P,C-palladacycle complexes of triphenylphosphite: Synthesis, characterization and catalytic activity in the Suzuki cross-coupling reaction

Article abstract of DOI:10.1016/j.poly.2013.10.035

Five-membered P,C-palladacycles were synthesized through the reaction of the binuclear triphenylphosphite complex [Pd(μ-Cl)P(OPh)₂(OC ₆H₄)]₂ (1) with different monodentate ligands [L = triphenylphosphine (PPh₃), thiourea (tu), 2,4,6- trimethylpyridine (Me₃Py), pyridine (Py)] and bidentate ligands [N^N = 1,10-phenanthroline (Phen), 4-methyl-1,10-phenanthroline (MePhen)], giving mononuclear P,C-palladacycles [Pd(L)(Cl){P(OPh)₂(OC₆H ₄)}] [L = PPh₃ (2a), tu (2b), Me₃Py (2c)], [PyH]⁺[Pd(Cl)₂{P(OPh)₂(OC₆H ₄)}]^- (3d) and [Pd(N^N){P(OPh)₂(OC ₆H₄)}]⁺ NO₃^- [N^N = Phen (4a), MePhen (4b)]. The synthesized complexes were characterized by ¹H, ¹³C-{¹H} and ³¹P-{¹H} NMR spectroscopy, elemental analysis and FT-IR techniques. Structural details of complexes 2a, 2c and 3d were determined by X-ray crystallography, which showed these complexes crystallize in the triclinic (2a) and monoclinic (2c and 3d) crystalline systems. According to the structural data, the orthopalladated complex 3c has two different conformers (R,S) in the solid state, although it appeared as one isomer in the solution state, as confirmed by the NMR spectroscopy. Complex 2a, containing PPh₃, was evaluated as a homogeneous catalyst for a variety of substrates, affording coupled products in good to excellent yields, importantly at room temperature. Furthermore, the use of the lesser reactive aryl chloride as a substrate in the Suzuki reaction under mild conditions, has received much attention.