Influence of various P/N and P/P ligands on the palladium-catalysed reductive carbonylation of nitrobenzene

Article abstract of DOI:10.1016/S0022-328X(97)00029-6

A series of bidentate phosphorus-nitrogen ligands was synthesised for the palladium-catalysed reductive carbonylation of nitrobenzene in order to combine the favourable influence of the phosphorus atom on the stability of the catalyst complex with the stimulating effect of the nitrogen atom on the catalytic activity. The nitrogen atom of the P/N ligand was either incorporated in an imine function, yielding the JV-(2′-diphenylphosphinobenzylidene)-R-amine ligands (R = phenyl, 4-chlorophenyl, 2,4-dimethoxyphenyl, 2,4-dimethylphenyl, tert-butyl), or in a heteroaromatic ring system which gave 2-(2′-(diphenylphosphino)ethyl)pyridine and 8-(diphenylphosphino)quinoline. Complexes of the type Pd(ligand)₂(BF₄)₂ were prepared for these ligands. Additionally, a series of bidentate phosphorus ligands was tested: dppm, dppe, dppp, dppb, dppf, 1,2-bis(diphenylphosphino)benzene, 1,8-bis(diphenylphosphino)naphthalene, bis(2-diphenylphosphinophenyl)ether, and 9,9-dimethyl-4,6-bis(diphenylphosphino)xanthene. The P/N ligands containing the imine function did not yield any conversion of the nitrobenzene in combination with Pd. On the use of the second type of P/N ligand, moderately active palladium catalysts were obtained. This different behaviour is ascribed to the relatively low π *-level of the imine-containing ligands. Oxidation of the phosphorus donor atom by the nitro substrate inactivated the catalysts derived from the P/N ligands as well as from a series of P/P ligands. For the bidentate phosphorus ligands the bite angle and flexibility of the ligand turned out to be of crucial influence due to the different geometries required for the Pd(II) and Pd(0) intermediates of the catalytic cycle.