MECHANISM OF NUCLEOPHILIC-ATTACK BY DIETHYLAMINE ON CATIONIC PALLADIUM(II) ALLYL COMPLEXES CONTAINING ALPHA-DIIMINE LIGANDS

The reactions of the cationic complexes [Pd(eta3-allyl)(N-N')]ClO4 (allyl = 4-methoxycyclohexenyl, allyl or 2-methylallyl; N-N' = 1,2-bis(imino)ethanes or pyridine-2-carbaldimines) with diethylamine, in the presence of an activated olefin, in chloroform at 25-degrees-C have been studied. They involve a fast equilibrium displacement of the co-ordinated alpha-diimine to yield [Pd(eta3-allyl)(NHEt2)2]+, accompanied by slow nucleophilic attack at the allyl ligand of the [Pd(eta3-allyl)(N-N')]+ substrate producing [Pd(eta2-olefin)(N-N')] (olefin = dimethyl fumarate or fumaronitrile) and allyldiethylamine. As shown by the stereochemical course of the reaction with [Pd(1-3-eta3-C6H8OMe)(C5H4N-2-CH=NC6H4OMe4)]ClO4, the nucleophilic attack takes place on the allyl face opposite the metal. The equilibrium constants for alpha-diimine displacement have been determined. They are strongly affected by the structure of the N-N' ligand and decrease in the order RN=CH-CH=NR much greater than RN=C(Me)-C(Me)= NR almost-equal-to C5H4N-2-CH=NR > C5H4N-2-CH=NCMe3 (R = C6H4OMe-4). Kinetic studies showed that the pseudo-first-order rate constants (k(obs)) for the slow amination path display both a first- and second-order dependence on the NHEt2, concentration of type k(obs) = k2[NHEt2] + k2'[NHEt2]2. The k2 term is related to direct bimolecular attack of NHEt2 on the terminal allyl carbon, whereas the k2' term is ascribed to a parallel nucleophilic attack by a hydrogen-bonded diethylamine dimer arising from amine self-association.