Lawessons Reagent

LR converts carbonyl compounds to the corresponding thiocarbonyl compounds and alcohols to the corresponding thiols, as shown below  

For carbonyl thiation, the mechanism involves nucleophilic attack by one of the sulfurs of the ArPS2 intermediate on the carbonyl carbon. The alkoxide oxygen might then be expected to form a bond with the cationic phosphorus  

The four-membered ring then falls apart to produce the desired thiocarbonyl compound  

Clearly, the chemistry of LR is a fascinating blend of phosphorus and sulfur chemistry. Its success hinges as much on the nucleophilicity of sulfur as on the oxophilicity of pentavalent phosphorus. 

LR is commercially available but can also be readily synthesized in the laboratory by heating anisole with phosphorus pentasulfide (P4S10), until the mixture is clear and no more HjS evolves, followed by recrystallization from toluene or xylene. Suggest a mechanism for the reaction. 

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On treatment with an equivalent amount of (Et2N)3P, thiotelluroxide 67a is reduced to give telluroazocine 65 in almost quantitative yield, which can be converted into telluroxide 68 by the usual procedures for a diorganyl telluride to telluroxide transformation. It is worth noting that when treated with Lawesson reagent, the telluroxide 68 forms thiotelluroxide 67a, whereas diaryl telluroxides are reduced by this reagent to diaryl tellurides. 

Dithio-squaraines 37 can be synthesized via thionation of the correspondent oxo-squaraines 5 with phosphor pentasulphide or Lawesson reagent (Fig. 8) [103, 104], However, dithio-squaraines containing carboxylic groups cannot be obtained by this method. Symmetrical indolenine- and benzothiazole-based monothio- (38) and dithio- (37) squaraines containing carboxy and sulfo groups are synthesized using 1,2-dithio- and tetrathiosquaric acid, respectively, while the reaction of these... 

A convenient preparation of dithiophosphonato metal complexes of transition metals, M[S2PR(OR )]2 (M = Ni, Pd, Pt) uses the direct reaction of the Lawesson reagent, [4-MeOC6H4P(S)S]2, with metal salts in alcohols, without... 

Telluropyran-4-thiones are prepared by treating the pyranones with Lawesson reagent. ... 

Thiazine compounds 830, prepared from 829 by the Lawesson reagent, were converted to the corresponding (2-acetoxyethoxymethyl)imidazothia-... 

Finally Ebersbach s group succeeded in synthesizing the first (and, up to now, only known) authentic pyrido[l,2-a]azepine base 315 (Scheme 70) from their lactam 290 by thionation with Lawesson reagent (LR), methylation of thiolactam 313, and deprotonation of sulfenium salt 314 (91HCA1095). 

Thionylation with Lawessons Reagent General Procedure 517 ... 

Enaminoamides with phosphorus trichloride yielded diazaphosphorins (92JPR711) (Scheme 48). Diazaphosphorins were also obtained by treatment of enaminonitriles, enaminoamides, and enaminothioamides with phosphorus pentasulfide in pyridine (96MC191) (Scheme 49). Fused 3-cyano-2-pyridones react with the Lawesson reagent giving phosphorus-containing heterocycles (95MC191) (Scheme 50). 

Quinoline-4-thiones274 (equation 32), 3-benzyl-l,4-dithiochromones275 (equation 33), 4-thioisoflavones276 and different pyrrole277 and indole derivatives277-279 (equation 34) have also been obtained with Lawesson reagent. 

Again in the field of natural products synthesis, Lawesson reagent has found application in the obtention of sulfur carotenoids285 (equation 39) and thionated steroids286 (equation 40). 

The versatility and selectivity of the Lawesson reagent is also evidenced in the synthesis of the highly complex alkaloid 19-hydroxytubotaiwine287, in which a ketolactam is transformed into a thionethiolactam (equation 41). Also, Diels-Alder cycloadducts can be thionated with 52 and, upon thermolysis at high temperatures (FVT), afford the corresponding thioketenes which are trapped in situ2m (equation 42). 

In situ combination of Lawesson reagent and anthracene allows reactive thioaldehy-des to be formed and subsequently trapped as Diels-Alder cycloadducts, as illustrated in the case of pentafluorothiobenzaldehyde289 (equation 43), which is very reactive and polymerizes rapidly. 

The thionation of 240 with Lawesson reagent (LR) in boiling xylene led to 1,2-dithietane 241 as a single isomer in 45% yield <2006HCA991>. The dithietane 241 then underwent a gas-phase thermolysis at 850°C under a pressure of 1.5 X 10 3 Torr and 3-benzylidene-4-methyl-377-dithiolane 242 was obtained as a 1 3-E/Z mixture in 54% yield (Scheme 29). 

The only diorgano tellurium sulfide so far reported was prepared by reacting 2-phenyl-7-methoxy-4-oxo-4//-l-benzotellurin with the Lawesson reagent in benzene at 20° for 17 hl. 

Treatment of 2,6-diorgano-4-oxo-4/f-tellurins with an excess of the Lawesson reagent in hot toluene for five minutes produced the corresponding thiones in quantitative yields2-4. 

When the reaction was carried out with two moles of the Lawesson reagent per mole of the benzotellurin, 2-phenyl-7-methoxy-4-thioxo-4H-l-benzotellurin 1-sulfide was isolated1. 

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