Kinetics and mechanism of the oxidation of cyclohexanone by a nickel(III) macrocyclic ligand complex in CH3CN

Abstract

The oxidation of cyclohexanone by the Ni(III) complex of the macrocyclic ligand, 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane(meso-[14]ane) in acetonitrile solution containing CF3CO2H obeyed the rate equation: −d[Ni(III)]/dt = k[Ni(III)][cyclohexanone]/[CF3CO2H]. Kinetic and spectral studies in the presence of CF3CO2Na indicate the inhibition term in CF3CO2H is not due to complexation of trifluoroacetate but is a real acid inhibition term. The isotope effect, kH/kD using 2,2,6,6-tetradeuterocyclohexanone as substrate, was 3.2. The main product found in greater than 100% yield based on Ni(III) was the non-oxidative product, 2(cyclohexen-1-yl)cyclohexanone, which must have been formed in a radical chain reaction. A mechanism involving deprotonation of one of the coordinated nitrogens followed by rearrangement to a Ni(II) ligand radical intermediate is proposed. The ligand radical extracts a hydrogen from cyclohexanone to give [Ni(II)meso-[14]ane] and a cyclohexanone radical which initiates a radical chain reaction to give the observed dimeric product. Support for the mechanism is provided by recent studies of the base catalyzed decomposition of Ni(III) complexes in aqueous and non-aqueous solvents.In the presence of CF3CO2Na, the Ni(III) complex decomposes rapidly even in the absence of cyclohexanone. This is believed to occur via trifiuoroacetoxy radicals, a mechanism which has analogy in other systems.