH2Ph)2

S SR1= R2=

NH

COOEt

EtOOC

R1=R2=

R1

OO NR2R3

R1

S

S

NR2R3

S

S

R1 NR2R3

S

HS

R1 NR2R3SR1 NR2R3 -H2S

LR

H Transfer69

70

R3

O

R1

O

R2S

R2 R1

R3

a

R1= H, R2= CH2Cl, R3= (CH3)2CHCH2; 64%R1= CH3, R2= H, R3= Ph; 83 %

73 74

Me

Me

O

CHOMe

Me

O

S

a

75 76

N

Me

O

H

OH

OMe

N

Me

S

OMe

a

77 78

Scheme 24 2,5-Disubstituted thiophenes 71 from 1,6-dioxo-2,4-dienes 72; yields from 2–93% with different ratios of a:b105

SMe

O

R

NO2 OMe

LR

R= Me, Ph, OEt, ,

71a72

S R

O

Me+

71b

MeO

OR



Table 6 Syntheses of Higher Substituted and Annellated Thiophenes

Conditions Ref.

LR, quantitative 106

1.2 equiv LR, toluene, reflux, 6 h, 82% 107

LR, toluene, 83% 108

LR, benzene/dimethoxyethane, reflux, 99% 109

1.35 equiv LR, toluene, reflux, 2 h, 83% 110

2 equiv LR, toluene, reflux, 4 h, 61% 111

LR, xylene, reflux, 60% 112

0.5 equiv LR, reflux, 2 h, 78% 113

O O

R1 R2

R3 R4

SR2R1

R3 R4

a

ClR1= R2= R3= -S(CH2)3CH3 R4=H

SR1= R2= R3= -(CH2)3CN R4 = H

O2NR1= R2= R3= R4=

O

O

OMe

S

OMe

a

O OS S S SS

a

S

SH

Th

Th Th

H

Th

Th Th

O O

OO

ThTh a

Th = thiophene

N

S O

PhS

O

Cl

S

NS

S

Ph

Cl

a

Scheme 28 Preparation of thiophenes 79 from 1,8-diketones80118,120

Scheme 29 Preparation of trithiapentalenes 82.121 Reagents andConditions: a) LR, benzene or toluene, reflux, 0.5–1.5 h

N NMe Me

Me Me

R

SS S

R

a

R= Et (49%)R= Ph (39%)R= tBu (40%)R= CO2Et (37%)R= CF3 (32%)

83 82



Two research groups, Karakasa et al.123,124 (Scheme 31)and Moriyama and Motoki125 (Scheme 32), reported onthe thionation of a,b-unsaturated thioketones (e.g. 85) andfollowing dimerization yielding the novel compounds 86and a spiro-compound (Scheme 32), respectively.

Scheme 31 Thionation followed by dimerization.123 Reagents andConditions: a) LR, CS2, reflux, 5 h under nitrogen, 46%

Scheme 32 Thionation followed by dimerization.125 Reagents andConditions: a) LR, CS2, reflux

Hegab described a similar reaction for the preparation ofa spiro-compound (Scheme 33). [2+4]-Cyclization wasfollowed by desulfuration, the spiro-compound 87 couldthen be isolated.126

Scheme 33 Thionation and dimerization126

Rufanov et al. treated 1-indanone 88 with LR(Scheme 34). The product 89 was obtained in 95%yield.127

Nakayama et al. synthesized 2,6-diaryl-1,4-dithiins 90from the corresponding sulfide 91 with LR(Scheme 35).128

Scheme 35 Preparation of 2,6-dithiophenyl-1,4-dithiin 90.128 Con-ditions: a) LR, benzene, 65%

Thionation of keto aldehyde 92 followed by cyclizationwas described by Ishii et al. (Scheme 36).129

Scheme 36 Cyclization of keto aldehyde 92129

Nishio and Sekiguchi studied thionation reactions of d-and w-hydroxy amides 93 and 94 (Scheme 37). Treatmentof the alcohols with LR resulted in 5- and 6-memberedsulfur-containing heterocycles 95 and 96.130

Scheme 37 Cyclization of d- and w-hydroxy amides 93 and 94130

Thiazoles and other sulfur-containing heterocycles can beobtained readily with LR. Table 7 shows selected exam-ples for the synthesis of thiazoles and thiazolthions. Se-lected examples of additional sulfur-containingheterocycles prepared with LR are depicted in Table 8.

The preparation of pyridine-2(1H)-thiones was describedby Soto et al.141 Cyclization of the intermediate 2-cyan-othioacetamide (98) led directly to the otherwise not eas-ily accessible pyridinthione (e.g. 97 in Scheme 38).

Milwska et al. used LR for the preparation of thioimides99. Selective thionation of amide 100, as described inScheme 39, followed by cyclization yielded thioimide99.142

CHSPh

Ph O

CHSPh

Ph SS

S

SPhPh

SPh

Ph

a 1/2

85 86

O

SPhS

S

SPh SPh

a

OS

S

S

S

S

LR

87 (45 %)

SO

S

O

S

S

SS S

a

91 90

Ph O

Me

Me Me

Me

O H S

S

S

Me

Me Me

MeLR

(35 %)

92

Ph

OH

MeO

NH

Me

S

Me

Ph S

Ph

O

NHPh

OH

S SPh

LR

LR

(48 %)

93 95

94 96

(5 %)

Scheme 34 Cyclization of 1-indanone 88127

O

SSLR

89 (95 %)88

1/3

Scheme 30 Cyclization of an unsaturated 1,3-diester 84 with LR122



Table 7 Preparation of Thiazoles

Cyclization Reaction Conditions Yield (%) Ref.

