Palladium(II) complexes of chiral tridentate nitrogen pybox ligands

Abstract

A series of mono- and di-cationic palladium(II) complexes containing different chiral tridentate nitrogen ligands, pybox, have been prepared [pybox = 2,6-bis[4′-( S)- iPr (or Ph, or Bz or p-EtOC 6H 4)oxazoline-2′-yl]pyridine ( 1–4), respectively]. The molecular structures for two of these, [Pd(CH 3CN)( 2)](BF 4) 2 ( 6) and [Pd(PPh 3)( 3)](BF 4) 2 ( 21g), have been determined by X-ray diffraction and show no major steric hindrance in the fourth coordination position. In connection with the aldol reaction of CNCH 2CO 2Me with PhCHO, several new isonitrile Pd II complexes have also been prepared. It is shown that, under catalytic conditions, the chiral tridentate pybox ligand is completely displaced, thus explaining its failure as a chiral auxiliary. Preparative details for a series of chiral Pd(L)( 3) n+( BF 4) n ( 21) complexes [L = 4-methylpyridine, 2,6-dimethylpyridine, 4-methyl aniline, H 2NCH 2CH(OMe) 2, H 2NCH 2CH 2OH, H 2N(CH 2) 5CH 3, N 3 −, HCO 2 −] Cl −] are given, as are preparative details for some model Pd II acetonitrile complexes with chiral phosphorus and nitrogen chelating ligands. For 6, i.e. PdC 25H 22N 4O 2B 2F 8, the crystals are monoclinic with space group P2 1 (No. 4), a = 13.582(6) A ̊ , b = 13.826(6) A ̊ , c = 14.667(6) A ̊ , β = 97.28(3)°, V = 2732(2) A ̊ 3, Z = 4 . For 21g, i.e. C 43H 38B 2F 8N 3O 2P 2Pd, the crystals are orthorhombic with space group, P2 12 12 1, a = 10.616(4) A ̊ , b = 16.774(2) A ̊ , c = 23.086(4) A ̊ , V = 4111(3) A ̊ 3, Z = 4 .