THE MECHANISTIC BASIS FOR ELECTRONIC EFFECTS ON ENANTIOSELECTIVITY INTHE (SALEN)MN(III)-CATALYZED EPOXIDATION REACTION

Enantioselectivity in the (salen)Mn-catalyzed asymmetric epoxidation r eaction correlates directly with the electronic properties of the liga nd substituents, with complexes bearing electron-donating substituents affording highest ee's. Several lines of evidence point to a single f actor-control of the position of the transition state along the reacti on coordinate-as being responsible for the electronic effects on enant ioselectivity. Analysis of the epoxidation of cis-beta-deuteriostyrene reveals that electron-rich catalysts display a more pronounced second ary inverse isotope effect than electron-deficient catalysts. A strong correlation between Delta Delta H double dagger and the electronic ch aracter of the catalyst is also observed, The conclusion that enantios electivity is tied to the position of a transition state along the rea ction coordinate may hold general implications for the design of asymm etric catalysts, particularly those that effect reactions without subs trate precoordination.