Hydrogen-Atom Abstraction Reactions by Manganese(V)- and Manganese (IV)-Oxo Porphyrin Complexes in Aqueous Solution

摘要

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 Mn-V- and Mn-IV-oxo porphyrins in basic aqueous solution and investigated their reactivities in C-H bond activation of hydrocarbons. We now report that Mn-V- and Mn-IV-oxo porphyrins are capable of activating C-H bonds of alkylaromatics, with the reactivity order of Mn-V-oxo>Mn-IV-oxo; the reactivity of a Mn-V-oxo complex is 150 times greater than that of a Mn-IV-oxo complex in the oxidation of xanthene. The C-H bond activation of alkylaromatics by the Mn-V- 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 Mn-IV-oxo porphyrins to Mn-V-oxo and Mn-III 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 Mn-V- and Mn-IV-oxo species proceeds through a one-electron process, in which a Mn-IV-oxo porphyrin is formed its a product in the C-H bond activation by a Mn-IV-oxo porphyrin, followed by a further reaction of the Mn-IV-oxo porphyrin with Substrates that results in the formation of a Mn-III porphyrin complex. This result is ill contrast to the oxidation of sulfides by the Mn-V-oxo porphyrin. in which the oxidation of thioanisole by the Mn-V-oxo complex produces the starting Mn-III porphyrin and thioanisole oxide. This result indicates that the oxidation of sulfides by the Mn-V-oxo species occurs by means of a two-electron oxidation process. In contrast, a Mn-IV-oxo porphyrin complex is not capable of oxidizing sulfides due to a low oxidizing power in basic aqueous solution.