Bulky triarylarsines are effective ligands for palladium catalysed Heck olefination

The four arsines, As {C6H3(o - CH3) (p - Z)}(3) {Z = H(2a) orOMe(2b)} and As {C6H3(o - CHMe2) ( p - Z)}(3) {Z = H( 2c) or OMe (2d)} react with [PdCl2(NCPh)(2)] or [PtCl2(NCBut)(2)] to give trans - [MCl2L2] or trans - [M2Cl2(mu-Cl)(2)L-2]. The crystal structures of trans - [PdCl2( 2a) 2] and [PtCl2( 2d)(2)] have been determined, the latter as its dichloromethane solvate. The structures show that in these complexes, the ligands adopt gga type conformations as do all analogous tri-o-tolyl- and tri-o-isopropylphenylphosphines in square-planar and octahedral complexes. The variable-temperature NMR behaviour of the complexes shows that they are fluxional due to restricted As-C bond rotation. The rate of the fluxionality is more rapid than in the analogous phosphine complexes and this is associated with longer As-C and As-M bonds allowing more free movement. The catalytic activity of the palladium complexes of the arsines and their phosphine analogues for the reaction of 4-bromoacetophenone and n- butyl acrylate has been screened. The results show that the arsines are generally superior to the phosphines as ligands for this catalysis. Tri(o - isopropylphenyl) phosphine and tri(o-isopropylphenyl) arsine are superior to tri-o-tolylphosphine as ligands for this Heck reaction and a p - methoxy substituent improves the arsine catalyst but not the phosphine catalyst. The phosphine catalysts are superior to the arsine catalysts for the reaction of 4-chloroacetophenone and n- butyl acrylate. These observations are discussed in the context of ligand stereoelectronic effects, as measured by the Tolman electronic parameter, nu(CO) of the [NiL(CO)(3)] {L = AsAr3 or PAr3}.