The dioximatoiron complexes [Fe(Hdmed)](+), [Fe(Hdmpd)](+) and [Fe(H(2)dmdt)](2+) catalyze the oxidation of 3,5-di-tert-butylcatechol (H(2)dtbc) by O-2 to the corresponding o-benzoquinone (dtbq) at room temperature in MeOH solution. The reaction was followed by measuring the rate of dioxygen absorption as a function of catalyst, substrate and dioxygen concentration. Kinetic measurements reveal first-order dependence on the catalyst and O-2 concentration and saturation type behavior with respect to the substrate. The proposed reaction mechanism involves prior binding of the substrate H2dtbc and O-2 to the iron complex, forming a ternary active intermediate, decomposing in the rate-limiting step to a semiquinonato anion radical (dbsq(.-)), detected by ESR spectroscopy. It is then rapidly oxidized to the dtbq product. Added triethylamine accelerates the reaction to an extent much greater than that expected from the parallel base-catalyzed oxidation route. The kinetic behavior is similar to the TEA-free systems except for a saturation type dependence on TEA. This feature is due to a novel iron-enhanced oxidation path in which Hdtbc(-) binds O-2 to form the hydroperoxide HdtbcO(2)(-), coordinating to the iron(II) complexes as a hydroperoxo ligand. Subsequently, the hydroperoxo complex eliminates dbsq(-), which is directly oxidized by O-2 to dtbq. According to Mossbauer spectroscopy, the catalyst species are predominantly low-spin iron(II) complexes. (c) 2006 Elsevier B.V. All rights reserved.