C-H oxidation by hydroxo manganese(v) porphyrins: a DFT study

摘要

A DFT study on toluene oxidation by Mn(v) porphyrins shows that both Mn~v(O) and, remarkably, Mn~v(OH) are potent H-atom abstractors.rnC-H oxidation catalyzed by metal porphyrins is one of the best approaches to the activation of inert C-H bonds. Iron porphyrins are used by nature in cytochrome enzymes, and manganese porphyrins are used as synthetic catalysts.rnThe rebound mechanism has been postulated for metalloporphyrin-catalyzed C-H oxidations involving iron, manganese, and ruthenium, although alternative mechanisms have also been proposed. The key step of this mechanism is a radical hydrogen abstraction from the substrate. It is generally accepted that this reaction is promoted by a Mn(v) oxo intermediate, which is reduced to a Mn(IV) hydroxo species (Scheme 1). A Mn(v) hydroxo intermediate can also be formed by protonation, but it is not known if this species can promote H abstraction leading to a Mn(IV) aqua species, as in the case of the Fe~(III)(OH) function in lipoxygenases.