Silk-Fibroin-Supported Palladium Catalyst for Suzuki-Miyaura and Ullmann Coupling Reactions of Aryl Chlorides

Recently, we have reported the preparation of a silk fibroin-supported Palladium catalyst (Pd/SF) and its use in the Suzuki-Miyaura cross-coupling of aryl iodides. Since its synthetic applicability and structural features are still far from being fully explored, a deeper investigation is reported here. Pd/SF is tested as a catalyst for the Suzuki-Miyaura and Ullmann coupling reactions of aryl chlorides, in HO/EtOH solvent mixture under atmospheric conditions. Starting from the analysis of the products of the Pd/SF promoted Ullmann reactions of poly-haloarenes, we can hypothesize the existence of catalytic pockets where monoatomic palladium species can form stable complexes with SF. To shed light on this hypothesis, Pd/SF is characterized by Wide-Angle X-ray Scattering (WAXS) analysis, which supports the supposition of catalytic pockets. Finally, using a computational model developed by Molecular Mechanic Energy Minimization we have estimated the dimension of the catalytic pocket of Pd/SF (about 15 Å), in good agreement with the experimental results.