SCHIFF-BASE LIGANDS CARRYING 2 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 va nadium-catalyzed asymmetric sulfoxidation of thioethers with hydrogen peroxide. The product enantioselectivities revealed matched-mismatched effects that depended strongly on the structures of the chiral salicy lic aldehydes. In particular, thioanisole and 2-bromothioanisole could be oxygenated in almost quantitative yields and with enantiomeric exc esses up to 78 % ee. The latter values are the highest ones ever achie ved for these substrates using transition metal catalysts and hydrogen peroxide as terminal oxidant. (C) 1998 Elsevier Science Ltd. All righ ts reserved.