The Mechanistic Basis for Electronic Effects on Enantioselectivity in the (salen)Mn(III)-Catalyzed Epoxidation Reaction

Abstract

Enantioselectivity in the (salen)Mn-catalyzed asymmetric epoxidation reaction correlates directly with the electronic properties of the ligand substituents, with complexes bearing electron-donating substituents affording highest ee's. Several lines of evidence point to a single factor control of the position of the transition state along the reaction coordinate as being responsible for the electronic effects on enantioselectivity. Analysis of the epoxidation of cis -β-deuteriostyrene reveals that electron-rich catalysts display a more pronounced secondary inverse isotope effect than electron-deficient catalysts. A strong correlation between ΔΔ H ⧧ and the electronic character of the catalyst is also observed. The conclusion that enantioselectivity is tied to the position of a transition state along the reaction coordinate may hold general implications for the design of asymmetric catalysts, particularly those that effect reactions without substrate precoordination.