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.