Asymmetric Oxidation of Sulfides Catalyzed by Vanadium(IV) Complexes of Dibromo- and Diiodo-Functionalized Chiral Schiff Bases

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 has 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 exhibit considerably higher enantioselectivity than those derived from (S)-phenylalaninol and (R)-leucinol. The enantiomeric excess values are 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 catalytic efficiency of VO(acac)2/Schiff base systems cannot be improved by the addition of carboxylic acids or carboxylate salts.