Atropochiral (C,C)-chelating NHC-ylide ligands: synthesis and resolution of palladium(II) complexes thereof

Atropochiral NHC-phosphonium ylides based on the naphthyl-benzimidazolyl core are targeted as strongly sigma-donor C,C-chelating ligands of transition metals. The ligand core is shown to act in either a monodentate (through the carbene center) or bidentate (through the carbene and the ylide centers) fashion in seven palladium(II) complexes obtained by two sequential strategies. In particular, the enantiomerically pure ortho-palladated (S)-dimethyl(1-phenylethyl) amine co-ligand is used as the chiral resolving agent for the separation of diastereoisomeric NHC-ylide complexes by fractional crystallization. Hydrochloric treatment of either diastereoisomer was found to keep intact the two carbon-palladium bonds, while affording the corresponding enantiomeric beta-zwitterionic NHC-ylide palladate complexes. The absolute configuration of the chirally isolated C,C-chelating ligand was finally confirmed by X-ray crystallography.