Studies on development of the selective oxygenation catalysts based on mimicking function of nonheme monoiron oxygenases

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Following studies were performed on the development of oxygenase-like catalytic functions by nonheme monoiron complexes and manganese complexes and on the clarification of mechanisms of electron transfer that is essential for activation of substrates.1)Intradiol and extradiol oxygenative cleavage of catechols by monoiron complexesCatalytic activity of oxygenations by monoiron complexes ligated by tripodal ligands and tridentate ligands were studied. Effect of electronic and steric effects of substituents on the selective intradiol cleavage gave important information of the oxygen activation and oxygen insertion process into the C-C bonds. In the extraiol cleavage by iron complexes with tridentate ligands also clarified the mechanism of oxygenation via the direct oxygen binding to the iron center. Not only ligands with only the N binding sites but also ligands with N and O binding sites were used for clarification of the O2 insertion into catechols.2)Intra and extradiol oxygenative clea … More vage of catechols by manganese complexesBy mimicking manganese-containing extradiol cleaving oxygenases, Mn(II)-semiquinone complexes were first synthesized. The complexes were found to be oxygenated to give the intradiol cleavage productws. Interestingly, the reversible uptake of oxygen was observed, indicating that the molecular oxygen binds first to the metal center (iron and manganese) prior to binding to the catechol carbon.3)Oxygenations in water by iron complexes with water-soluble ligandsAlthough enzymatic oxygenations proceeds in aqueous media, models oxygenations have been performed in organic solvents. This is partly due to the ligands soluble only in organic solvents, and partly on exclusion of oxygen saurce other than molecular oxygen. We have first developed a model system that works in water to give the selective intradiol cleavage of mono-alkyl catechols, e.g. 4-butyl catechols, catechols with the electron-withdrawing substituents, such as chloro and carboxylate, by using the TPA ligands with sulfonate subsitutients. It is important to find that the protocatechuic acid was found to be oxygenatively cleaved by monoiron complexes, because the model oxygenation has not be developed in spite of the most popular enzyme, protocatechuic oxygenases. It was also found that the pH control of the media is essential for the aqueous system. We also found that the TPA complexes without the solfonate substituents are also effective for the reaction in aqueous media. Less