COMPLEXES AND MECHANISMS OF METAL-CATALYZED C-C COUPLING REACTIONS .2. OXIDATIVE ADDITION OF CHLOROAROMATICS TO PD-0 COMPLEXES - SYNTHESIS, STRUCTURE AND STABILITY OF ARYLPALLADIUM(II) CHLORIDES OF THE PHOSPHORANE SERIES

Chlorobenzene and electron-poor aryl chlorides ClC(6)H(4)X (X = 4-NO2, 4-CHO, 4-CN, 4-H) add oxidatively to Pd(PPh(3))(4) or Pd(PPh(3))(2)(dba) at 100-140 degrees C to give complexes of the type trans-Pd(PPh(3))(2)(C6H4-p-X)Cl (1a-d). Oxidative addition of electron-rich aryl chlorides ClC6H4Y (Y = 4-CH3, 4-CH3O) to Pd(PPh(3))(4) was expected to lead to the complexes trans-Pd(PPh(3))(2)(C6H4Y)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(6)H(4)p-Y)}(Ph)Cl (3e,f). The stability of the oxidative addition products (1a-f) in solution was examined by means of in situ H-1 and P-31 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.