Solvent effects on the activation barriers of olefin epoxidation - A density functional study

Solvent effects reduce the activation energies calculated for olefin epoxidation by dioxiranes and percarboxylic acids in the gas phase. Association of a proton donor to the oxygen atom of the peroxo group not attacked by the olefin activates the peroxo group and lowers the reaction barrier by 2-6 kcal/mol, in agreement with experimental findings. A polarizable solvent reduces the activation barrier further by about 2 kcal/mol. Overall, satisfactory agreement with experimental activation energies is achieved if calculated intrinsic (electronic) barriers are corrected for both types of solvent effects. As an application we also discuss the electrostatic solvent effect on the activation barrier of the olefin epoxidation by different base adducts of a rhenium bisperoxo complex and we rationalize opposite trends with different solvent dielectric constants.