Catalytic Substrate Oxidations by Iron/Hydrogen Peroxide: Generation of High-valent Iron-oxo Intermediates by Proton-Induced O-O Bond Cleavage

Abstract

Non-heme iron enzymes are responsible for various stereospecific hydroxylations under ambient conditions. Pterin-dependent hydroxylases hydroxylate the aromatic rings of aromatic amino acids while Rieske dioxygenases perform cis-dihydroxylation of aromatic double bonds. The most important step in the functioning of these enzymes is the cleavage of the O–O bond. The goal of this research is to investigate the reactivity of synthetic model complexes with H2O2 towards selective oxidative transformations. Iron complexes with tetradentate ligands (TPA, BPMEN where TPA = tris(2- pyridylmethyl)amine and BPMEN = N,N¢-bis(2-pyridylmethyl)-N,N¢-dimethyl-1,2- diaminoethane)) are synthesized and tested in the acid-catalyzed oxidation reactions with H2O2 as the oxidant. The addition of aliphatic acids is found to increase the yield of both alkane and olefin oxidations. Interestingly, the use of benzoic acids, instead, leads to the hydroxylation of the aromatic ring, which is found to be in competition with olefin oxidation. Further investigation reveals FeTPA/H2O2-catalyzed regioselective ortho- (producing salicylates) and ipso-hydroxylation (producing phenolates) of benzoates. Detailed kinetic studies of these reactions – to get a better understanding of the reaction mechanism – show an acid-induced H/D kinetic isotope effect, suggesting the involvement of proton in the rate limiting O–O bond heterolysis of iron(III)-peroxos towards the formation of the high-valent FeV=O species (postulated as the active oxidant). The tuning of ligand (TPA and BPMEN) electronics, by introducing electrondonating substituents in the backbone, exerts a 'push-effect' and increases the rate of O–O bond cleavage and affords a higher yield of catalytic oxidation products.