A kinetic study of the coordination template effect in the nickel-catalysed formation of arylphosphonium salts

Abstract

A kinetic study is reported of the nickel(II) halide-catalysed reactions of o-halogenoarylimine- and o-halogenoarylazo-template systems with tertiary phosphines in refluxing ethanol, which result in the replacement of the halogen ortho to the imino or azo function with formation of the related arylphosphonium salt. At a fixed concentration of catalyst, a second-order rate law is observed. The reaction rate also shows a first-order dependence on the concentration of the catalyst. Significant variations in rate have been observed which can be attributed to (a) the nature of halogen replaced by phosphorus (1 > Br > Cl), (b) the donor / acceptor character of the phosphine, (c) the effects of “remote” substituents in the template substrate, and (d) the nature of the halogen of the nickel(II) halide catalyst. It is suggested that the rate data can be accommodated in terms of a mechanism involving initial reduction of nickel(II) to nickel(I), followed by oxidative insertion of the latter into the carbon-halogen bond to form an intermediate arylnickel(III) complex. Reductive elimination from the latter results in the arylphosphonium salt and regeneration of the nickel(I) species.