Mechanism(s) of the cobalt-catalyzed electrochemical coupling between aromatic halides and allylic acetates

We have studied the mechanism of the electrochemical coupling between aromatic halides and allylic acetates catalyzed by CoBr2 in acetonitrile + pyridine mixed solvent. The early steps involve the electrochemical reduction of the Coll precursor into Col followed by a fast complexation with the allyl acetate. We have established that an oxidative addition takes place between the aromatic halide Ar-X and electrogenerated cobalt(I). The determination of the apparent oxidative addition rate constants for several aromatic halides and for different allylOAc/Co ratios shows that the rate constants depend strongly on the nature of both the halide and the substituent on the aromatic ring. The constants are dramatically affected by the concentration of allylic acetate, suggesting that oxidative addition occurs competitively rather than consecutively with respect to complexation. The resulting sigma-arylcobalt(III) species is reduced at the same potential as the original Co-II/Co-I couple. The ultimate steps leading to the coupling product are discussed according to the results obtained in both preparative-scale electrolyses and cyclic voltammetry. (C) 2003 Elsevier B.V. All rights reserved.