CoI- and Co0-bipyridine complexes obtained by reduction of CoBr2bpy: electrochemical behaviour and investigation of their reactions with aromatic halides and vinylic acetates

The electrochemical behaviour of CoBr(2)bpy (bpy=2,2'-bipyridine) catalyst precursor in acetonitrile has been studied, revealing its possible reduction into the corresponding Co(I) and Co(0) complexes. These low-valent cobalt species appear to be stable on the time scale of cyclic voltammetry. In the presence of aromatic halides, both complexes undergo oxidative addition, the latter Co(0) species allowing the activation of poorly reactive aromatic chlorides. The arylcobalt(III) and arylcobalt(II) obtained are reduced at the same potential as the original Co(II) and Co(I) complexes, respectively, resulting in the observation of overall ECE mechanisms in both cases. The electrochemical study shows that vinylic acetates competitively react with electrogenerated Co(0) species, leading to a labile complex. Preparative scale electrolyses carried out from solutions containing aromatic halides (ArX), vinyl acetate (vinylOAc) and a catalytic amount of CoBr(2)bpy lead to a mixture of biaryl (Ar-Ar) and arene (ArH) as long as the potential is set on the plateau of the Co(II) right arrow over left arrow Co(I) reduction wave. The coupling product (Ar-vinyl) is formed only if the electrolysis is performed on the plateau of the Co(I)/Co(0) reduction wave. A mechanism is proposed for the overall cobalt-catalyzed coupling reaction between aromatic halides and allylic acetates.