Asymmetric Oxidation Of Sulfides Catalyzed By Chiral Schiff Base-Mn And V Complexes

Although catalytic asymmetric oxidation of sulfides has been extensively investigated using transition metal complexes of various chiral ligands as catalysts. So far, the development of highly efficient and practical catalysts for this type of reactions is still an interesting challenge in modern catalytic and synthetic chemistry. Four chiral (pyrrolidine salen)Mn(III) complexes and eight Schiff-base ligands are synthesized. The structures of prepared ligands and complexes are characterized by many analytical methods such as IR, MS ¹H NMR and Elemental analysis. Under different reaction conditions, the catalytic performances in the asymmetric oxidation of sulfides are explored using two catalytic systems, chiral (pyrrolidine salen)Mn(III) complexes and VO(acac)₂/Schiff bases, with PhIO and H₂O₂ as oxidants respectively.With a series of chiral (pyrrolidine salen)Mn(III) complexes as catalysts for asymmetric oxidation of aryl methyl sulfides, moderate activity, good chemical selectivity and low to moderate enantioselectivity are attained using PhIO as oxidant. Enantioselectivity of sulfide oxidation is affected slightly by polar solvent and the sulfoxidation carried out in THF for thioanisole and in CH₃CO₂Et for electron deficient sulfides give better enantioselectivities. The addition of the donor ligand such as 4-phenylpyridine N-oxide (PPNO) or trimethylamine N-oxide (MNO) exhibits a minor positive effect on the enantioselectivity. Also we investigate the steric effect of the N_aza-substituent in the backbone of (pyrrolidine salen)Mn(III) complexes on the enantioselectivity of sulfide oxidation. An increase in the size of the R group on the N atom of the (pyrrolidine salen)Mn(III) complexs leads to an observable increase in the enantioselectivity. The sulfides' access pathway is discussed based on the catalytic results.The catalytic system of VO(acac)₂ and chiral Schiff base ligands, derived from 3,5-dibromo- or 3,5-diiodosalicylaldehyde and inexpensive chiral amino alcohols, displays good yields and moderate to high enantioselectivities for the asymmetric oxidation of arylmethyl sulfides at room temperature using 1 mol% catalyst (VO(acac)2:ligand = 1:2, mol/mol) with H2O2 as oxidant. The ligands derived from S-valinol give considerably higher enantioselectivity than others derived from i'-phenylalaninol and i?-leucinol. Moreover, the ee values can be improved up to 88% for methyl phenyl sulfoxide and 92% for methyl /?-bromophenyl sulfoxide by slow dropwise-addition of H2O2 with the ligand prepared from S-valinol and 3,5-diiodosalicylaldehyde. Present studies show that the catalytic performance of VO(acac)2/Schiff base system can not be improved by addition of carboxylic acids or carboxylate salts.