CATALYTIC ASYMMETRIC OXIDATION OF SULFIDES TO SULFOXIDES WITH TERT-BUTYL HYDROPEROXIDE USING BINAPHTHOL AS A CHIRAL AUXILIARY

The asymmetric oxidation of sulfides to chiral sulfoxides in moderate yield with tert-butyl hydroperoxide (TBHP) was catalyzed very effectively by a titanium complex produced in situ from a titanium alkoxide and a chiral binaphthol. The highest enantioselectivities (up to 96% ee) were obtained with commercial aqueous 70% TBHP under atmospheric conditions at 25-degrees-C with 2.5 mol % of the catalyst. The presence of more than 1.0 mol equiv of water relative to sulfide was essential for the oxidation, and it was found that the water was necessary not only to produce an effective catalyst for the highly enantioselective oxidation but also to maintain the catalytic activity of the titanium-binaphthol complex for a longer time. The nature of the solvent had a remarkable effect upon the ee of the sulfoxide. Carbon tetrachloride was proven to be the solvent of choice; the use of other chlorinated solvents resulted in low enantioselectivity. A moderate level of asymmetric amplifiication was observed in this catalytic system. From a mechanistic standpoint, it was revealed that the initial asymmetric oxidation to the chiral sulfoxide (approximately 50% ee) is followed by the kinetic resolution of the sulfoxide; that is, the titanium-binaphthol complex catalyzes not only the asymmetric oxidation but also the subsequent kinetic resolution.