Schiff-base ligands carrying two elements of chirality: Matched-mismatched effects in the vanadium-catalyzed sulfoxidation of thioethers with hydrogen peroxide

Eight chiral Schiff-base ligands were prepared from (S)-tert.-leucinol and four chiral, racemic salicylic aldehydes. The resulting pairs of diastereomers were separated by preparative HPLC and applied to the vanadium-catalyzed asymmetric sulfoxidation of thioethers with hydrogen peroxide. The product enantioselectivities revealed matched-mismatched effects that depended strongly on the structures of the chiral salicylic aldehydes. In particular, thioanisole and 2-bromothioanisole could be oxygenated in almost quantitative yields and with enantiomeric excesses up to 78 % ee. The latter values are the highest ones ever achieved for these substrates using transition metal catalysts and hydrogen peroxide as terminal oxidant. (C) 1998 Elsevier Science Ltd. All rights reserved.