Abstract: The catalyst system of VO(acac)2 and Schiff base ligands derived from 3,5-dibromo- or 3,5-diiodosalicylaldehyde and inexpensive chiral amino alcohols was prepared. This catalyst displayed good yields and moderate to high enantioselectivity for the asymmetric oxidation of aryl methyl sulfides at room temperature when 1% catalyst (VO(acac)2/ligand molar ratio of 1∶2) and H2O2 oxidant were used. The ligands derived from (S)-valinol exhibited considerably higher enantioselectivity than those ligands derived from (S)-phenylalaninol and (R)-leucinol. The enantiomeric excess values were improved up to 88% for methyl phenyl sulfoxide and 92% for methyl p-bromophenyl sulfoxide by slow dropwise addition of H2O2 with the ligand prepared from 3,5-diiodosalicylaldehyde and (S)-valinol. The present study showed that the catalytic efficiency of VO(acac)2/Schiff base systems could not be improved by the addition of carboxylic acids or carboxylate salts.

Key words: bromine, iodine, Schiff base, chiral sulfoxide, vanadium, thioether, asymmetric oxidation