Equilibrium studies of α-diimine displacement in cationic allylpalladium(II) complexes by monodentate N-donors and the mechanism of allyl amination by triethylamine and pyridine

Abstract

In the cationic complexes [Pd(η³-allyl)(L–L)]ClO₄[L–L = 1,2-bis(imino)ethanes or 2-(iminomethyl)-pyridines] the chelated α-diimine was rapidly and reversibly displaced by secondary amines (N-methylaniline, morpholine or piperidine), triethylamine and 4-substituted pyridines. The observed equilibrium constants K_e, increased with increasing basicity and decreasing steric requirements of the entering N-donor. They strongly depend on the α-diimine and decrease in the order RNCHCHNR RNC(Me)C(Me)NR ≈ NC₅H₄(CHNR)-2 (R = C₆H_aOMe-4). The cationic complex [Pd(η³-C₃H₅){NC₅H₄(CHNC₆H₄OMe-4)-2}]⁺ underwent a slow allyl amination by triethylamine or pyridine (L′) in the presence of fumaronitrile (fn), yielding [Pd(η²-fn){NC₅H₄(CHNC₆H₄OMe-4)-2}] and Et₃[graphic omitted]CH₂CHCH₂ or C₅H₅[graphic omitted]CH₂CHCH₂. Kinetic studies showed that the pseudo-first-order rate constants for amination (k_obs) are given by k_obs=k₂[L′], suggesting a direct bimolecular attack of L′ on the η³-allyl ligand. Amination hardly proceeds in the presence of the less-activated olefin dimethyl fumarate (dmf). The π-accepting properties of the olefinic ligands play an important role also in the reaction of Et₃[graphic omitted]CH₂CHCH₂ or C⁵H₅[graphic omitted]CH₂CHCH₂ with [Pd(η²-olefin){NC₅H₄(CHNC₅H₄OMe-4)-2}](olefin = fn or dmf), i.e. the reverse of the amination reaction.