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

Abstract

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 [PdCl 2(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.