Hydrogen-atom abstraction reactions by manganese(V)- and manganese(IV)-oxo porphyrin complexes in aqueous solution

Abstract

High-valent manganese(IV or V)-oxo porphyrins are considered as reactive intermediates in the oxidation of organic substrates by manganese porphyrin catalysts. We have generated MnV- and MnIV-oxo porphyrins in basic aqueous solution and investigated their reactivities in C-H bond activation of hydrocarbons. We now report that MnV- and Mn IV-oxo porphyrins are capable of activating C-H bonds of alkylaromatics, with the reactivity order of MnV-oxo>Mn IV-oxo; the reactivity of a MnV-oxo complex is 150 times greater than that of a MnIV-oxo complex in the oxidation of xanthene. The C-H bond activation of alkylaromatics by the MnV- and Mn IV-oxo porphyrins is proposed to occur through a hydrogen-atom abstraction, based on the observations of a good linear correlation between the reaction rates and the C-H bond dissociation energy (BDE) of substrates and high kinetic isotope effect (KIE) values in the oxidation of xanthene and dihydroanthracene (DHA). We have demonstrated that the disproportionation of MnIV-oxo porphyrins to MnV-oxo and MnIII porphyrins is not a feasible pathway in basic aqueous solution and that Mn IV-oxo porphyrins are able to abstract hydrogen atoms from alkylaromatics. The C-H bond activation of alkylaromatics by MnV- and MnIV-oxo species proceeds through a one-electron process, in which a MnIV-oxo porphyrin is formed as a product in the C-H bond activation by a MnV-oxo porphyrin, followed by a further reaction of the MnIV-oxo porphyrin with substrates that results in the formation of a MnIII porphyrin complex. This result is in contrast to the oxidation of sulfides by the MnV-oxo porphyrin, in which the oxidation of thioanisole by the MnV-oxo complex produces the starting Mn III porphyrin and thioanisole oxide. This result indicates that the oxidation of sulfides by the MnV-oxo species occurs by means of a two-electron oxidation process. In contrast, a MnIV-oxo porphyrin complex is not capable of oxidizing sulfides due to a low oxidizing power in basic aqueous solution. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.