Dithioacetals, alkylation reduction

The reaction of methylenesulphones with allyl halides in the presence of quaternary ammonium salts produces the 1-allyl derivatives [52], unlike the corresponding reaction in the absence of the catalyst in which the SN- product is formed (Scheme 6.5). In contrast, alkylation of resonance stabilized anions derived from allyl sulphones produces complex mixtures [51] (Scheme 6.6). Encumbered allyl sulphones (e.g. 2-methylprop-2-enyl sulphones) tend to give the normal monoalkyl-ated products. Methylene groups, which are activated by two benzenesulphonyl substituents, are readily monoalkylated hydride reduction leads to the dithioacetal and subsequent hydrolysis affords the aldehyde [61]. 

Dithioacetals (see also dithianes and dithiolanes) alkylation of 98 as acyl anion equivalents 75 carbanions of 87,97-102 cleavage of 14-18,98,102 desulfurization of 78 metal-catalysed coupling 127 reaction with Grignard reagents 127 reductive lithiation of 89 synthesis of 12-19,97-102 Dithioacids synthesis of 40... 

Dithioesters can be reduced in anhydrous acetonitrileor in methanol,the best results being obtained with dimethyl sulfate as alkylating agent. The most convenient procedure involves a simple electrolysis cell with a lead cathode and methanol as solvent. Eight substrates (68 X = SMe) were tested, with R equal to phenyl, chloro- and methoxy-substituted phenyls, and f-butyl. Unlike the corresponding reductions of thioamides, the reactions were not entirely clean and gave substantial amounts of side products such as (70). The product dithioacetals (69 X = SMe, R = Me) were formed in yields of 40-60%. On two of the substrates, better yields (ca. 70%) could be obtained in anhydrous acetonitrile, but a more sophisticated apparatus was required. One enolizable substrate (71) was tested, but the yield was only 30%. ... 

Dithioacetals can also be desulphurized under radical conditions using tributyltin hydride (TBTH) (equation 65) or by using metal-ammonia solutions (equation 66). Lithium aluminium hydride has also been used in some cases for reductive desulphurization. In a manner analogous to the preparation of dithioacetals, ketones can be transformed to diselenoacetals with aryl or alkyl selenol. These in turn have been reduced with Raney or Li-EtNH2 and under radical conditions with TBTH or... 

A small number of papers that cover mechanistic aspects deal with the factors that stabilize the developing carbonium ion in the hydrolysis of 0,S-thioacetaIs. An electron-transfer, mild reduction system (iron polyphthalocyanine) has been shown to reduce benzil dithioacetal to the / -keto-sulphide [PhCOCPh(SPh)2 PhCOCHPhSPh]. Homolysis of dithioacetals on heating with BuKDOBu in PhCI followed by a 1,2-shift of an alkylthio-group leads to l,2-bis-(alkylthio)alkyl compounds/ ... 

Sulphur.—Electrochemical methods are proving useful in the synthetic manipulation of sulphur compounds. For instance, dithioacetals may be synthesized by electrolytic reduction of alkyl dithiocarboxylates, and can be unmasked to... 

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