The absolute stereochemistry of isotopic exchange and methylation in benzyl methyl sulphoxide

Abstract

α-Sulphinyl carbanions generated from diastereomeric monodeuteriobenzyl methyl sulphoxides were quenched with water and methyl iodide and the kinetic isotope effect also determined. Analyses of the isomer distributions and the deuterium content of the resulting mixture revealed that the stereochemical pathway of these reactions consists of three distinct steps, namely (i) the stereochemistry of the formation of the carbanions, (ii) the stereochemistry of the configurational inversion equilibrium, and (iii) the stereochemistry of the approach of the electrophile. It was concluded that the pro-R hydrogen in the (S)-sulphoxide is abstracted preferentially, k(R)/k(S) ca. 1·5, and that the equilibrium constant between the configurationally inverted carbanions is ca. 20 in favour of the carbanion having the carbon lone pair which bisects the sulphur lone pair and the S–O bond. It is also suggested that the isotopic exchange occurs with retention and the methylation with inversion during the final substitution step in the light of Wolfe's theoretical requirement from extensive non-empirical calculations on α-sulphinyl carbanions.