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) an d 4,4'-dinitro-2,2'-bipyridyl (dNO(2)bpy), when combined with copper(II) ni trate render active catalysts. The highest activity was found for bpy as th e ligand. The ligands dMeObpy and dMebpy, with their electron-donating subs tituents, render the copper ions less electrophilic than unsubstituted bpy and so decrease the oxidation rate of 2,6-dimethylphenol (DMP). One would h ave expected an increase in the oxidation rate of DMP with the ligands dClb py and dNO(2)bpy, having electron-withdrawing substituents, which render th e copper ions more electrophilic. However, probably due to a stabilisation of the copper(I) species, which retards the reoxidation to Cu(II), the over all reaction rate decreases with dClbpy and dNO(2)bpy. (C) 2001 Elsevier Sc ience B.V. All rights reserved.