Thiane 5-oxides carbanions

The enantioselective reaction of a-sulfinyl carbanions involves both desym-metrization of the prochiral alkyl groups and diastereoselection. Simpkins and coworkers have reported the asymmetric induction of a traws-thiane oxide with a camphor-derived chiral lithiiun amide giving products with complete diaster-eoselectivity and with good enantioselectivity (Table 6) [59,60]. 

Even the widely accepted assumption of pyramidal sp -hybridized a-sulfmyl carbanions has been challenged. A NMR study provides evidence for a chelated a-lithio thiane 5-oxide with a planar met-alated carbon. The stereoselectivity observed in methylation with Mel is completely reversed with (MeO)3PO. These results are explained in terms of trans approach of Mel to the chelated face and cis approach of (MeO)3PO, which requires coordination to Li. It was also reported that the carbanion carbon of PhCHLiS(0)Me or Bu S(0)CH2Li is primarily sp -hybridized or intermediate between sp - and sp -hybridized, respectively... 

Reactions.— The stereoselectivity of the abstraction of diastereotopic protons a to trico-ordinated sulphur and the stereochemistry of electrophilic reactions at the carbanion centres of cFlithio-thian 1-oxides still excite considerable interest. The rate factors for base-catalysed H-D exchange of a-protons in several conforma-tionally biassed or rigid thianium cations have been measured with increasing rigidity, the reactivity of decreases, while that of remains about constant. It... 

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