PHENYLATION OF CATIONIC ALLYL PALLADIUM(II) COMPLEXES BY TETRAPHENYLBORATE - SYNTHESIS OF ALPHA-DIIMINE OLEFIN PALLADIUM(O) COMPLEXES AND MECHANISTIC ASPECTS

The cationic allyl complexes [Pd(eta-3-2-R1C3H4)(N-N' = alpha-diimine ligand; R1 = H or Me) react with BPh4- in the presence of activated olefins to give [Pd(eta-2-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 pi-accepting ability of the alpha-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.