LR 56 131

LR 92 132

LR 90 133

LR 47, <1 134

LR 94 134

LR 30 134

1.1 equiv LR, toluene, reflux, 115 h 70 136

LR 137

LR 71–74 138

LR, THF, reflux 66 139

LR 52 140



Table 8 Preparation of Other Sulfur-Containing Heterocycles


LR, toluene, 110 °C 90 143

LR 59 144

LR 56 144

LR 64 145,146

LR, toluene, reflux, 45 min 34 147

LR, toluene, 100 °C, 2 h 93 148

LR, benzene, reflux, 24 h 46 149

LR, benzene, reflux, 18h 76 149



Scheme 38 Synthesis of pyridine-2(1H)-thione 97141

Scheme 39 Preparation of a thioimide 99142

5 Synthesis of Heteroatom-Containing Com-pounds

5.1 Sulfur-Containing Heterocyclic Compounds

This chapter discusses the preparation of sulfur-contain-ing heterocyclic compounds with LR other thanthiophenes and thiazoles.

El-Barbary et al. formed a mixture of 1,3-benzodithiane-4-ones 101 and 1,2-benzodithiole-3(H)-thiones 102 afterthe reaction of 1,3-benzoxathian-4-ones 103 with LR(Scheme 40).152

In 1990, Adam et al. reported the treatment of b-lactone104 with LR. The postulated mechanism in Scheme 41shows a [4+2]-cyclization to a six-membered ring 105.Ring-openning between the sulfur-phosphorus-bondleads to a 1,6-dipole 106, which cyclizes to the b-S-thi-olactone 107.153

5.2 Phosphorus- and Sulfur-Containing Hetero-cycles

The ability of LR to produce 4-, 5-, 6- and 7-memberedP,S-heterocycles is shown below. Although mostly 5- and6-membered ring-systems are described in the literaturethere are some examples for 7-membered and few exam-ples for 4-membered rings.

4-Membered P,S-Heterocycles

Mahran and co-workers treated a,b-unsaturated nitriles108 with LR. They received the corresponding thionated

LR 50 150

LR 70 151

Table 8 Preparation of Other Sulfur-Containing Heterocycles (continued)


OPh

Ph

CN

CN

OPh

Ph

CSNH2

CN

NH

Ph

CN

SPh

LR

98 97 (68 %)

NH

Me Me

Me

O

S

COOH

CONH2Me

Me Me

aa. LR, toluene, reflux, 3 h, 30 %

100 99

Scheme 41 Synthesis of a b-S-thiolactone 107153

O

O

R

RO

O

S P

Ar

S

O

PS

ArS

R

R

O

P

ArS

S

O

R

R

S

O

a

104 105

106107 (37 %)

Scheme 40 Treatment of 1,3-benzoxathian-4-ones 103 with LR152

O

S

O

R

HO

S

S

R

HS

S

S

R

H

S

S

S

R

HS

S

S

R

H SS

S

LR

LR+

R= a. (CH2)10CH3

b. CH3

c. C6H5

103

101a. 30 %b. 20 %c. 39 %

102a. 18 %b. 22 %c. 19 %

a. 41 %b. 49 %c. 25 %



nitriles 109a and 4-membered thiaphosphetanes 109b asbyproducts (Scheme 42).154


Ibrahim et al. reported on the preparation of thiaphospho-lotriazines 110 by heating 1,2,4-triazin-6(1H)-ones 111and LR for 3 hours in toluene in the presence of morpho-line.155 Scheme 43 shows an example for the synthesis of

thiaphospholotriazine 110 with an optimized yield of85%.

Two research groups, He et al.156 and Shabana et al.,157

prepared dioxaphospholanes by treatment of diols withLR (Table 9).

5-Membered P,N,S-heterocycles were synthesized by re-action of a-chlorothioamides 112, substituted 2-amino-1,4-naphthoquinones 113 and a-hydroxynitriles 114(Table 10).

Table 10 Synthesis of Thiazaphospholidines

Thiazaphospholidines Conditions Yield (%) Ref.

LR 55 158

LR 70 159

LR 85 160–162

Table 9 Preparation of Dioxaphospholanes

Synthesis of Dioxaphospholanes Conditions Yield (%) Ref.

LR 32 157

LR 85 156

Scheme 43 Synthesis of thiaphospholes110.155 Reagents and Con-ditions: a) LR, toluene, morpholine, 85%

N

N

HNO

p-MeO-C6H4

N

NHN

p-MeO-C6H4

SP

S

MeOa

111 110

Ph Ph

Scheme 42 Thionation of a,b-unsaturated nitriles 108; thiaphos-phetanes 109b as byproducts154

R

CH

CNC CN SH2N

NC

RS

P CNNC

MeO

R

109a

LR+

R= % %2-OH-C6H4 a:70 b:204-MeO-C6H4 a:70 b:254-NO2-C6H4 a:50 b:302-thienyl a:70 b:20

108 109b



Boulos and Ab-del-Malek demonstrated a novel route forthe preparation of benzodiphenoquinone-bisthiazaphos-pholine derivatives from 2-amino-1,4-naphthoquinone(113) and LR.159 Deng and co-workers synthesized 5-membered P,N,S-heterocycles in high yields by reactinga-hydroxynitriles with LR.161

Diazaphopholines were built by treatment of phenylhy-drazones 115, a-aminonitriles 116, and -aminoamides 117with LR (Table 11). Deng et al. showed an efficient meth-od for the synthesis of 1,3,2-thiazaphospholidine-4-thione2-sulfide. a-Aminonitriles 116 and LR led to the corre-

sponding 5-membered spiro-N,P,N-heterocycle in 70%yield.

