Komplexchemie und mechanismen metallkatalysierter cc-kupplungsreaktionen: II . Oxidative addition von chloraromaten an Pd 0-komplexe: synthese, struktur und stabilität von arylpalladium(II)-chloriden der phosphanreihe

Abstract

Chlorobenzene and electron-poor aryl chlorides ClC 6X (X = 4-NO 2, 4-CHO, 4-CN, 4-H) add oxidatively to Pd(PPh 3) 4 or Pd(PPh 3) 2(dba) at 100–140°C to give complexes of the type trans-Pd(PPh 3) 2(C 6H 4- p-X)Cl ( 1a- d). Oxidative addition of electron-rich aryl chlorides ClC 6H 4Y (Y = 4-CH 3, 4-CH 3O) to Pd(PPh 3) 4 was expected to lead to the complexes trans-Pd(PPh 3) 2(C 6H 4Y)Cl ( 1e, f), but instead gave, in almost quantitative total yield, mixtures consisting of 90% trans-Pd(PPh 3) 2(Ph)Cl ( 1d) and 10% trans-Pd(PPh 3){PPh 2(C 6H 4- p-Y)}(Ph)Cl ( 3e, f). The stability of the oxidative addition products ( 1e- f) in solution was examined by means of in situ 1H and 31P NMR spectroscopy and mass spectrometry. It is shown that the complexes 1e and 1f, synthesized independently by halogen-exchange from the iodo derivatives, undergo a facile aryl-aryl exchange between the palladium center and the coordinated phosphine ligands, just in contrast to 1a- c. Subsequent intermolecular phosphine scrambling leads to further isomerisation which results in the formation of the observed reaction products 1d and 3e, f. Complexes with chelating phosphine ligands (P-P) of the type cis-Pd(P-P)(Ph)Cl ( 2a, b; P-P = dppe, dppp) are obtained by oxidative addition of chlorobenzene to Pd(P-P)(dba) or by simple phosphine exchange from 1d. Their identity is supported by single-crystal X-ray diffraction of 2b.