Modification of polyhydroxyalkanoates: Evaluation of the effectiveness of novel copper(II) catalysts in click chemistry

Abstract

Copper(I) catalyzed azide-alkyne cycloadditions, click reactions, are an established synthetic tool to derivatize polymers. Only a few catalytic systems have been explored towards the derivatization of functionalized poly(3‑hydroxyalkanoate)s, PHAs, using click reactions. Here, the performances of three Cu(II)-catalysts supported by tetradentate polypyridyl ligands, [Cu(L1)ClO4]ClO4, [Cu(L2)ClO4]ClO4 and [Cu(L3)ClO4]ClO4, were examined in click reactions on functionalized PHAs carrying either terminal azido or alkyne groups in the side chain and the results were compared to the traditional CuSO4·5H2O/Na ascorbate and the organo-soluble Cu(I) bromotris(triphenylphosphine)copper(I), CuBr(PPh3)3 catalysts. It was determined that the effectiveness of the catalytic systems depended on the molecular architecture of the polymer and the nature of the small molecule reactants to be clicked onto the PHA. Click reactions on PHAs with terminal azido groups were catalyzed with Cu(II)-catalysts, but not with CuBr(PPh3)3. For alkyne-containing polymers CuBr(PPH3)3 effected 65% conversion in contrast to Cu(II) catalysts that were ineffective. While no strong trend was found, differences in the effectiveness were related to dissimilarities in the accessibility of the alkyne moiety for the reactive Cu(I) species. Propargyl benzoate was most effectively clicked onto a azido PHA (100% conversion) when catalyzed by CuSO4·5H2O/Na ascorbate, however the click reaction with a similar reactant, propargyl acetate, was more effectively catalyzed by a Cu(II)-catalyst supported by a tetradentate polypyridyl ligand (44% conversion).