Kinetic model for the reaction of cobalt porphyrins with olefins under free radical conditions

A general kinetic, model to describe the interaction of two dissimilar olefins with cobalt(II) tetrakis(anisyl)porphyrin complexes in the presence of organic radicals has been developed. The kinetic scheme presumes the formation of organometallic products both by radical addition to Co(II) and by reaction of hydridocobalt(III) complexes with the corresponding olefins. The proposed general mechanism also presumes that cobalt hydride is formed by two pathways-either by bimolecular reaction of the Co(II) porphyrin complex with the radical or by unimolecular beta-hydrogen atom elimination from the organocobalt(III) species. The general model is then modified to the specific case of competition between methacrylonitrile and aliphatic olefins. On application of the simplifying assumption that most of the hydride forms through the bimolecular reaction and that both olefins follow the same predominant reaction pathways, a model describing the concentration of organometallic species derived from both olefins is developed. This dependence allows the evaluation of some elementary relative reaction rate constants for organometallic dissociation and reactions of the olefins with the hydridocobalt complex. It is shown that, despite the observation that the products derived from simple olefins such as cyclopentene are formed at much higher steady-state concentrations, methacrylonitrile reacts with hydridocobalt(III) at more than 25 times the rate of those simple olefins. Limitations on applying the developed methods of rate constant measurements have also been considered.