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Catalytic C-H Bond Oxidation Using Dioxygen by Analogues of Heme Superoxide.

Heme active sites are capable of oxidizing organic substrates by four electrons using molecular oxygen (heme dioxygenases), where a dioxygen (O2) adduct of heme (FeIII-O2•-) acts as the primary oxidant, in contrast to monooxygenases, where high-valent species are involved. This chemistry, although lucrative, is difficult to access using homogeneous synthetic systems. Over the past few years using a combination of self-assembly and in situ resonance Raman spectroscopy, the distribution of different reactive intermediates formed during the electrochemical reduction of oxygen has been elucidated. An FeIII-O2•- species, which is the reactive species of dioxygenase, is an intermediate in heterogeneous electrochemical O2 reduction by iron porphyrins and its population, under electrochemical conditions, may be controlled by controlling the applied potential. Iron porphyrins having different axial ligands are constructed on a self-assembled monolayer of thiols on an electrode, and these constructs can activate O2 and efficiently catalyze the dioxygenation of 3-methylindole and oxidation of a series of organic compounds having C-H bond energies between 80 and 90 kcal mol-1 at potentials where FeIII-O2•- species are formed on the electrode. Isotope effects suggest that hydrogen-atom transfer from the substrate is likely to be the rate-determining step. Axial thiolate ligands are found to be more efficient than axial imidazoles or phenolates with turnover numbers above 60000 and turnover frequencies over 60 s-1. These results highlight a new reaction engineering approach to harness O2 as a green oxidant for efficient chemical oxidation.