Addition of LR to hydrazones 118 or a hydroxyethy-lamine 119 led to their corresponding azaoxaphospho-lidines 120 in moderate yields (Table 12).

Boulos et al. used p-quinone monoimines 121 (1 equiv)and LR (0.5 equiv) for the preparation of oxathiophospho-lines 122, refluxing in toluene gave the colorless crystal-line adduct 122 (Scheme 44).176

Table 11 Preparation of Diazaphospholines

Synthesis of Azaoxaphospholines Conditions Yield (%) Ref.

LR 72 160,163,164

LR 70 165,166

LR 25 167,168

LR 51 169–173

Table 12 Preparation of Azaoxaphospholines 120

Synthesis of Azaoxaphospholines Conditions Yield (%) Ref.

LR 72 163,174

LR 73 175



In 2001, Hegab et al. were able to prepare dithiaazaphos-pholes 123 in low yields by adding LR to thione-S-imides(Scheme 45).177

Dubau-Assibat described a 1,3-dipolar cyclization of N-(t-butyl)-C-phenyl nitrone 124 with LR giving oxathiaza-phospholines 125 (Scheme 46).178 Two equivalents of 124were added to a suspension of LR in THF at room temper-ature yielding oxathiazaphospholines 125 after a few min-utes.

Access to thiadiazaphospholines 126 can be obtained byreaction of LR with phenylhydrazone or 3-mercapto-4-amino-1,2,4-triazoles 127 (Scheme 47).179

Scheme 47 Preparation of thiadiazaphospholine 126179


Testa et al. used LR for the cyclization of 3-aminopro-penenitriles 128 (Scheme 48).180 A mixture of enaminoni-trile 128 (1 equiv) and LR (1.2 equiv) in anhydroustoluene was refluxed for 1 hour yielding the 1,3,2-thiaza-phosphorin-4-thione 130a.

Hafez et al. described the cyclization of 5-amino-1-meth-yl-1H-pyrazole-4-carboxylate 129 with LR to thiazaphos-phorines 130b (Scheme 49).181

Scheme 49 Cyclization of 5-amino-1-methyl-1H-pyrazole-4-car-boxylate 129181

In 1997, He and Zhuo reported on the [4+2]-cycloadditionof 2-methyl-1,3-butadiene (131) with LR (0.5 equiv)yielding a 6-membered phosphorus and sulfur containingheterocycle with potential herbicidal activity(Scheme 50).182

Scheme 50 Cyclization of 2-methyl-1,3-butadiene 131182

Shabana et al. synthesized 6-membered P,O-heterocycles132a,b in good yields by cyclization of o-hydroxy-ace-tophenones 133a,b with LR (Scheme 51).183 A mixture ofvisnaginone (133a) or khillinone (133b) (1 equiv) togeth-er with LR (0.5 equiv) was refluxed in toluene for 8 hoursto give the corresponding 1,2-oxaphosphinane derivatives132a,b.

NN

N

NH2

SHMe

NN

N

HN P

SS

OMe

MeLR

127 126 (38 %)

Scheme 48 Cyclization of 3-aminopropenenitriles 128.180 Reagentsand Conditions: a) 1.2 equiv LR, toluene, reflux, 1 h, 31%

CN

NH2O

MeNH

P

S

S

S

OMe

OMea

128 130a

NN

Me

NH2

O

OC2H5

NH

P

SN

N

Me

S

OMe

LR

129 130b (85 %)

MeP

SS

OMe

Me

LR(53 %)

131

Scheme 44 Synthesis of oxathiophospholines 122176

O

NSO2CH3 NHSO2CH3

S

PO

S

OMeLR

121 122 (80 %)

Scheme 45 Synthesis of dithiaazaphospholes 123177

O

O

Me

Me

S

N1-adamantyl

O

O

Me

MeS

NP

S

S

1-adamantyl

OMe

LR

123 (9 %)

Scheme 46 Synthesis of oxathiazaphospholine 125178

N

Ph

H

Me Me

Me

O S P

ON

S

MeO

Me

MeMe

PhLR

124 125 (95 %)



He et al. described another route for the preparation ofspiroheterocycles (Scheme 52). Treatment of pentaerythi-tol 134 (2.2 equiv) with LR (1 equiv) in anhydrous MeCNat reflux led to the corresponding spiro-compound diox-aphorphorinane 135.184

In 1987, Bryce and Matthews prepared the benzophos-phadiazine 136 by cyclization of 2-aminobenzamide 137(1 equiv) with LR (4 equiv) (Scheme 53).185 The reactionmixture was dissolved in toluene and stirred at room tem-perature for 24 hours.

Scheme 53 Cyclization of an o-aminobenzamide 137185

Bertrand and co-workers used 2-phosphorus-substituted2H-azirines 138 and LR for the synthesis of 1,3-thiazad-iphosphines 139 (Scheme 54).186 Compound 138 under-goes ring-extension reactions, in which the C,N-singlebond is broken and enables an efficient route for the syn-thesis of 6-membered P,S,P,N-heterocycles.

