Electrocatalytic oxidation of alkenes by water-soluble manganese porphyrins in aqueous media: a comparison of the reaction products at different oxidation states

Abstract

Manganese(III) tetrakis(N-methyl-4-pyridyl)porphine (MnTMPyP) is water-soluble and exhibits a reversible MnIII/II redox couple in acidic solutions. However, the MnIV/III couple cannot be seen by cyclic voltammetry in aqueous solutions of pH < 8. Spectroelectrochemical methods have shown that (H2O)MnIIITMPyP undergoes a one-electron oxidation with slow heterogeneous electron transfer rate to form OMnIITMPyP, which then oxidizes cyclopent-2-ene-1-acetic acid (1) catalytically to give cyclopent-2-ene-4-one-1-acetic acid (2). Further electrochemical oxidation of OMnIVTMPyP causes the deactivation of the catalyst, presumably due to porphine ring degradation. A sterically hindered porphyrin, manganese(III) tetrakis(sulfonatomesityl)porphine (MnTSMP), is first oxidized at the morphine ring and is stable as a radical cation (H2O)MnIIITSMP+. in pH < 2 solutions. In pH 8.5 buffer solutions, MnIIITSMP is oxidized totally by two electrons at Eappl = + 1.05 V. The electrogenerated OMnIVTSMP+. reacts rapidly with 1 to give 2 and cyclopent-2,3-diol-1-acetic acid (3). The reaction mechanisms are proposed.