Olefins epoxidation by Ph(4)PHSO(5) catalyzed by manganese porphyrins: A novel mechanistic proposal

The epoxidation of three olefins, cyclooctene, styrene and cis-stilbene by Ph(4)PHSO(5), catalyzed by Mn(TMP)Cl, has been studied in dichloroethane at 0 degrees C. In the absence of imidazole the reaction does not proceed, When the axial ligand is present the oxidation provides the 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 concentration the rates increase up to a plateau. This occurs when all the manganese-porphyrin has been complexed by imidazole to form the bis-coordinated adduct. Also by increasing the olefin initial concentration the rates reach a plateau. This behavior, which might be taken as evidence of the formation of a Michaelis-Menten type intermediate, is alternatively rationalized here on the basis of a balance of the rates of the various steps of the overall process. In particular it is envisaged that at high olefin concentration the oxygen transfer to the double bond from the ore-manganese porphyrin becomes at least as fast as the oxygen transfer from Ph(4)PHSO(5) to the manganese porphyrin.