Pd nanosized particles supported on chitosan and 6-deoxy-6-amino chitosan as recyclable catalysts for Suzuki-Miyaura and Heck cross-coupling reactions

Several chitosan and 6-deoxy-6-amino chitosan-Schiff base ligands (1-4) were prepared by condensation of either 2-pyridinecarboxaldehyde or 2-(diphenylphosphino)benzaldehyde with the amino group(s) on chitosan and its derivative (6-deoxy-6-amino chitosan). The supported ligands were reacted with [PdCl₂(COD)] to form chitosan-supported Pd^II catalysts (5-8). All the supported catalysts were air- and moisture-stable and have been characterized using elemental analysis, ICP-MS, UV-vis, FT-IR, PXRD, TGA, ³¹P solid state NMR and TEM. As models for the heterogenized catalysts (5 and 6), mononuclear Pd^II complexes (9 and 10) were also prepared via the Schiff-base condensation reaction of 1,3,4,6-tetra-O-acetyl- β-d-glucosamine hydrochloride to form 1,3,4,6-tetra-O-acetyl-β-d- glucos-2-pyridylimine and 1,3,4,6-tetra-O-acetyl-β-d-glucos-2- (diphenylphosphino)imine which were subsequently reacted with [PdCl ₂(COD)]. Complexes (9 and 10) and their precursors were characterized by ¹H and ³¹P NMR, UV-vis, FT-IR spectroscopy and elemental analysis. Catalytic Suzuki-Miyaura and Heck carbon-carbon cross-coupling reactions were carried out using the supported Pd catalysts and their mononuclear analogues. The immobilized and homogeneous catalysts showed high activity for both the Suzuki-Miyaura and Heck cross-coupling reactions in organic and aqueous media. Homogeneous catalysts (9 and 10) decomposed during the first run, while the supported catalysts could be recycled and reused up to five times.