Kinetics and mechanism of the cobaloxime(II)-catalysed oxidative dehydrogenation of 3,5-di-tert-butylcatechol by O2. A functional oxidase model

Abstract

Triphenylphosphinecobaloxime(II) has been found to be a selective catalyst for the oxidative dehydrogenation of 3,5-di-tert-butylcatechol to the corresponding 1,2-benzoquinone at room temperature and atmospheric dioxygen pressure. In a rapid initial phase the catalyst is reversibly transformed to a previously characterised catecholatocobaloxime(III) species, which persists throughout the reaction, keeping Co^II at a low level. The semiquinone anion radical (dbsq^˙–) and its cobaloxime(III) complex Co^III(dbsq^˙–) have been detected as intermediates by ESR spectroscopy. The kinetics was followed by volumetry. According to the proposed mechanism, in the rate-determining step superoxocobaloxime (Co^IIIO₂) abstracts an H atom from the cathechol via an hydrogen-bonded intermediate. The system investigated is a functional model of catecholase (oxidase) activity, based on free-radical intermediates, a possibility recently demonstrated for certain oxidoreductases.