Bulky triarylarsines are effective ligands for palladium catalysed Heck olefination

Abstract

The four arsines, As{C₆H₃(o-CH₃)(p-Z)}₃ {Z = H (2a) or OMe (2b)} and As{C₆H₃(o-CHMe₂)(p-Z)}₃ {Z = H (2c) or OMe (2d)} react with [PdCl₂(NCPh)₂] or [PtCl₂(NCBu^t)₂] to give trans-[MCl₂L₂] or trans-[M₂Cl₂(µ-Cl)₂L₂]. 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, ν_CO of the [NiL(CO)₃] {L = AsAr₃ or PAr₃}.