COPPER(II) COMPLEXES OF POLYSTYRENE-BOUND DMAP - EFFECT OF CHAIN LOADING ON THE CATALYTIC ACTIVITY IN THE OXIDATIVE COUPLING OF 2,6-DIMETHYLPHENOL

The effect of the degree of loading, α, of polystyrene with DMAP ligands on the catalytic activity of 'polystyrene-bound DMAP'-copper catalysts in the oxidative coupling of 2,6-dimethylphenol was studied. The intrinsic activity increases upon enhancing α from 0.096 to 0.23. This increase proved to be mainly brought about by an increasing 'strain' in the polymeric catalyst. An additional accelerating effect is the increase of the amount of catalytically active mononuclear complexes CuL4(OH)Cl with increasing α up to α=0.134. This is due to a stronger polydentate effect for higher α because of the higher local ligand concentration within the polymer coils, which can be regarded as separate micro-reactors. For α>0.23 the interligand distance becomes too short to link adjacent ligands to the same copper ion. Consequently, some ligands have to be skipped in favor of next ones, the strain is somewhat released and the intrinsic activity slightly decreases. For α≥0.096 the phenol oxidation step proved to be rate limiting. However, for very low chain loadings, e.g. α=0.044, the local concentration of mononuclear copper complexes within the coils becomes too low and the dimerization which is needed for the Cu(I) reoxidation becomes rate determining. The catalytic specificity proved to be independent of α under our reaction conditions.