Reductive elimination from organometals. Thermal and induced decomposition of arylmethylnickel(II) complexes

Abstract

The thermal decomposition of arylmethylbis(triethylphosphine)nickel(II), ArNiMeL 2, is studied in hydrocarbon solutions, both in the presence and absence of aryl halide. The direct thermolysis affords methylarenes without aryl scrambling, by first-order kinetics. The inverse phosphine dependence of the rate is related to a dissociative mechanism proceeding via reductive elimination directly from the coordinatively unsaturated ArNiMeL intermediate. In contrast, the reductive elimination of methylarene induced by aryl halide is a radical chain process in which there is extensive scrambling of aryl groups, consistent with paramagnetic nickel(I) and nickel(III) species, and not aryl radicals, as reactive intermediates. The induced reductive elimination is a significantly more facile process than direct thermolysis. However, the relative contributions from these pathways in the reductive elimination of ArNiMeL 2 can be deliberately manipulated by additives (inhibitors and promoters) which control the induction period relating the generation of nickel(I, III) intermediates required for chain initiation. The radical chain mechanism for the formation of carboncarbon bonds by reductive elimination in this system is essentially the same as that previously deduced for aryl coupling to biaryls from arylhalonickel(II) and aryl halides.