A mndo study of olefin epoxidation by peroxyacids. : Hammond effects on the computed transition structures

Abstract

MNDO calculations of potential energy surfaces for the interaction between peroxyformic acid and ethylene or substituted ethylenes do not support the electrophilic mechanism preferred in the interpretation of this reaction. While the calculated interaction between neutral molecules of olefin and peroxyformic acid is repulsive over the entire range of studied reaction coordinates, protonated peroxyformic acid is predicted capable of addition to olefin, but with an activation energy of 39 kcal/mol. Peroxyformate anions, according to the MNDO calculations, should add readily to olefins. Electron withdrawing subntituents in the latter have the effect that adducts are formed without activation energy, while ethylene or olefins with donor substituents are predicted to react in a two-step mechanism, the first step being the formation of metastable intermediates with activation energies of 16 – 20 kcal/mol. The rate determining step of the reaction is the epoxide ring closure. She activation energies predicted for this step increase with the electron withdrawing capacity of substituents in line with the observed negative slopes of Hammett-like relationships. The nucleophilic mechanism suggested by the present calculations does not conflict with observed stereo-specificity of this reaction. Olefin substituents cause Hammond effects on the predicted transition structures, which are particularly pronounced on those geometrical parameters having the dominating contributions in the corresponding transition vectors. These predictions are in line with qualitative reaction surface models, but in contrast with VB considerations.