EPOXIDATION OF OLEFINS BY CYTOCHROME-P-450 MODEL COMPOUNDS - KINETICS AND STEREOCHEMISTRY OF OXYGEN ATOM TRANSFER AND ORIGIN OF SHAPE SELECTIVITY

An analysis of the mechanism of the lithium hypochlorite epoxidation of olefins catalyzed by Mn(TPP[meso-tetraphenylporphyrinato])Cl is presented. It was previously reported that the O2 atom transfer from Mn porphyrin to olefin occurs via an oxo-olefin intermediate whose breakdown is the rate-determining step of the catalytic cycle. Stereochemical and kinetic evidence presented here indicate that both formation of this intermediate and its subsequent breakdown to form epoxide are, in general, concerted processes. The rarely observed loss of stereoselectivity represents the presence of a minor leakage pathway in the breakdown of the oxo-olefin complex of form epoxide. Shape selective olefin epoxidation is achieved when the sterically hindered porphyrin, Mn(TMP[meso-tetramesitylporphyrinato])Cl, is used as a catalyst in the hypochlorite system. The observed shape selectivity parallels the large differences in binding energies for formation of the oxo-olefin complex.