Asymmetric Mn(III)-salen catalyzed epoxidation of unfunctionalized alkeneswith in situ generated peroxycarboxylic acids

Unfunctionalized aromatic alkenes were enantioselectively epoxidized with p eroxycarboxylic acids prepared in situ from urea-H2O land other anhydrous a dducts of H2O2) and carboxylic acid anhydrides (maleic, phthalic, and aceti c anhydride) using chiral Mn(III)-salen complexes as catalysts and N-methyl morpholine N-oxide (NMO) as an additive. Experimental results were compared with those reported earlier that employed aqueous hydrogen peroxide as the primary oxidant and the method presented here was found to offer both high er enantioselectivities and shorter reaction times. This novel epoxidation system was also compared with the Jacobsen's MCPBA/NMO system, and some dif ferences in reactivity and selectivity were observed. These differences cou ld possibly be explained assuming the presence of alternative mechanistic p athways during the catalytic cycle of the asymmetric epoxidation. (C) 2001 Elsevier Science B.V. All rights reserved.