Mechanism of nucleophilic attack by diethylamine on cationic palladium(II) allyl complexes containing α-diimine ligands

The reactions of the cationic complexes [Pd(η3-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 °C have been studied. They involve a fast equilibrium displacement of the co-ordinated α-diimine to yield [Pd(η3-allyl)(NHEt2)2]+, accompanied by slow nucleophilic attack at the allyl ligand of the [Pd(η3-allyl)(N-N′)]+ substrate producing [Pd(η2-olefin)(N-N′)] (olefin = dimethyl fumarate or fumaronitrile) and allyldiethylamine. As shown by the stereochemical course of the reaction with [Pd(1-3-η3-C6H8OMe)(C5H 4N-2-CH=NC6H4OMe4)]ClO4, the nucleophilic attack takes place on the allyl face opposite the metal. The equilibrium constants for α-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 ≫ RN=C(Me)-C(Me)=NR ≈ C5H4N-2-CH=NR > C5H4N-2-CH=NCMe3 (R = C6H4OMe-4). Kinetic studies showed that the pseudo-first-order rate constants (kobs) for the slow amination path display both a first- and second-order dependence on the NHEt2 concentration of type kobs = 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.

The reactions of the cationic complexes [Pd(q3-allyl) (N-N')] CIO, (allyl = 4-methoxycyclohexenyI, 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 "C have been studied. They involve a fast equilibrium displacement of the co-ordinated a-diimine to yield [Pd(q3-allyl) (NHEt,),] +, accompanied by slow nucleophilic attack at the ally1 ligand of the [ Pd(q3-allyl) (N-N')] + substrate producing [Pd(q2-olefin) (N-N')] (olefin = dimethyl fumarate or fumaronitrile) and allyldiethylamine. As shown by the stereochemical course of the reaction with [Pd(l -3-q3-C,H,0Me) (C,H4N-2-CH=NC6H,OMe- 4)]CIO,, the nucleophilic attack takes place on the ally1 face opposite the metal. The equilibrium constants for a-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 >> RN=C(Me)-C(Me)= NR E C5H,N-2-CH=NR > C,H,N-2-CH=NCMe, (R = C,H,OMe-4). Kinetic studies showed that the pseudo-first-order rate constants (kobs) for the slow amination path display both a first- and second-order dependence on the N H Et, concentration of type kobs= k2[N H Et,] + k,'[ N H Et,],. The k, term is related to direct bimolecular attack of NHEt, on the terminal ally1 carbon, whereas the k,' term is ascribed to a parallel nucleophilic attack by a hydrogen-bonded diethylamine dimer arising from amine self-association.