STEREOCHEMISTRY OF SQUARE-PLANAR PALLADIUM(II) COMPLEXES

Abstract

Three diastereomeric palladium(II) complexes, [(S)-1-[1-(dimethylamino)ethyl-C,N] [N ,N,N' ,N' -tetramethyl-(R,R)-2,3-butanediamine-N ,N']palladium(Il) perchlorate ((S,R,R)-8), (S,S,S)-8 and (S,R,S)-8 have been prepared separately as their perchlorate salts from stoichiometric amounts of [bis-acetonitrile][(S)-1-[1-(dimethylamino)ethyl]-2- naphthalenyl-C,N]palladium(II) perchlorate and the (R,R)-, (S,S)- and (R',S')- forms of N ,N,N' ,N' -tetramethyl-2,3-butanediamine respectively. All the three diastereomers have a distorted square-planar geometry around palladium, and the relative degree of distortion is in the order of (S,R,R)-8 < (S,R,S)-8 < (S,S,S)-8. The stereochemistries of the three complexes due to ligand-ligand interactions are studied in solid state and in solution. The (S,R,S)-8 isomer crystallizes as a sole conformer while existing as an equilibrium mixture of two internal diastereomeric structures in solution at room temperature. The ratios of the two conformers are temperature dependent. Another feature is that in solution, the diamine ligands of these complexes are labile, such that intermolecular ligand redistribution between diastereomers occur readily under ambient conditions. The novel bidentate ligand, [2-(ethenylthio)ethyl]diphenylphosphine was synthesized and the palladium(II) complex carrying this ligand and the ortho-metalated (S)-dimethyl-[ 1-(1-naphthyl)ethyl]amine was prepared as a perchlorate salt. The Pd(Il) complex crystallizes as a single conformer, whereby the absolute configuration at the coordinated sulfur stereocentre is S. In solution, the complex undergoes facile intramolecular equilibration between diastereomers epimeric at sulfur. From the variable temperature NMR spectroscopy, a coalescence temperature of -50°C was recorded for such interconversion. As an application of the above stereochemical studies, investigations of the potential of the Pd(II) complexes containing the (S)-dimethyl-[1-(1-naphthyl)ethyl]amine as catalysts in several asymmetric synthesis have been carried out These investigations are targeted on assessing their role as asymmetric catalysts in Diels-Alder reaction.