Substitution effects on the catalytic activity of Mn(III)-porphyrins in epoxidation of alkenes with iodosylbenzene: A comparison between the electron-rich and electron-deficient porphyrins

Oxidation of different olefins with iodosylbenzene in the presence of Mn(III) complexes of meso-tetra(para-tolyl)porphyrin, meso-tetra(ortho-tolyl)porphyrin, meso-tetra(thien-2-yl)porphyrin and beta-hexaboromo-meso-tetra(thien-2-yl)porphyrin as catalyst has been studied. Oxidation of cis- and trans-stilbene in a competitive reaction strongly suggests the involvement of a high valent (porphyrin)Mn=O as the active oxidant intermediate, in the case of each catalyst. Clear observation of the band relevant to a (porphyrin)Mn(IV)=O species in the presence of excess amounts of styrene shows the stability of this moiety towards reaction with olefins. Although, the stability of metalloporphyrins towards oxidative degradation decreases in the order MnT(o-tolyl)P(OAc) > MnT(thien-2-yl)PBr(6)(OAc) > MnT(p-tolyl)-P(OAc) >= MnT(thien-2-yl)P(OAc), a complex pattern of catalytic activity and product (epoxide) selectivity has been found for the Mn-porphyrins in oxidation of various alkenes. (C) 2011 Elsevier Ltd. All rights reserved.