MECHANISM OF BIARYL SYNTHESIS WITH NICKEL-COMPLEXES

The mechanism of the nickel-catalyzed coupling of aryl halides to afford biaryls is examined by focusing on the reactions of arylnickel(II) halide as the important organometallic intermediate. A variety of triethylphosphinenickel(ll) complexes, trans-ArNiX(PEt3)2 (1), are synthesized and found to yield biaryls only upon treatment with aryl halide (ArX). Biaryl formation is shown to involve a radical-chain process in which paramagnetic nickel(I) and arylnickel(III) species are reactive intermediates. The propagation steps include the oxidative addition of ArX to nickel(I) to produce the reactive arylnickel(III) species, which undergoes aryl transfer with 1 to afford a diarylnickel(III) intermediate, followed by reductive elimination of biaryl and the regeneration of nickel(I). This series of chain reactions provides an efficient mechanism for the cross coupling of 1 and ArX selectively to ArAr, except for a competition from a halogen exchange process which, in effect, scrambles aryl groups between 1 and an arylnickel(III) species. The initiation of the catalytic cycle is associated with electron transfer from 1 to ArX, and it can be manipulated by a rational choice of initiators and inhibitors. In the course of biaryl formation, the triethylphosphine ligand reacts with excess ArX to produce arylphosphonium salts, ArPEt3+, by a second catalytic process induced by the nickel(I) intermediate. The phosphine levels in the reaction are critical to initiation and inhibition of both of these catalytic or chain processes, which are discussed in relation to nickel ligation. © 1979, American Chemical Society. All rights reserved.