The electronic effect of 4,4′-disubstituted 2,2′-bipyridine ligands on the copper-catalysed oxidative coupling of 2,6-dimethylphenol

The ligands bipyridine (bpy), 4,4'-dimethoxy-2,2'-bipyridyl (dMeObpy), 4,4'-dimethyl-.2,2'-bipyridyl (dMebpy), 4,4'dichloro-2,2'-bipyridyl (dClbpy) and 4,4'-dinitro-2,2'-bipyridyl (dNO(2)bpy), when combined with copper(II) nitrate render active catalysts. The highest activity was found for bpy as the ligand. The ligands dMeObpy and dMebpy, with their electron-donating substituents, render the copper ions less electrophilic than unsubstituted bpy and so decrease the oxidation rate of 2,6-dimethylphenol (DMP). One would have expected an increase in the oxidation rate of DMP with the ligands dClbpy and dNO(2)bpy, having electron-withdrawing substituents, which render the copper ions more electrophilic. However, probably due to a stabilisation of the copper(I) species, which retards the reoxidation to Cu(II), the overall reaction rate decreases with dClbpy and dNO(2)bpy. (C) 2001 Elsevier Science B.V. All rights reserved.