Application of palladium(II) complex with bidentate phosphine sulfide ligands to palladium-catalyzed C-C coupling reaction

A phosphine sulfide Pd(II) complex, [Pd(p₂S₂)₂](BF₄)₂ (1) (p₂S₂ = 1,2-bis(diphenylphosphino)ethane disulfide), was synthesized and characterized by an X-ray crystal structure analysis and ³¹P NMR spectroscopy. The p₂S₂ ligand exchange rate of 1 with free p₂S₂ in chloroform was revealed to be comparable to the general solvent exchange rate on Pd(II). The catalytic activity of 1 was evaluated by carrying out the Heck reaction. The diminishing of the induction period and acceleration of the reaction were observed for 1 by comparing the phosphine Pd(II) complexes with a leaving chloro ligand, [PdCl(p₃)]Cl (p₃ = bis[2-(diphenylphosphino)ethyl]phenylphosphine) and [PdCl(pp₃)]Cl (pp₃ = tris[2-(diphenylphosphino)ethyl]phosphine), and the catalytic activity was comparable to that of the phosphine Pd(0) complex, [Pd(PPh₃)₄]. Such a high catalytic activity of 1 is attributed to the π-accepting ability of the phosphine sulfide S atom which stabilizes the catalytically active Pd(0) species electronically and weak σ-donation of the S atom which does not block the formation and a subsequent reaction of the Pd(II) substrate adduct in the catalytic cycle.