Carbanion sulfoxide-stabilized

Fluonnation of a sulfoxide-stabilized carbanion provided a route to fluorinated estrones after elimination of the sulfoxide [111] (equation 62). 

The few examples indicate that sulfone-stabilized carbanions should react normally with electrophilic animating reagents (Eqs. 145158 and 146465) with the caveat that free a-amino sulfones are unstable.158,465 The (3,y-unsaturated sulfone 74 is animated at the y-position (Eq. 147),250 presumably by an ene reaction. The preparation of a-tosyl azides from nitronates was shown above in Eq. 144. The scope of this reaction does not seem to have been determined. Reaction of the anions of nitrobenzyl aryl sulfones with l-oxa-2-azaspiro[2.5]octane (13a) gives nitrobenzaldehydes by cleavage of the initially formed amination products.466 Similarly, reaction of the lithium salt of benzyl phenyl sulfone with phenyl azide gives benzilydeneaniline and phenyl sulfinate.467 No reports on animations of sulfoxide-stabilized carbanions were found. 

The delicate effects which allow highly diastereoselective protonation of sulfoxide-stabilized carbanions are discussed in Section D. 11.1.5. A further example is provided by the intermediate anions produced by deacylation of the following substituted 2-alkanoyl-1.3-dithiane 1-oxide41. 

Sulfoxide-stabilized carbanions also undergo stereoselective conjugate addition to a,P-unsaturated esters [73,74]. One application has described the asymmetric synthesis of chiral indolizine and yohimbanoid alkaloids [75]. 

Asymmetric Carbon-Carbon Bond Fbnnation Using Sulfoxide-Stabilized Carbanions Walker. A.J. Tetrahedron Asymmetry, 1992, 3,961. 

Stereochemical constraints in cyclic sulfones and sulfoxides impart increased weight to strain and conformational factors in the generation of carbanions and their stability, causing distinct differences between the behavior of cyclic and open-chain systems233, due primarily to the prevention of extensive rotation about the C —S bond, which is the major way that achiral carbanions racemize. Study of the a-H/D exchange rate fce and the racemization rate ka may provide information concerning the acidity-stereochemical relationships in optically active cyclic sulfone and sulfoxide systems. 

The stabilization of the carbanionic center on the a carbon atom by the sulfone group is no doubt an important factor promoting such reactions. Sulfoxides are capable of undergoing such reactions but are rather less prone to do so than sulfones. 

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