Mechanism of Acid-Catalyzed Epoxidation of Alkenes with Peroxy Acids

A 6.8 fold increase in the rate of epoxidation of (Z)-cyclooctene with m-chloroperbenzoic acid is observed upon addition of the catalyst trifluoroacetic acid. Kinetic and theoretical studies suggest that this increase in rate is due to complexation of the peroxy acid with the undissociated acid catalyst (HA) rather then protonation of the peroxy acid. The transition structure for oxidation of ethylene with protonated peroxyformic acid exhibits a spiro orientation of the electrophilic oxygen at the QCISD/6-31G(d) level and the complexed peroxy acid (HCO3H·HA) transition state is also essentially spiro at the ab initio and density functional levels. At the B3LYP/6-311G(d,p) level the protonated transition structure exhibits a more planar approach where the O3−H9 of the peroxy acid lies in the plane of the π-system of ethylene, and the barrier for formation of protonated oxirane is only 4.4 kcal mol-1. Epoxidation with neutral and complexed peroxyformic acid also involves a symmetrical spiro orientation affording an epoxide, and the barriers for formation of oxirane at the same level are 14.9 kcal mol-1 and 11.5 kcal mol-1, respectively. The free energy of activation for the epoxidation of ethylene by peroxyformic acid is lowered by about 3 kcal mol-1 upon complexation with the catalyst.