Dioxygen-activated reductive epoxidation of cyclohexene using Mn(III) porphyrin as catalyst and hexylviologen as electron mediator

In a monooxygenation reaction of cyclohexene in an air-equilibrated dichlor omethane suspension containing 5,10,15,20-tetraphenylporphyrinatomanganese( III) chloride (Mn(III)(tpp)Cl) as a catalyst, insoluble zinc powder as a re ductant, and benzoic anhydride as a cleaving reagent of dioxygen double-bon d, a large amount of cyclohexene oxide was produced by adding an axial liga nd for Mn(tpp)Cl such as Cl- ion, Br- ion, and 1-methylimidazole (1-MeIm) b esides an electron mediator such as hexylviologen (HV2+). The role of the l igand and the mediator has been investigated in detail by cyclic voltammetr y. 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 re action cycle, to cyclohexene, and HV2+ functioned as the mediator of electr on transfer from zinc to Mn(II)(tpp)-dioxygen adduct, enhancing the product ion of epoxide. (C) 1999 Elsevier Science B.V. All rights reserved.