Seven-membered Pd(II) complexes containing symmetric phosphorus ylides: Synthesis, characterization and high catalytic activity toward Suzuki cross-coupling reactions

The reaction of 1,2-bis(diphenylphosphino)ethane (dppe) with various ketones in acetone produces the new phosphonium salts [RC(O)CH2PPh2(CH2)2PPh2CH2C(O)R]X2 (R = 2-naphtyl, X = Br (1); R = 2,4-dichlorophenyl, X = Cl (2); R = 3-nitrophenyl, X = Br (3)). Further treatment with a base gives the symmetrical phosphorus ylides, RC(O)CHPPh2(CH2)2PPh2CHC(O)R (R = 2-naphtyl (4), 2,4-dichlorophenyl (5), 3-nitrophenyl (6)). These ligands react with Pd(II) chloride to form C,C-chelated complexes with the composition [RC(O)CHPPh2(CH2)2PPh2CHC(O)R]PdCl2, where R = 2-naphtyl (7), 2,4-dichlorophenyl (8), 3-nitrophenyl (9). These compounds have been characterized by elemental analysis and spectroscopic methods and consist of seven-membered rings formed by the coordination of the ligands through the two ylidic carbon atoms to the metal center. The structure of compound 5 has been characterized crystallographically. The palladium complex 9 is employed in the Suzuki cross-coupling reaction between phenylboronic acid and several aryl halides. It was found to be a competent catalyst for a variety of substrates to afford the coupled products in high yields using DMF as a solvent. The biaryl products were obtained under aerobic conditions in short reaction times with a lower loading of the catalyst (0.001 mol%).

Graphical abstractThe three palladacycle complexes of the type [RC(O)CHPPh2(CH2)2PPh2CHC(O)R]PdCl2, where R = 2-naphtyl, 2,4-dichlorophenyl and 3-nitrophenyl, contain a seven-membered C,C-chelate ring. Palladium complexes were synthesized and investigated by physicochemical techniques. We used a palladium(II) complex (R = 3-nitrophenyl) as a highly efficient catalyst for the Suzuki coupling reaction of various aryl halides.Figure optionsDownload full-size imageDownload as PowerPoint slide