A mechanistic study of the oxidation of phenols in aqueous solution by oxoiron(IV) tetra(N-methylpyridyl)porphyrins. A model for horseradish peroxidase compound II?

Abstract

The reaction of oxoiron(IV) tetra(2-N-methylpyridyl)porphyrin (OFe^IVT2MPyP), generated from iron(III) tetra(2-N-methylpyridyl)porphyrin and tert-butyl hydroperoxide, with 3-cyanophenol in aqueous solution (pH 7.7) shows first-order dependence on the concentration of the phenol and the oxidant. The pH dependence of the measured second-order rate constant (pH 7.7–8.6) indicates that the phenol, and not the phenolate ion, is the substrate oxidised by OFe^IVT2MPyP.

Substituent effects on the second-order rate constant were obtained from the oxidation of phenol and six monosubstituted derivatives and these data were analysed by Hammett and modified Hammett equations. The ρ values obtained, in conjunction with the results from an EPR study of the oxidation of the water soluble hindered phenol, Trolox C, and the oxidation of [O-¹H₂]-4-fluorophenol, suggest that the rate-determining step in these reactions involves hydrogen atom abstraction from the phenol by the oxoiron(IV) species.

The Hammett analyses of rate data from the oxidation of phenols by horseradish peroxidase compound II have been reexamined and compared with those from the present study. This leads to the conclusion that the enzymatic process involves a rate determining electron transfer from the phenol to the oxo-haem.