ChemInform Abstract: Asymmetric Oxidation of Sulfides with H2O2 Catalyzed by Titanium Complexes of Schiff Bases Bearing a Dicumenyl Salicylidenyl Unit.

Abstract

The sterically hindered Schiff bases (L3–L5), prepared from 3,5-dicumenyl salicylaldehyde and chiral amino alcohols, were used in combination with Ti(OiPr)4 for asymmetric oxidation of aryl methyl sulfides with H2O2 as terminal oxidant. Among the ligands L3–L5, L4 with a tert-butyl group in the chiral carbon of the amino alcohol moiety gave the best result with 89% yield and 73% ee for the sulfoxidation of thioanisole under optimal conditions [with 1 mol% of Ti(OiPr)4 in a molar ratio of 100:1:1.2:120 for sulfide:Ti(OiPr)4:ligand:H2O2 in CH2Cl2 at 0 °C for 3 h]. The reaction afforded good yield (84%) with a moderate enantioselectivity (62% ee) even with a lower catalyst loading from 1.0 to 0.5 mol%. The oxidations of methyl 4-bromophenyl sulfide and methyl 4-methoxyphenyl sulfide with H2O2 catalyzed by the Ti(OiPr)4–L4 system gave 79–84% yields and 54–59% ee of the corresponding sulfoxides in CH2Cl2 at 20 °C. The chiral induction capability of the cumenyl-modified sterically hindered Schiff bases for sulfoxidation was compared with the conventional Schiff bases bearing tert-butyl groups at the 3,5-positions of the salicylidenyl unit. Copyright © 2011 John Wiley & Sons, Ltd.