7-Membered P-Heterocycles

In the literature there are few examples for the synthesisof 7-membered P-heterocycles. He et al. reported on thecyclization of salicylaldehyde hydrazones 140 (2 equiv)with LR (1 equiv) to oxadiazaphosphepin-2-sulfides 141(Scheme 55).187

Moustafa published the syntheses of oxadiazaphospho-rine-6-sulfide 142b and triazaphosphorine-6-sulfide 143bderivates.188,189 Reaction of compounds 142a and 143a (1equiv) with LR (0.5 equiv) in MeCN under reflux for 5hours afforded 142b and 143b. These compounds can alsoreact with halo compounds (e.g. methyl iodide or benzylchloride) to give the corresponding S-alkylated deriva-tives (Scheme 56).

5.3 Other Phosphorus- and Sulfur-Containing Compounds

Non-heterocyclic P,S-containing compounds can be ob-tained directly from LR. Shabana et al. studied the reac-tion of LR with sodium salts of different alcohols andPhSCl (Scheme 57).190,191 A study on the reactions of LRwith alcohols, phenols, and thiols was also published byShabana et al.192,193

Scheme 52 Cyclization of pentaerythitol 134184

HO OH

HO OH

P

O

O

O

P

O S

OMe

MeO

S

LR, CH3CN, reflux

134

135 (36%)

NH2

O

NH2 NH

P

HN

S

OMe

LR, toluene,

137 136 (28 %)

r.t., 24 h

Scheme 54 Preparation of 1,3-thiazadiphosphines 139186

NPh

SiMe3P

(c-hex)2N

(c-hex)2NP

S P

N

S

OMe

Me3Si Ph

(c-hex)2N

(c-hex)2N

LR

138 139 (83 %)

Scheme 55 Cyclization of hydrazones 140187

N

OH

NHR

O P

N

N

S

OMe

RLR, benzene, reflux, 5 h

140 141R=H 30 %Me 26 %Ph 35 %

Scheme 56 Syntheses of oxadiazaphosphorine-6-sulfide (X = O)142b and triazaphosphorine-6-sulfide (X = NH) 143b188,189

N

XH

N

N

NH2

SHX

PNH

N

NN

S

MeO

SHLR

X=O 142b (85 %)NH 143b (82 %)142a, 143a

Scheme 51 Cyclization of o-hydroxy-acetophenones 133a,b183



The reaction of LR with disulfides 144 was described byNizamov et al. and yielded (4-methoxy-phenyl)phospho-notrithiolothionates 145 (Scheme 58).194

Scheme 58 Treatment of LR with disulfides 144194

The reaction of LR with alkyl borates,195,196 trialkylsilyland stannyl derivatives,197,198 and with arsenic(III)alkoxides199 was examined by Nizamov et al. He et al.studied the treatment of phenylthiourea and oxamide withLR.200

6. Synthesis of Organometallic Compounds

6.1 Transition Metals

Sato and Asai thionated diferrocenyl ketone 146 with LRunder conventional conditions to give diferrocenylthioketone 147 as violet crystals in 71% yield(Scheme 59).201 Diferrocenyl thioketone 147 then reactedwith Zn–TiCl4 in THF to give the coupling product 148 ina low yield.

Scheme 59 Thionation of diferrocenyl ketone 146 with LR201

They also showed that it is possible to react [1.1]ferro-cenophane-1,12-dione (149) with LR. Using 1.5 equiva-lents of LR leads to the dithio derivative 150 in 90% yield(Scheme 60). Using half an equivalent of LR, the mono-thio derivate 151 together with 150 were produced in 46%and 13% yields, respectively.

Beer et al.202 converted [(n-butylamino)carbonyl]fer-rocene (152) to [(n-butylamino)-thiocarbonyl]ferrocene(153) by refluxing with LR in toluene (Scheme 61). Theyfound that the thioamide-based receptor 153 binds halide

anions more effectively than its carboxamide analogue152.

A paper on the chemistry of ferrocenoyl derivatives hasbeen published by Imrie et al. (Scheme 62).203 They reportthat ferrocenoyl imidazole 154 reacts with LR to give di-ferrocenoyl disulfide 155.

Chiral ferrocenyl-thiazoline ligands 157 have been syn-thesized starting from a-azidoacetyl ferrocene and 1,1¢-bis(2-azidoacetyl)ferrocene by a four-step sequence in-volving enantioselective borane reduction, catalytic hy-drogenation, acylation, and cyclization promoted byLR.204,205 The last step of this sequence is shown inScheme 63.

Scheme 63 Conversion of ferrocenyl b-hydroxyamides 156 to thecorresponding ferrocenyl-thiazolines 157

2 HS-SR MeO P

S

S-SR

SR

LR R=a. Et 40 %b. i-Bu 30 %

144 145

O

Fe Fe

Fc

Fc

Fc

Fc

Zn-TiCl4

LR

146 147 (71 %)

148 (10 %)

S

Fe Fe

Scheme 60 Thionation of [1.1]ferrocenophane-1,12-dione 149 withLR

+

150 (90%, 13 %) 151 (46 %)

149

O

Fe Fe

S

Fe Fe

O

S

S

Fe Fe

O

LR

Scheme 61 Synthesis of [(n-butylamino)-thiocarbonyl]ferrocene153201

NH

NH

SLR, toluene

152 153 (66 %)

O

Fe Fe

Me Me

NH-COR

OH

N

SHR

a: R= Phb: R=Fec

LR (2 equiv.)

