Phenylation of cationic allyl palladium(II) complexes by tetraphenylborate. Synthesis of α-diimine olefin palladium(0) complexes and mechanistic aspects

Abstract

The cationic allyl complexes [Pd(η³-2-R¹C₃H₄)(N–N′)]⁺(N–N′=α-diimine ligand; R¹= H or Me) react with BPh₄^– in the presence of activated olefins to give [Pd(η²-olefin)(N–N′)](olefin = fumaronitrile, dimethyl fumarate or maleic anhydride) and PhCh₂C(R¹)CH₂. The palladium(0) derivatives can be isolated in good yield and have been characterized by elemental analysis, molecular weight measurements and standard spectroscopic techniques. The reaction rates increase with increasing π-accepting ability of the α-diimine, with decreasing steric requirements of the imino carbon substituents and with decreasing stability towards palladium–nitrogen bond breaking in the parent cationic compounds. The rates also increase with decreasing relative permittivity and co-ordinating properties of the solvent. Kinetic measurements in aqueous (2% v/v) methanol provide pseudo-first-order rate constants that are independent of both BPh₄^– and olefin concentrations. This has been interpreted on the basis of extensive ion pairing between the cationic substrate and the BPh₄^– anion, followed by rate-determining phenyl transfer to the palladium centre and fast reductive elimination of allylbenzenes.