Ferroxime(II)-catalysed oxidation of 3,5-di-tert-butylcatechol by O2. Kinetics and mechanism†

Abstract

The complex [Fe(Hdmg)₂(MeIm)₂] 1, referred to as ferroxime(II), was found to be the precursor of a selective catalyst for the oxidative dehydrogenation of 3,5-di-tert-butylcatechol (H₂dbcat) to the corresponding 1,2-benzoquinone (dtbq) at room temperature and atmospheric dioxygen pressure. The observed kinetic behaviour in MeOH is consistent with solvolysis of one MeIm ligand and binding of dioxygen to the five-co-ordinate intermediate to form a superoxo complex. The latter abstracts an H atom from H₂dbcat via a hydrogen-bonded ternary active intermediate, affording the semiquinone anion radical dbsq^˙– and [Fe^III(Hdmg)₂(MeIm)(O₂H)]. Utilising an electron and a proton from H₂dbcat, the latter undergoes heterolytic cleavage to yield a ferryl species, which rapidly oxidises a second H₂dbcat. Complexation of dbsq^˙– with [Fe^II(Hdmg)₂(MeIm)] affords a strongly coloured paramagnetic species [Fe^II(Hdmg)₂(MeIm)(dbsq^˙–)], which persists throughout the catalytic reaction, acting as buffer for dbsq^˙–. The proposed mechanism involves steps similar to those of the known cytochrome P450 cycle, with H₂dbcat acting as both ancillary electron source and substrate.