Sulphenyl Carbanions

In contrast to the early theoretical work of Rank and coworkers , C-NMR investigations had revealed that the metallated carbon atom in the a-sulphinyl carbanion is nearly planar . A four-centre chelate structure 315 has been proposed for a-lithiosulphoxides, and it is believed to be responsible for the planar configuration of the anionic carbon atom and for the greater stability of o(-sulphinyl carbanions in comparison with a-sulphenyl carbanions This chelation favours one of the two diastereoisomeric carbanions and for this reason a-sulphinyl carbanions react with electrophiles in a highly stereoselective manner (see below). 

Uses of Saturated Sulphides, Selenides, and Tellurldes in Synthesis.— These applications are mostly based on the C—C bond-forming reactions of sulphenyl carbanions the extensive uses of vinyl sulphides and their allyl and propargyl analogues are discussed in a later section. Some methods for C—S or C—Se bond cleavage, which are used to terminate a use in synthesis, have been discussed in an earlier section other methods, notably the conversion of the sulphide into a sulphoxide followed by pyrolytic elimination of the sulphenic acid, are covered in later sections. 

In contrast with the spate of activity in the area of theoretical calculations on sulphur carbanions and ylides reported in Volume 5, only a single, brief paper has been published in this area in the past two years. This interesting study predicts (and experimentally substantiates) the greater stability of cr-disulphide carbanions compared to cr-sulphenyl carbanions. 

Change of personnel has led to the division of the wide-ranging chapters on thiocarbonyl compounds and on ylides into more manageable parts. In keeping with more recent work, the short sections on sulphines and sulphenes have been confined to the chapter on thiocarbonyl compounds. To avoid possible inconsistencies, the chemistry of sulphonyl, sulphinyl, and sulphenyl carbanions has been largely included in the chapter on ylides. The order of the chapters has been revised so that the chemistry of acyclic sulphur compounds and sulphur-containing functional groups is covered before cyclic systems. 

Steps (a) and (b) of Scheme 6 constitute the Arens mechanism. Therefore, equation (244) was simply a case of attack on Cl in which the ion-molecule (PhC=C CISC,-H,-p) was the key intermediate. Attack of acetylide on the sulphur of the sulphenyl chloride leads to the product given in equation (244). Support for this step is the well-known reaction of sulphenyl chloride with carbanions to yield sulphides . The disulphides which sometimes turn up in the haloalkyne-thiolale processes (see Table 6) are easily explained by the sulphenyl halide reacting with the thiolates. 

Sato has reported a new route to allylic sulphides through the reaction of a-sulphenyl aldehydes (35) with phosphonium ylides or phosphonate carbanions.25 Similar reactions have been used to achieve a two-carbon... 

A related process consists initially in the reaction between diethyl methylphosphonate carbanion and a nitrile, RCN, to give the species 318, and the alkylation (R = Me, CH2CH=CH2, CH2Ph) of the latter. Work-up of the product under basic conditions leads to the enamines 319. Work-up with acid conditions affords the (2-oxoalkyl)phosphonic diesters 320 the latter were also obtained through acidolysis (3 m H2SO4) of the enamines 319. A wide choice of alkylating species is possible alkyl halides, disulphides RSSR (R = Me or Ph), sulphenyl chlorides RSCl (R = Me or Ph), PhSeBr, MeS02SMe and PhSOoCl have all been used ". ... 

Preparation from Disulphides, Diselenides, and Ditellurides.—The use of disulphides as sulphenylating agents towards alkenes and carbanions continue to offer a distinctive alternative to the use of sulphenyl halides for the same purpose. Although reaction conditions are more severe, cleaner results are usually obtained. The special case of the conversion of disulphides into corresponding sulphides is covered in the later section on Reactions of Disulphides. 

Another example of the addition of sulphur-stabilized carbanions to carbonyl groups is shown in the synthesis of a-sulphenylated aldehydes (179) from ketones (RR CO) and PhSCHOMe, and of vinyl sulphides or thioacetals from ketones and R C(SR)2 or C(SR)3, respectively. a-Sulphenylated aldehydes are also formed by electrolysis of vinyl sulphides as in the conversion of PhC=CSR to PhCH(SR )CHO. The carbanion PhSCH2 adds specifically to the ketone in (180) to provide methylenecyclohexenes by further elaboration as shown in Scheme 33. °... 

Methods for the introduction of an a-alkylthio-group into an aliphatic ketone have been established, involving either treatment of the silyl enol ether derived from the ketone with MeLi and a sulphenyl chloride, or a-carbanion formation from the ketone with the Li salt of a secondary amine, followed by treatment with a disulphide or a sulphenyl halide. ... 

---