Synthesis and characterization of palladium(II) complexes with chiral aminophosphine ligands: Catalytic behaviour in asymmetric hydrovinylation. Crystal structure of cis-[PdCl2(PPh((R)-NHCHCH3Ph)2)2]

Optically active ligands of type Ph2PNHR (R = (R)-CHCH3Ph, (a); (R)-CHCH3Cy, (b); (R)-CHCH3Naph, (c)) and PhP(NHR)2 (R = (R)-CHCH3Ph, (d); (R)-CHCH3Cy, (e)) with a stereogenic carbon atom in the R substituent were synthesized. Reaction with [PdCl2(COD)2] produced [PdCl2P2] (1) (P = PhP(NHCHCH3Ph)2), whose molecular structure determined by X-ray diffraction showed cis disposition for the ligands. All nitrogen atoms of amino groups adopted S configuration. The new ligands reacted with allylic dimeric palladium compound [Pd(η3-2-methylallyl)Cl]2 to gave neutral aminophosphine complexes [Pd(η3-2-methylallyl)ClP] (2a–2e) or cationic aminophosphine complexes [Pd(η3-2-methylallyl)P2]BF4 (3a–3e) in the presence of the stoichiometric amount of AgBF4. Cationic complexes [Pd(η43-2-methylallyl)(NCCH3)P]BF4 (4a–4e) were prepared in solution to be used as precursors in the catalytic hydrovinylation of styrene. 31P NMR spectroscopy showed the existence of an equilibrium between the expected cationic mixed complexes 4, the symmetrical cationic complexes [Pd(η3-2-methylallyl)P2]BF4 (3) and [Pd(η3-2-methylallyl)(NCCH3)2]BF4 (5) coming from the symmetrization reaction. The extension of the process was studied with the aminophosphines (a–e) as well as with nonchiral monodentate phosphines (PCy3 (f), PBn3 (g), PPh3 (h), PMe2Ph (i)) showing a good match between the extension of the symmetrization and the size of the phosphine ligand. We studied the influence of such equilibria in the hydrovinylation of styrene because the behaviour of catalytic precursors can be modified substantially when prepared ‘in situ’. While compounds 3 and bisacetonitrile complex 5 were not active as catalysts, the [Pd(η3-2-methylallyl)(η2-styrene)2]+ species formed in the absence of acetonitrile showed some activity in the formation of codimers and dimers. Hydrovinylation reaction between styrene and ethylene was tested using catalytic precursors solutions of [Pd(η3-2-methylallyl)LP]BF4 ionic species (L = CH3CN or styrene) showing moderate activity and good selectivity. Better activities but lower selectivities were found when L = styrene. Only in the case of the precursor containing Ph2PNHCHCH3Ph (a) ligand was some enantiodiscrimination (10%) found.