Synthesis and reactivity of aryl- and alkyl-palladium(II) complexes with functional phosphines and phosphinoenolate ligands: first analogues of model nickel catalysts

Abstract

Phenyl- and methyl-palladium(II) complexes analogous to model nickel(II) catalysts were prepared from readily available precursors. The methods used allow different ligands to be introduced in the co-ordination sphere. For example, the chelating phosphinoenolate ligand in [[graphic omitted])NPh2}L2][L2= Ph2PCH2C(O)NPh2] was displaced by 1 equivalent of Ph2PCH2C(O)Ph (L1) to give [[graphic omitted])Ph}L2] whereas the terminal functional phosphine was displaced by P(C6H11)3 to give [[graphic omitted])NPh2}{P(C6H11)3}]. Owing to favourable ligand-redistribution reactions, treatment of a mixture of complexes trans-[PdMe(Cl)L22], trans-[PdMe(Cl)L12] and trans-[PdMe(Cl)L1(L2)](which cannot be isolated pure) with an excess of NaOMe in toluene selectively afforded the phosphinoenolate complex [[graphic omitted])Ph}L2]. The enolate moiety of [[graphic omitted])NPh2}L2] and of [[graphic omitted])NPh2}L2] reacted with R′NCO (R′= Ph or p-tolyl) with formation of a carbon–carbon bond in a Michael-type addition and the products were shown to exist in the form of two isomers a and b, characterised by a N–H ⋯ O or a N–H ⋯ N hydrogen bond within the ligand system. Insertion of CO into the Pd–Me bond of [[graphic omitted])NPh2}L2] or [[graphic omitted])NHPh]C(O)NPh2}L2] yielded the corresponding acyl complexes. Although [[graphic omitted])Ph}(PPh3)] inserted ethylene into its Pd–Me bond, as evidenced by quantitative formation of propylene, the palladium hydride that must be generated by the β-elimination reaction decomposes before further ethylene insertion can occur.