Asymmetric Kharasch reaction: catalytic enantioselective allylic oxidation of olefins using chiral pyridine bis(diphenyloxazoline)-copper complexes and tert-butyl perbenzoate

Abstract

Copper complexes of chiral pyridine bis(diphenyloxazoline)-type ligands have been studied as catalysts for the enantioselective allylic oxidation of olefins. Using 2.5-5 mol % of these chiral catalysts and tert-butyl perbenzoate as oxidant, optically active allylic benzoates were obtained in up to 86% ee. A variety of copper salts was studied under different conditions and in different solvents. Acetone was found to be a superior solvent for the reaction. Use of phenylhydrazine in conjunction with the chiral copper complex played a crucial role in increasing the rate of the reaction. Use of 4 Å molecular sieves increased the optical yield of product in almost every case.