Catalytic asymmetric oxidation of aryl sulfides with a Ti/H2O/(R,R)-diphenylethane-1,2-diol complex: a versatile and highly enantioselective oxidation protocol

A new catalytic procedure for the asymmetric oxidation of aryl alkyl and ar yl benzyl sulfides to optically active sulfoxides by hydroperoxides is desc ribed. This oxidation of sulfides is mediated by a chiral Ti complex formed in situ by reacting Ti(i-PrO)(4), (R, R)-diphenylethane-1,2-diol (1), and water. The conditions of the reaction (stoichiometric composition of the ca talyst, temperature, and the presence of additives and solvent) have been d etermined in order to reach the highest enantioselectivity and avoid the in tervention of a kinetic resolution process. The oxidation protocol describe d herein is quite versatile as the values of chemical yields (60-73%) and o f enantioselectivity (ee 70-80%) achieved for aryl alkyl sulfides are almos t independent of the nature of the aryl substituent and of the size of the alkyl group. Notably, aryl benzyl sulfides, which are poor substrates for t he Ti/DET catalyzed oxidations, afforded very high ee's (92-99%) with this oxidation system.