Abstract
In a monooxygenation reaction of cyclohexene in an air-equilibrated dichloromethane suspension containing 5,10,15,20-tetraphenylporphyrinatomanganese(III) chloride (Mn(III)(tpp)Cl) as a catalyst, insoluble zinc powder as a reductant, and benzoic anhydride as a cleaving reagent of dioxygen double-bond, a large amount of cyclohexene oxide was produced by adding an axial ligand for Mn(tpp)Cl such as Cl − ion, Br − ion, and 1-methylimidazole (1-MeIm) besides an electron mediator such as hexylviologen (HV 2+). The role of the ligand and the mediator has been investigated in detail by cyclic voltammetry. The Cl − and Br − ions promoted this epoxidation probably by assisting the oxygen transfer from Mn(V)-oxo complex, that is an intermediate in this reaction cycle, to cyclohexene, and HV 2+ functioned as the mediator of electron transfer from zinc to Mn(II)(tpp)–dioxygen adduct, enhancing the production of epoxide.
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