Chiral Mn(III) salen complex-catalyzed enantioselective epoxidation of nonfunctionalized alkenes using urea–H 2O 2 adduct as oxidant

Abstract

Enantioselective epoxidation of chromenes, indene, and styrene mediated by manganese salen complexes 1a–b, 2a–b (1 mol%) as catalysts with urea–H 2O 2 adduct as an oxidant is observed to give excellent epoxide yield (>99%) in 0.5–4 h with enantiomeric excess (ee) in the range 56–99% except for styrene in which case 23–39% ee was obtained in 20 h. Even with a catalyst loading of 0.4 mol%, the system works efficiently with retention of enantioselectivity, albeit with an increase in reaction time. Kinetic investigations of a representative substrate, indene, with these catalysts indicated a kinetic profile having first-order dependence with respect to the concentrations of the catalyst and oxidant and independent of-initial concentration of the substrate. Based on kinetic, catalytic and experimental evidence, the mechanism of the epoxidation reaction is suggested.