Iron catalyzed competitive olefin oxidation and ipso-hydroxylation of benzoic acids: Further evidence for an Fe^V=O oxidant

The iron complex [Fe^II(TPA)(CH₃CN) ₂](OTf)₂ (1) [TPA = tris(2-pyridylmethyl)amine] with H₂O₂ as an oxidant performs ipso-hydroxylation of electron-withdrawing benzoic acids at room temperature, leading to multiple turnovers of corresponding phenols. ipso-Hydroxylation competes with olefin epoxidation and cis-dihydroxylation in the presence of olefins, with the product ratios being modulated by the relative amounts of benzoic acid, olefin, and water. It is proposed that benzoic acid and water compete for the available sixth site on the [(TPA)Fe^III(OOH)] intermediate, which undergoes O-O bond heterolysis to form, respectively, the Fe^V(O)(O₂CAr) and Fe^V(O)(OH) oxidants that determine the product outcome. The putative Fe^V(O)(O₂CAr) oxidant decays either by undergoing oxidative decarboxylation and subsequent ipso-hydroxylation to form the observed phenol product or by oxo-transfer to olefins to form epoxide. The observed higher yield of phenol over epoxide or cis-diol in all cases studied, where an electronwithdrawing benzoic acid is present in the reaction mixture, suggests that intramolecular decay of the putative Fe^V(O)-(O ₂CAr) oxidant is favored over intermolecular olefin oxidation. These results support the mechanistic framework postulated for Fe(TPA) oxidative catalysis and further strengthens the notion that oxoiron(V) species are the key oxidants in these reactions.