Enantioselective oxidation of sulfide to sulfoxide on Ti-containing mesoporous silica prepared by a template-ion exchange method

Ti-containing MCM-41 (Ti-M41) showed the highest catalytic activity for the asymmetric oxidation of sulfide with hydrogen peroxide in the presence of optically active tartaric acid among several metal-ion-containing MCM-41 sa mples. The chemical and optical yields of methyl 4-methylphenyl sulfoxides reached 54% and 30% ee, respectively, in dichloromethane at a tartaric acid /titanium ratio of 2 at 273 K and 72 h reaction time. No titanium ion was d issolved into the CH2Cl2 solution from the Ti-M41 catalyst, concluding that the asymmetric oxidation indeed proceeded on the surface of the solid cata lyst. The COOH and OH groups in optically active tartaric acid are both ess ential to the enantioselective oxidation. The degree of enantiomeric excess gradually increased with the reaction time. This was clarified to be due t o the asymmetric induction in the sulfide oxidation and the kinetic resolut ion in the subsequent sulfoxides oxidation. With (R,R)-tartaric acid as a c hiral modifier, the formation rate of (R)-sulfoxide was 1.5 times larger th an that of (S)-isomer and the oxidation rate of (R)-sulfoxide to sulfone wa s 0.59 times that of (S)isomer. (C) 2001 Elsevier Science B.V. All rights r eserved.