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 β-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)MnO 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)PBr6(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.

Oxidation of cyclic and acyclic olefins bearing hindered and unhindered double bonds with iodosylbenzene in the presence of different Mn-porphyrins shows that the electron rich Mn-porphyrins may have comparable or higher catalytic efficiencies compared with the electron-deficient ones.Figure optionsDownload as PowerPoint slideHighlights
► Comparable or higher catalytic efficiency of electron-rich Mn-porphyrins.
► Complex pattern of activity of Mn-porphyrins in the presence of different alkenes.
► Unusual cis to trans-stilbene oxide isomerization using an ortho substituted Mn-porphyrin.