Enantioselective epoxidation of olefins catalyzed by chiral dimeric and partially water-soluble monomeric salen-Mn(III) complexes in the presence of novel co-catalysts

Abstract

New chiral salen-Mn(III) complexes analogous to Jacobsen's catalyst ([( R, R)- N, N′-bis(3,5-di- tert-butylsalicylidene)-1,2-cyclohexanediaminato(2-)]manganese(III)chloride) have been synthesized and employed in the enantioselective epoxidation of non-functionalized alkenes. Two dimeric chiral salen-Mn(III) complexes proved to be effective catalysts in the asymmetric epoxidation of some cyclic alkenes, and the two other monomeric chiral salen-Mn(III) catalysts should have certain inherent phase-transfer capability during the epoxidation because of their weak water-solubility. Based on a new synergetic strategy of catalysts with their corresponding co-catalysts, pyridine N-oxide and Fujita's porous coordination polymer were used as co-catalyst. In particular, Fujita's porous coordination polymer ({[Cd(4,4′-bpy) 2](NO 3) 2} ∞) accelerated epoxidation without depressing the chiral induction as well as conversions in the fresh catalytic cycle. In general, moderate to high enantioselectivity and acceptable yields were achieved when NaClO was used as oxidant under CH 2Cl 2/H 2O biphasic media in the presence of these co-catalysts. Additionally, the recovery and recycling of one dimeric salen-Mn(III) complex were tested in order to further apprehend the effects of different co-catalysts on the alkene epoxidations.