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 (Fe^III-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 Fe^III-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 Fe^III-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.