Manganese Catalysts with Molecular Recognition Functionality for Selective Alkene Epoxidation

Selective epoxidation of alkenes is possible with a new manganese porphyrin catalyst, C_PMR, that uses hydrogen bonding between the carboxylic acid on the substrate molecule and a Kemp’s triacid unit. For two out of three olefin substrates employed, molecular recognition prevents the unselective oxidation of C−H bonds, and directs oxidation to the olefin moiety, giving only epoxide products. Weak diastereoselectivity is observed in the epoxide products, suggesting that molecular recognition affects the orientation of the catalyst-bound substrate. The previously reported manganese terpyridine complex C_TMR is shown to be a superior epoxidation catalyst to the porphyrin catalyst C_PMR. Good conversion of 2-cyclopentene acetic acid (substrate S2) with C_PMR is consistent with molecular modeling, which indicates a particularly good substrate/catalyst match. Evidence suggests that hydrogen bonding between the substrate and the catalyst is critical in this system.