Ligand-Free Polymeric Catalysts for Suzuki, Sonogashira and Heck Cross-Coupling Reactions: Influence of Support Hydrophobicity.

In this study, new data are presented on the behavior of Pd nanoparticles (NPs) stabilized in naphthalene-based polymers containing different functional groups (hydroxyl, sulfonate, and nitro groups) in cross-coupling reactions (Suzuki, Sonogashira, and Heck). Polymers were synthesized by the direct knitting strategy using naphthalene or naphthol as the monomers and iron chloride as the polymerization catalyst with following sulfonation or nitration. The resulting polymeric materials were used to support the formation of Pd NPs. It was shown that the sizes of Pd NPs depend on the relative hydrophobicity of the polymeric environment. All the catalytic materials were shown to be highly active and selective: using low amounts of catalysts (0.2 mol.% of Pd with respect to an aryl halide) in environmentally benign solvents (ethanol, dimethyl sulfoxide) and in the absence of any ligands and co-catalysts more than 90 % conversion was achieved in 60 min in the case of Suzuki and Sonogashira cross-coupling. Catalytic activity was found to go through a maximum corresponding to the sizes of Pd NPs of about 10 nm. The Heck cross-coupling required the use of N,N-dimethylformamide as a solvent and the addition of triethylamine, which allowed achieving more than 80 % aryl halide conversion for 3 h.