The mechanistic basis for electronic effects on enantioselectivity in the (salen)Mn(III)-catalyzed epoxidation reaction

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-beta-deuteriostyrene reveals that electron-rich catalysts display a more pronounced secondary inverse isotope effect than electron-deficient catalysts. A strong correlation between Delta Delta H double dagger 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.