Orthometalated palladium(II) complex-catalysed reduction of nitroalkanes and nitriles

Abstract

Dihydrogen reduction of nitroalkanes and aromatic nitriles to the corresponding amines was efficiently achieved at (4.0 – 12.0) × 10 3 kN m −2 in the temperature range 65 – 80 °C using a series of di- and mononuclear orthopalladated complexes of the type trans-Pd 2L 2X 2 and trans-PdLL'Cl [L = 2-(2-pyridylcarbonyl)phenyl-C 1,N(pcph), 2-(2-picolyl)phenyl-C 1,N-(piph); ( N,N-dimethylbenzylamine)-C 2,N(dmba); L'= pyridine, triphenyl-phosphine; X = OAc, Cl] as catalysts in DMF medium. The existence of metal-arene σ-bonds in the complexes was found essential for their catalytic activities. Six-membered orthopalladated complexes were generally superior catalysts to the five-membered ones, and the binuclear benzoylpyridine complex was the best in the series. The complexes were the catalyst precursors, and the actual catalytic species was always found to be the corresponding mononuclear palladium hydride-DMF adduct. Kinetic studies indicate first-order rate dependence on catalyst concentration and hydrogen pressure, and zero-order on substrate concentration. A tentative mechanism for this reduction process has been suggested on the basis of kinetic data and other experimental observations.