156 a,b 157 a,b (65, 84 %)

THF, refluxFeFe

Scheme 62 Synthesis of ferrocenoyl disulfide 155 with LR203

C

O

C

O

C

O

SS

154 LR

155 (52 %)

2 FeN

N

Fe Fe

Scheme 57 Treatment of LR with NaOR and PhSCl191,192

NaOR MeO P

S

SNa

OR

PhSCl MeO P

S

SSPh

Cl

LR

LR

R=a. CH3 85 %b. C2H5 81 %c. n-C3H7 85 %d. i-C3H7 84 %e. n-C4H9 72 %



Iron carbonyl complexes can also react with LR. Initially,Raubenheimer and co-workers206 wanted to convert metalcarbonyl complexes into thiocarbonyl complexes by re-acting them with LR. However, they were unsuccessfulsince the reaction of LR with diiron nonacarbonyl[Fe2(CO)9] led to new complexes containing theFe2(CO)6S2 butterfly unit.

Wood and Woollins207 reported on the reaction of LR withbis-phosphine-dihalide complexes of nickel, palladium,and platinum proceeding with asymmetric bridge cleav-age to result in M(PR3)2(S2(S)PC6H4OMe). In this reac-tion, halide atoms, and not oxygen atoms, are replaced bysulfur atoms. Woolins and co-workers208 used LR togeth-er with Pt(C2H4)(PPh3)2 in benzene to synthesize cis-Pt{S2P(S)(C2H4OMe)}(PPh3)2 in 63% yield.

Verani and co-workers209 successfully synthesizedphosphonodithioate nickel(II) complexes. They devel-oped an easy one-step synthesis that consists of the directreaction between NiCl2 and LR in the appropriate alcoholR¢OH (R¢ = Me, Et, i-Pr, Bu, Bz) as solvent giving bis-[O-alkyl/aryl-(4-methoxyphenyl)phosphonodithioato] nick-el (II) complexes 158 with high yields (Figure 7 andTable 13).

Figure 7 Bis-[O-alkyl/aryl-(4-methoxyphenyl)phosphonodi-thioato] nickel (II) complexes 158a–e

Williams and co-workers210 prepared a new class ofnickel-dithiolenes that show high absorption in the NIR,accompanied by high photochemical stability thatmakes these complexes promising NIR dyes. The directaddition of nickel powder to the reaction mixtures of 1,3-dialkyl-4,5-dioxoimidazolidine-2-thione with LR pro-duces [NiII(1,3-dialkylimidazolidine-2,4,5-trithione)2]. In1999, Aragoni et al.211 synthesized several new Ni, Pd,and Pt dithiolenes belonging to the general class[M(R,R¢timdt)2] (R,R¢timdt = monoanion of di-substitut-ed imidazolidine-2,4,5-trithione) by thionating the disub-stituted imidazoline-2-thione-4,5-diones with LR inpresence of the appropriate metal either as powder or as

chloride. All these complexes absorb in the NIR region inthe range 991–1030 nm with high extinction coeffcients.

Dicobalt octacarbonyl [Co2(CO)8] reacts with LR to formthe hexanuclear cobalt carbonyl cluster [Co6(m3-S)2(CO)14(m4-h2-SPC6H4OMe)] and the trinuclear cluster[Co3(m3-S)(m3-PSMe)(CO)7] (Scheme 64).212

Hill and Malget213 synthesized new thioketenyl complex-es [M(h2-SCCR)(CO)L(Tp)] 160a–d in high yields by re-acting [M(h2-CCR)(CO)L(Tp)] 159a–d with 1 equivalentof LR in THF.

Coffey et al.214 attempted to generate molybdenum(VI)imido-sulfido complexes [Mo(h2-S2CNEt2)2(NR)(S)] byreaction of bis(imido) complexes [Mo(h2-S2CNEt2)2

(NR)2] with LR in toluene. Instead, they synthesized imi-do-disulfido complexes [Mo(h2-S2CNEt2)2(h2-S2) (NR)]in moderate yields.

6.2 Main Group Metals

In 1996, Takaguchi and Furukawa215 reported the synthe-sis of a new spirotellurane, 1,1¢-spirobi(3H-2,1-benzo-thiatellurole)-3,3¢-dione [10-Te-4-(C2S2)] 161. The newspirotellurane 161 was prepared by reaction of thespirotellurane 162 with LR in toluene under an argonatmosphere (Scheme 65).

Scheme 65 Synthesis of the new spirotellurane 1,1¢-spiro-bi(3H-2,1-benzothiatellurole)-3,3¢-dione [10-Te-4-(C2S2)] 161215

Table 13 Melting Points and Yields of Phosphonodithioato Nick-el(II) Complexes 158a–e

Entry R¢ Mp (°) Yield (%)

158a Me 184 91

158b Et 131 78

158c i-Pr 168 64

158d n-Bu 79 93

158e Bz 148 80

CH3O

PHS

SNi

S

SP

OR'

OCH3R'O

158a-e

Scheme 64 Synthesis of thioketenyl complexes 160a–d with LR.



In order to explain the formation of 161 from 162 theyproposed that 162 is initially converted into a thiocarbonylcompound 163, rearrangement of 163 then gives 161.Scheme 65 shows also that the new spirotellurane 161 hastwo sulfur atoms as apical ligands and two carbon atomsas equatorial ligands.

