Electrocatalytic Epoxidation of Cyclohexene by Manganese(III) Porphyrin Using Electron Mediator

Abstract

Abstract For an electrocatalytic oxidation of cyclohexene in an air-equilibrated acetonitrile solution containing (5,10,15,20-tetraphenylporphyrinato)manganese(III) as a catalyst, benzoic anhydride as an acylating reagent and 1-methylimidazole as a ligand to Mn porphyrin, the addition of the compounds such as Methylene Blue, Safranine-T, and viologens promotes the production of cyclohexene oxide. The result implies that these compounds act effectively as mediators for the electron transfer from electrode to Mn porphyrin because they have more negative reduction potential than Mn porphyrin. The faradaic efficiency reaches about 100% by the use of benzylviologen under an optimum condition. But, in the presence of methylene blue, the maximum faradaic efficiency gives low values of about 30%. On the other hand, cyclohexene oxide is also produced by the use of zinc powder as a reductant, but the utilization efficiency of zinc gives relatively low values of 30—40% regardless of the kind of the mediator. This result is ascribed to the irreversible reduction of the mediator and the direct reaction of oxomanganese complex, which is an intermediate in this epoxidation reaction, with zinc powder.