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

The reaction of oxoiron(IV) tetra(2-N-methylpyridyl)porphyrin (OFe(IV) T2MPyP), generated from iron(III) tetra(2-N-methylpyridyl)porphyrin an d 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 c onstant (pH 7.7-8.6) indicates that the phenol, and not the phenolate ion, is the substrate oxidised by OFe(IV)T2MPyP.Substituent effects on the second-order rate constant were obtained from the oxidation of ph enol and six monosubstituted derivatives and these data were analysed by Hammett and modified Hammett equations. The rho values obtained, in conjunction with the results from an EPR study of the oxidation of th e water soluble hindered phenol, Trolox C, and the oxidation of [O-H-2 (1)]-4-fluorophenol, suggest that the rate-determining step in these r eactions involves hydrogen atom abstraction from the phenol by the oxo iron(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 con clusion that the enzymatic process involves a rate determining electro n transfer from the phenol to the oxo-haem.