Mechanism of the reaction of allyl amination of Pd(II) allyl complexes containing chelating pyridine-chalcogen ligands. A surprisingly low influence of the chalcogen atom

The rates of amine nucleophilic attack on the allyl ligand (k(2)) and the equilibrium constants (K-E) for the displacement of bidentate ligands in Pd(II) allyl complexes of chelating pyridine-chalcogen ethers [Pd(eta (3)-allyl)(RN-XPh)](+) (R = H, Me; X = S, Se) are shown to depend strongly on the steric and electronic requirements of the reactants but are hardly affected by the nature of the chalcogen. Results about the reactivity and solution behaviour of these systems help build up a fairly complete mechanistic picture for this important class of reactions involving coordinated allyl species. In particular the reactivity of the complexes bearing pyridine-thioether ligands is close to that of their pyridine-selenoether analogues, probably owing to a balance of a and ti capabilities of the chalcogen atom. The associative nature of the ligand displacement is markedly affected by steric requirements which depend on the allyl bulkiness. The complexes bearing the ligands with methyl substituted pyridine are the more reactive, due to the destabilisation of the complex ground state induced by the distortion of the starting substrate. We also describe the fluxional behaviour of these species in terms of inversion of the chalcogen absolute configuration and apparent rotation of the allyl ligand. (C) 2001 Elsevier Science Ltd. All rights reserved.