Nizamov et al.216 used LR and one of its homologuestogether with antimonyl(III) alkoxides to form S-dialkyloxyantimony(III) O-alkylaryldithiophosphonates.Nizamov et al.217 reacted LR and its 4-ethoxy homologuetogether with triethyl- and triphenyl(alkoxy)plumbanes tosynthesize triethyl- and triphenyllead(IV) S-(O-alkyl-4-methoxyphenyl dithio-phosphonates).

In 1998, McBurnett and co-workers218 found that LRundergoes cleavage reactions with bis[bis(trimethyl-silyl)amino]germanium(II) and bis[bis(trimethyl-silyl)amino]tin(II) whilst with 1,3-di-tert-butyl-1,3,2-diazagermol-2-ylidene the product is a novel spirocyclicgermanium derivate.

7. Usage of Lawesson’s Reagent for Special Syntheses

7.1 Usage as Reagent for Glycosidations

LR was also used as catalytic reagent for different gly-cosidation reactions. Shimomura and Mukaiyama report-ed that LR can be used together with silver salts in a 1:2mol ratio for the stereoselective synthesis of b-D-ribofura-nosides 164.219,220 Scheme 66 and Table 14 show the ste-reoselective glycosidation of 2,3,5-tri-O-benzyl-D-ribofuranose 165 with different alcohols in high yields.Yields and selectivity decreases when sugars with an un-protected hydroxyl group are employed. Glycosylation ofL-serine showed a high a-selectivity.

Scheme 66 LR and silver perchlorate as catalyst mixture for glyco-sidation of 2,3,5-tri-O-benzyl-D-ribofuranose 165. Reagents andConditions: a) Toluene, 3 Å MS, 10 mol% LR, 20 mol% AgClO4, r.t.,2 h219,220

Furthermore, Mukaiyama co-workers used the same cata-lyst combination (LR and silver salts) in different ratiosfor the stereoselective synthesis of b-D-ribonucleosides166. The nucleobases are usually TMS-protected. Afterglycosidation, removal of the TMS-groups was performedin situ (Scheme 67). Selected examples are described inTable 15.221

OOH

OBnOBn

BnO

R OH

OOR

OBnOBn

BnO

a1.2 equiv.

165 164

Table 14 Conditions and Yields for the Glycosidation of 2,3,5-Tri-O-benzyl-D-ribofuranose 165219,220

R-OH Yield (%)

a:b Ref.

93 5:95 219

97 5:95 219

90 4:96 219

79 24:76 219,220

61 30:70 220

77 9:91 220

OH

OH

HOH

H3C

H H

H

H3CH3C

CH3

CH3

OBnO

BnO

OMeBnO

OH

OHOBnO

OMeBnO

OBn

HO

CO2Me

HN

O

OBn

Table 15 Stereoselective Synthesis of β-D-Ribonucleosides 166 221

(TMS)n-Base Conditions Yield (%)

MeCN,5 mol% LR, 10 mol% AgOTf,60 ºC, 6.5 h

97

MeCN,5 mol% LR, 10 mol% AgOTf, 80 ºC, 5 h

95

Theophylline MeCN,5 mol% LR,10 mol% AgOTf, 80 ºC, 6.5 h

95

N4-Benzoylcytosine MeCN,15 mol% LR, 30 mol% AgClO4, 80 ºC, 4 h

94

N

N

OTMS

TMSO

N

N

OTMS

TMSO

Me



Scheme 67 Stereoselective synthesis of β-D-ribonucleosides 166,for conditions see Table 15

7.2 Transformation of Alcohols to Thiols

LR is also used to transform alcohols to thiols. Tomichand co-workers published a study on the preparation ofthiol 167 with LR. Using different solvents and reactiontimes gave optimized conditions (Scheme 68).222

Eberle et al. were able to isolate sulfhydryl cyclosporineA as a byproduct. The hydroxyl group of cyclosporine Acould be replaced by addition of LR.223

Nishio postulated a mechanism of formation of thiol, de-scribed in Scheme 69.224,225

The formation of thiols took place with retention of con-figuration, 0.5 equivalents of LR were usually used for thetransformation. Higher equivalents led to alkenes as elim-ination products and in some cases rearrangement prod-

ucts were observed. Nishio et al. published a range ofexamples of conversion of alcohols into thiols (Table 16).Some heterocyclic thiols were synthesized by Tagawa etal.226 (phenylpyrazole-5-thiols) and Nishio et al. [4-ethoxy-1,5-dihydro-2H-pyrrol-2-thiones (keto-form),227

3-mercapto-isoindolin-1-one].228

O

OBnOBn

BnO O

O

OMeO

OBnOBn

BnO Base

(TMS)n-Basea1.2 equiv.

166

Table 16 Selected Examples of Conversion of Alcohols into Thiols

Conditions Yield (%) Ref.

