Carbanions thioether synthesis

On the other hand, the enantioselective 1,4-addition of carbanions such as enolates to linear enones is an interesting challenge, since relatively few efficient methods exist for these transformations. The Michael reaction of p-dicarbonyl compounds with a,p-unsaturated ketones can be catalysed by a number of transition-metal compounds. The asymmetric version of this reaction has been performed using chiral diol, diamine, and diphosphine ligands. In the past few years, bidentate and polydentate thioethers have begun to be considered as chiral ligands for this reaction. As an example, Christoffers et al. have developed the synthesis of several S/O-bidentate and S/O/S-tridentate thioether... 

In the unconventional synthesis of thioethers (Scheme 4.11), cyanide ion is displaced from thiocyanates by carbanions [52, 53], which have been generated under phase-transfer catalytic conditions (cf. 4.1.12). Thiocyanates are readily obtained by a standard catalysed nucleophilic substitution reaction [4, 54-58] (see Table 4.19). Aryl thiocyanates are obtained from activated aryl halides [4, 57] (see Chapter 2). 

In the reaction of the phenylacetonitrile carbanion with thiocyanates, a major side reaction leads to the formation of the dialkyl disulphides, as a result of the base-catalysed decomposition of the thiocyanate. This side reaction is reported to be insignificant in the reactions of the other carbanions. Phenylacetonitrile reacts with 1,2-ethanyl bisthiocyanate to produce 2-cyano-2-phenyl-1,3-thiolanes [52] under conditions analogous to those used for the synthesis of the thioethers (Scheme 4.12). 

The use in organic synthesis of carbanions stabilized by one or more adjacent thioether groups is now a well-established technique [272-275]. A landmark in this development was the work of Corey and Seebach on 1,3-dithianes [276]. They pointed out the use of the anions of 1,3-dithianes as... 

Carey, F.A., and Court, A.S., Silicon-containing carbanions. Part 1. Synthesis of vinyl thioethers and vinylphosphonates via silicon-modified organolithium reagents, J. Org. Chem., 37, 939, 1972. 

Use of the methylsulphinyl carbanion as base is recommended for the synthesis of ethers from alcohols and alkyl halides. The catalysed formation of dimethyl ether from hydrogen and carbon dioxide has been reported. Carey has described the use of the silicon-modified organolithium reagent (73) in the preparation of vinyl thioethers (Scheme 151). In order to extend the scope of this reaction to include vinyl ethers themselves, an attempt was made to metalate trimethylsilylmethyl ether use of n-buty 1-lithium resulted in nucleophilic attack on silicon, whereas t-butyl-lithium abstracted the wrong proton, as shown in Scheme 151. 

A number of phase transfer reactions of sulfur containing substrates have been reported. These include substitution reactions in which sulfur is nucleophile, electrophile or an ancillary function. The synthesis of a variety of thioethers has been reported, as have the alkylation of numerous sulfur stabilized carbanions. In addition, reduction of both disulfides and N-tosylsulfilimines have been conducted under phase transfer catalytic conditions. The unifying theme of this chapter is, therefore, the synthesis and reactions of sulfur containing molecules. Inherent in this approach is some redundancy, but it is hoped that the ability to locate reactions of interest compensates. 

An alternative to the synthesis of thioethers in which sulfur is a nucleophile, is a synthesis in which sulfur bears a leaving group. The reaction of a carbanion with a sul-fenyl halide should, in principle, constitute a synthesis of thioethers. Such a reaction of sulfenyl halides has not yet been reported but the case in which sulfur is rendered electrophilic by attached cyano, has been [10]. Thus, benzylthiocyanate reacts with trichloromethide ion (generated from chloroform under phase transfer conditions) according to equation 13.6 to yield (80%) of benzyl trichloromethyl thioether [10]. 

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