Coupling of organic halides electrocatalyzed by the NiII/NiI/Ni0–PPh3 system. A mechanistic study based on an electroanalytical approach

Abstract

The coupling of the organic halides bromobenzene, 1-iodobutane, 1-iodo-2,2-dimethylpropane, and benzyl chloride has been carried out by electrochemically generating and continuously recycling the nickel(0) complex promoter [Ni(PPh₃)₄]. This species undergoes oxidative addition by organic halides leading to σ-bonded organometallic nickel(II) derivatives. In these complexes the metal–carbon bond can be cleaved either by a straightforward thermal decomposition or by a cathodic reduction, depending upon whether the co-ordinated organic group is an alkyl or an aryl one. In the former case a high yield of the coupling product is obtained only by employing organic halides not bearing hydrogen atoms in the β position; when a β-elimination reaction can occur, the yield of the coupling product is, on the contrary, very low and can be significantly improved only by carrying out the reductive process at more negative potentials, at which a proposed nickel hydride intermediate can be reduced. When the co-ordinated organic group is an aryl one, reductive elimination of the organic group does not occur by the thermal decomposition and the coupling product can be formed only by means of a cathodic reduction of the relevant organometallic nickel(II) derivative. An overall mechanism is proposed which is consistent with the data.