Oxidative Decarboxylation of Benzilic Acid by a Biomimetic Iron(II) Complex: Evidence for an Iron(IV)-Oxo-Hydroxo Oxidant from O2

摘要

The existence of dioxygen-activating nonheme iron enzymes that carry out four-electron substrate oxidations requires a mechanism where the initially formed iron(III)-superoxide species must be involved as an oxidant to initiate the reaction. Such a step has been proposed for many aromatic and aliphatic C-C bond cleaving dioxygenases. Recently discovered examples include 2-hydroxyethylphosphonate (HEP) dioxygenase (HEPD) which catalyzes the cleavage of the HEP CI-C2 bond to form hydroxymethylphosphonate and formate, and CloR, which is involved in the conversion of a mandelate moiety to benzoate in the biosynthesis of chlorobiocin, an aminocoumarin antibiotic. For the above examples, the substrate provides all four electrons needed for the reduction of O2 to water. In contrast, the Rieske dioxygenases require two electrons from NADH to carry out the cis-dihydroxylation of aromatic C=C bonds. A high-valent iron-oxo-hydroxo intermediate has been proposed to carry out this transformation.