R1 = R2 = R3 = Ph DME, 2 equiv LR, r.t., 15 h quantative 224,225

Toluene, 2 equiv. LR, reflux, 0.2 h, quantative

R1 = PhCH=CH; R2 = R3 = H Toluene, 2 equiv. LR, reflux, 2 h 73 224,225

Toluene, 1 equiv LR, reflux, 0.5 h 72 224,225

Toluene, 1equiv LR, reflux, 0.5 h 67 224,225

Toluene, 0.55 equiv LR, reflux, 10 min 53 228

DME, 0.5 equiv LR 64 227

Toluene, 0.6 equiv LR, reflux, 3.5 h 58 226

R1R2R3COHLR

R1R2R3CSH

XH

XH

N Ph

O

XH

N X

EtO

N XH

EtO

Ph Ph

NN

Me

XH

NO2

Scheme 68 Thionation-reduction of 4-(α-hydroxybenzyl)phen-oxyacetic acid 168. Reagents and Conditions: a) DME, 0.55 equivLR, 2 h, under argon, other solvents and conditions are described inthe paper222

a

168 167 (99 %)

PhHO

O

O

OH

PhHS

O

O

OH



7.3 Reduction of Sulfoxides

The reducing activity of LR was investigated by Lawes-son et al.229 They described the formation of the corre-sponding sulfides and disulfides from DMSO andtetrahydrothiophene sulfoxide. Table 17 shows selectedexamples of different sulfoxides prepared by using LR.Sulfoxides and lactam sulfoxides can be reduced underoptimized conditions.230 Erker and Bartsch231 reported onsulfoxide containing lactams that are reduced selectivelywithout thionating the carbonyl group.

Kaiser and Anderson used this method to change the po-larity of the stationary phase in a liquid chromatographycolumn.232 The sulfoxide phase was reduced by LR in

dichloromethane or tetrahydrofuran. After reduction of S-oxide containing campher derivatives, Shimada et al. ob-tained kinetically stabilized dithiiranes.233

7.4 Catalyst for Aldol Reactions

Mukaiyama et al. reported that a mixture of LR and silverperchlorate in a ratio of 1:2 can be used as a catalyst foraldol reactions (Scheme 70). Aldehydes and silyl enolethers were used for different systems.234

Scheme 70 Aldol reaction catalyzed by a combined system of LRand AgClO4, for conditions see Table 18

OTMS

R2

OH

R1

O

R2R1CHO a

Table 17 Reduction of Sulfoxides and Sulfoxide Containing Lactams with LR

Conditions Yield (%) Ref.

R1 = R2 = PhR1 = R2 = BzR1 = Ph; R2 = CH2=CHR1 = Ph; R2 = CH2CO2Me

One or more than one equiv of LR, r.t., THF or toluene or xylene, 5–60 min (monitored by TLC)

quantitative

R = H or Ph

2.5 equiv LR, r.t. or –5 °C, THF or CH2Cl2 quantitative 231

R1 = PhOCH2; R2 = p-Nitrobenzyl; R3 = OH 1.2–1.5 equiv LR, 15–30 min, CH2Cl2 80 230

R1 = PhCH2; R2 = p-Nitrobenzyl; R3 = Cl 78 230

R1 = PhCH2; R2 = CHPh2; R

3 = CH3 70 230

R1 S R2

O

R1 S R2

a

NH

S

OR

O

N

S

OR1COHN

OCOOR2

R3

Table 18 Aldol Reaction Catalyzed by LR and AgClO4234

Aldehydes Silyl Enol Ether Conditions, a Yield (%)

0.25 mol% (LR/AgClO4, 1:2), 3 h, CH2Cl2, –78 ºC

89

5 mol% (LR/AgClO4, 1:2), 3 h, CH2Cl2, –78 ºC, 80%

80 (syn/anti1:1.1)

1 mol% (LR/AgClO4, 1:2), 3 h, CH2Cl2, –78 ºC

78

PhCHO OTMS

Ph

PhCHO OTMS

OMe

CHO

OTMS

Ph

Scheme 69 Postulated mechanism of formation of thiols withLR224,225



7.5 Preparation of Other Compounds

Wu and co-workers reported the stepwise synthesis ofmono-, di-, tri- and tetrathiacage compounds.235 InScheme 71, the mechanism is described for the mono-

thionated cage compound 169. Electrophilic attack of theP-atom of the ionic form of LR on the oxygen atom O-4of tetraoxacage compound 170 followed by cleavage ofC-3-O-4 bond gives zwitterion 171. Intermediate 172 re-sults after nucleophilic addition of the negatively charged

Scheme 73 N-Dimethyl ylide chromium complexes and LR.237 Reagents and Conditions: a) H2O, b) pyridine, then H2O.

NMe Me

O

Ph Ph

R1

Cr(CO)3

NMe Me

O

Ph Ph

R1

Cr(CO)3

P

S SAr

N

Me

O

Ph Ph

R1

Cr(CO)3

P

S SAr

Me

N

Me

O

Ph Ph

R1

NO

Ph Ph

R2

Cr(CO)3

NHO

Ph Ph

R2

Cr(CO)3

(CH2)4SP

SAr

NO

Ph Ph

R2

(CH2)4HS

LR

R1=Me, Ph

LR

a

b

Scheme 71 Stepwise synthesis of thiacagecompounds;235 Reagents and Conditions: a) LR, CH2Cl2, 25 ºC

H3CO P

S

SO

O

O

O

OO

O

H3CO P

S

S

O

OO

O

H3CO P

S

S

O

OO

O

H3CO P

S

S

O

O

S

O

OO

S

S

OS

S

S

OS

S

S

S

12

3

45

6 7

89

10

11 12

a a a

170 171

172173

169

Scheme 72 Olefins from secondary phosphates;236 Reagents and Conditions: a) 1.2 equiv LR, xylene, reflux, quantitative; b) 1.2 equiv LR,xylene, reflux, 79%

OP(OEt)2

O

(EtO)2PO

O

n-C9H19 n-C9H19 n-C9H19n-C8H17

a

b

175

174



sulfur anion to the positively charged carbon of the oxoni-um ion. Cleavage of the C(5)–O-bond leads to zwitterion-ic intermediate 173 and finally cleavage of the P–S-bondgives monothiacage 169. The same mechanism leads todi-, tri- and tetrathiacage compounds. Shimagaki et al. de-veloped a method for preparation of olefins using second-ary phosphates and LR. Under optimized conditionscholesterol derivative 174 could be synthesized in 79%

yield and olefin 175 could be prepared quantitatively(Scheme 72).236

Daran and co-workers discovered a protonation-dealkyl-ation reaction with LR and N-dimethyl ylide chromiumcomplexes (Scheme 73).237 Demetallation of the chromi-um complexes yielded unsaturated N-monoalkylated lac-tams.

