A new evidence of the high-valent oxo-metal radical cation intermediate and hydrogen radical abstract mechanism in hydrocarbon hydroxylation catalyzed by metalloporphyrins

Forty metalloporphyrins with different peripheral substituents (EP) on the porphyrin ring, (RTPPMCl)-Cl-III and [RTPPMIII](2)O (M = Fe, Mn), were synthesized, and were used to catalyze cyclohexane hydroxylation with iodosobenzene under mild conditions. A Hammett relationship, log k = p sigma + c, was shown to exist between the rate constants k of the cyclohexane hydroxylation and Hammett constants (sigma of substituents on the porphyrin rings. The value of the reaction constant rho changed from 0.3 to 0.5 for the cyclohexane oxidation catalyzed by metalloporphyrins, and electron-withdrawing groups increased the reaction speeds. This means that the reaction mediator was a radical cation, and that the rate-determining step of the reaction was a radical reaction. These research results provided a new evidence for the high-valent ore-metal radical cation intermediate and hydrogen radical abstraction mechanism in hydrocarbon hydroxylation catalyzed by metalloporphyrins. (C) 2000 Elsevier Science B.V. All rights reserved.