Acidities of Sulfoximines and Related Oxosulfonium Cations. Cyclopropyl Effects and Structures of α-Sulfonyl-Type Carbanions

Article abstract of DOI:10.1021/jo01307a030

(Dimethylamino)methylphenyloxosulfonium cation, PhS(O)(NMe2)CH3⁺, was found to be more acidic than the parent sulfoximine, PhS(O)(NMe)CH3, by about 19 pK_a units.The latter is about equal in acidity to PhS(O)CH3 and about 5 pK_a units less acidic than PhSO2CH3.The cyclopropyl compounds, c-PrG, with G equal to Ph(NMe2)SO⁺, p-MeC6H4(NMe2)SO⁺, F3CSO2, and Ph(PhSO2N)SO, were found to be about 4-7 pK_a units less acidic than their acyclic methyl analogues, CH3G.This is interpreted as evidence for a demand on the part of sulfone-type functions for p character in cyclopropyl carbanions.Evidence is presented to show that carbanions of the type GCH2^-, where G is an electron-withdrawing function such as RSO2, RS(O)(NR), RSO, Ph2PO, (RO)2PO, and the like, are planar or nearly planar.Acidity data indicate that the preferential generation of chiral, rather than achiral, planar α-sulfonyl carbanions is a consequence of a preferred kinetic pathway rather than an inherent greater thermodynamic stability.Factors controlling the stereoselective generation and reactions of carbanions are reviewed.