OLEFINS EPOXIDATION BY PH(4)PHSO(5) CATALYZED BY MANGANESE PORPHYRINS- A NOVEL MECHANISTIC PROPOSAL

The epoxidation of three olefins, cyclooctene, styrene and cis-stilben e by Ph(4)PHSO(5), catalyzed by Mn(TMP)Cl, has been studied in dichlor oethane at 0 degrees C. In the absence of imidazole the reaction does not proceed, When the axial ligand is present the oxidation provides t he corresponding epoxides. Cis-stilbene stereospecifically yields cis- stilbene oxide. The epoxidation rates linearly depend on Ph(4)PHSO(5) and catalyst concentrations. By increasing the axial ligand concentrat ion the rates increase up to a plateau. This occurs when all the manga nese-porphyrin has been complexed by imidazole to form the bis-coordin ated adduct. Also by increasing the olefin initial concentration the r ates reach a plateau. This behavior, which might be taken as evidence of the formation of a Michaelis-Menten type intermediate, is alternati vely rationalized here on the basis of a balance of the rates of the v arious steps of the overall process. In particular it is envisaged tha t at high olefin concentration the oxygen transfer to the double bond from the ore-manganese porphyrin becomes at least as fast as the oxyge n transfer from Ph(4)PHSO(5) to the manganese porphyrin.