Mechanistic analysis of metal-oxygen active species and their synthetic application to new catalytic oxidation

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This study is intended to clarify the dioxygen activation process by artificial model complexes for heme enzymes through the spectroscopic characterization of the reactive intermediates such as peroxo-and hydroperoxo heme species, which are generated by the cryoradiolysis with γ-ray (one-electron reduction) of the corresponding dioxygen adducts. The employed complexes have the mimics of the essential functional groups, a) axial ligand, b) hydrogen bondings toward dioxygen, found in the active sites of heme enzymes. The resulting species from the 2K-cryoradiolysis of oxy-complex was assignable to a low-spin six-coordinated peroxoferric heme. After annealing at 133K, a hydroperoxo species was formed. These results confirm that the present complex can reproduce the first important part of the dioxygen activation process composed of the reduction of the coordinated dioxygen and the protonation of the resulting peroxo group.Iron porphyrin complexes, which bear four or two naphtholic hydroxyl groups in the vicinity of the metal center, were synthesized as models of prostaglandin H synthase. In the catalytic cycle of this enzyme, the tyrosyl residue is oxidized to the phenoxyl radical, which abstracts the specific hydrogen atom of the polyolefinic substrate. The present model complexes were oxidized with peroxy acid to give the active species composed of naphthoxyl radical and ferryl ion, as observed in the enzyme. The active species can oxidize non-conjugated diene substrate to the corresponding hydroperoxide with the recovery of the starting naphthol-appended ferric complex. The structure of one of the model complexes was determined by X-ray crystallography.