Effects of methoxy-substituted metalloporphyrins in catalytic alkene epoxidation by n-Bu4NHSO5

TPPMnOAc and four different kinds of manganese tetraphenylporphyrin acetates were synthesized using different numbers of methoxy substituents in various positions of the phenyl rings. These porphyrins were used as catalysts in the epoxidation of various alkenes with tetra-n-butyl-ammonium hydrogen monopersulfate (n-Bu4NHSO5) as the oxidant and imidazole as the axial base. The following order of catalytic activity was obtained: TPPMnOAc >= T(2,3-OMeP)PMnOAc > T(4-OMeP) PMnOAc > T(3,4-OMeP)PMnOAc > T(2,4,6-OMeP)PMnOAc. By studying the UV-vis spectra in the reaction solution, the stability of the applied methoxy porphyrins and the effect of this factor on obtained yields were investigated. Lower catalytic activity in some of the methoxy porphyrins emphasized steric effects and special hydrogen bonding among the reaction elements. However, the stability of T(2,3-OMeP)PMnOAc under our reaction condition was considerable and high activity was observed. By adding small amounts of alcohol to the reaction solution, the effect of the solvent mixture was previewed and steps were taken to identify the active intermediate of the catalyst in these conditions.