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 synt hesized, and were used to catalyze cyclohexane hydroxylation with iodosoben zene under mild conditions. A Hammett relationship, log k = p sigma + c, wa s shown to exist between the rate constants k of the cyclohexane hydroxylat ion and Hammett constants (sigma of substituents on the porphyrin rings. Th e value of the reaction constant rho changed from 0.3 to 0.5 for the cycloh exane oxidation catalyzed by metalloporphyrins, and electron-withdrawing gr oups increased the reaction speeds. This means that the reaction mediator w as 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 abs traction mechanism in hydrocarbon hydroxylation catalyzed by metalloporphyr ins. (C) 2000 Elsevier Science B.V. All rights reserved.