Macromolecular imidazole copper complexes as catalysts for the oxidative coupling of phenols

Abstract

In order to extend the investigation on soluble macromolecular imidazole derivatives as ligands for copper salts, two types of polymers were synthesized. Copolymerization of styrene and N-vinylimidazole yielded macromolecular ligands of type II in which the imidazole moiety is bound directly to the polyethylene backbone. Chloromethylation of polystyrene and amination with imidazole yielded macromolecular ligands of type III in which the imidazole moiety is bound via a benzyl group to the polyethylene backbone. Both polymers gave greenish-blue soluble copper complexes. These complexes are good catalysts for the oxidative coupling of 2,6-disubstituted phenols. They have been compared to their low molar mass analogue, N-methylimidazole (I). As substrate, 2,6-di-t-butylphenol and 2,6-dimethylphenol were chosen. Kinetic analysis were performed. The results for all three types of catalysts and for both substrates are discussed. The activities of the macromolecular catalysts were fairly high compared to other systems. Moreover the polymeric catalysts showed enhancement of re-oxidation rates compared to the low molar mass analogue ( I) due to the nonpolar field formed by the polymer ligand. Binding of ligands to macromolecules increases the steric hindrance towards substrate complexation at active sites. This can be decreased by binding the ligands via spacers to the macromolecule (as in III).