Butler et al. published a rearrangement sequence withLR.238 Different triazolothiazaphospholes 176 were ob-tained from the reaction of substituted 1,2,3-triazolium-1-imide 177 (Scheme 74).

Dorn and Kreher were able to obtain a range of 3-thioxopyradolidine-azomethineimines 178 from their 3-oxo analogs 179.239 LR transforms the 1,3-dipoles 3-oxo-azomethineimines 179 to the corresponding 3-thioxo-azomethineimines 178 directly (Scheme 75).

8. Unexpected Reactions Following the Usage of Lawesson’s Reagent

Several papers report on unexpected reactions followingthe usage of LR. Sharp and Heathcock observed an unusu-al isomerization of compound 180 under Lawesson thion-ation conditions (Scheme 76).240 Thionation did notoccur; instead the diasteromeric lactam ester 181 was ob-tained in good yield. Molecular mechanics calculationsshowed that the energy conformation of the diasteromericlactam ester is lower than the thioesters. The postulatedmechanism involves a 1,3-dipolar cycloreversion.

Cava and co-workers investigated the reactions of o-ph-thalaldehydes 182 with LR.242 The suggested mechanismshows that monothioaldehyde was obtained first, then o-dithiophthalaldehyde 183 was formed quickly. Finally,after intramolecular cyclization of the two thione func-tions, followed by a 1,3-hydride shift, dithiophthalides184 were obtained (Scheme 77). Thionolactone did notreact to o-dithiophthalide.

Scheme 77 Treatment of o-dialdehydes 182.242 Reagents and Con-ditions: a) LR, CHCl3, r.t., 30 min, 96%

CHO

CHO

C

C

H

S

S

HS

S H

S

S

CHO

CHO

S

S

LR

a

182a 183

184a

182b 184

Scheme 74 Preparation of triazolothiazaphospholes 176238

N

NN

R1

R1

R2

NH

R2

N

NN

R1

R1

R2

N R2S

P

SAr

N

SP

NN

NR2

R1

R1R2

S

Ar

N

NN

R1

R1

R2

LR

R1=Ph; R1,R1=(CH2)4

R2= Ph, p-MeC6H4, p-BrC6H4, p-NO2C6H4

177

176

Scheme 75 Selected examples of transformation of 3-oxoazome-thineimines 179.239 Reagents and Conditions: a) 0.5 equiv LR, anhydCH2Cl2 or benzene, 20–40 ºC.

NN

OR1

R2

R3 R4

R5

NN

SR1

R2

R3 R4

R5

a

R1=R3=R4=R5=H, R2=PhR1=R4=R5=H, R2=Ph, R3=MeR1=R3=R4=R5=H, R2=p-Cl-C6H4

R1=R3=R4=R5=H, R2=p-MeO-C6H4

179 178

Scheme 76 Isomerization of 180.240,241 Reagents and Conditions: a)LR, toluene, 100 °C, 62–79%

N NBu

CO2EtO

Me

H

Me

Me

N NBu

CO2EtO

Me

H

Me

Me

180

181

a



Joule et al. published a mechanistic study on reactions ofnitroacetamides with LR. A range of mono- and dithioox-alic acid diamides have been synthesized under optimizedreaction conditions.241

Beckert and co-workers used another unexpected reactionof aryl-substituted 4H-imidazoles 185 with LR for thesynthesis of 6-azapentafulvenes 186 (Scheme 78).243

Scheme 78 Synthesis of 6-azapentafulvenes 186243

Garrat et al. tried to thionate cyclobutanone 187. As an un-expected result a rearranged cyclopentanone 188 and notthe expected corresponding cyclobutandithione wasformed (Scheme 79).244

Scheme 79 Thionation of cyclobutanone 187244

Hesse and co-workers described the synthesis of fu-rophane 189 with LR. Thionation of furophane 190 didnot occur. Instead, LR acted as a Lewis acid. Furophane189 was formed in 91% yield (Scheme 80).245

Scheme 80 Formation of furophane 189 with LR245

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1980, 21, 5015.

N

NPh

NH

NH

R1

R2N

NPh

NH

R1

S

N

NPh

H2N

N

R1

LR

- H2S

N

NPh

N

R1

NH

N

N

Ph

N

R1

a. + H2S, -1/8S8 b. CH3I, -HI R1=R2=4-CH3-C6H4

a,bNH

NPh

N

R1

NH

N

N

Ph

NH

R1

Me

185

186

MeMe

Me

Me

Me Me

Me Me

MeMe

O

MeMe

Me

Me

Me Me

Me

Me

Me Me

O

LR

187 188

O

OOLR

190 189 (91 %)



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applications of lawesson’s reagent in organic and ... · 6 synthesis of organometallic compounds...

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