Nature of the interactions between the beta-silyl substituent and allyl moiety in (eta(3)-allyl)palladium complexes. A combined experimental and theoretical study

Three new beta-silyl-substituted (eta(3)-allyl)palladium complexes were pre pared (3-5) in order to study the substituent effects of the silyl function ality on the allyl-metal system. Complexes 3-5 show a downfield C-13 NMR sh ift for the more substituted allylic terminus (C3). The deshielding effect of the silyl group on C3 is strongest when PPh3 ligands are coordinated to palladium. Both internal and external nucleophiles attack the less substitu ted allylic terminus (C1) with high selectivity. A theoretical analysis of the structure and stability of slightly simplified model compounds 7-10 was performed by applying density functional theory at the B3PW91 level. The t heoretical results indicate hyperconjugative interactions between the pi-al lyl-metal system and the C-Si sigma-bond. The intensity of these interactio ns depends on the conformation of the silyl substituent and on the sigma-do nor/pi-acceptor character of the ancillary ligand on palladium. The beta-su bstituent effects increase the thermodynamic stability of the complexes, we aken the Pd-C3 bond, and facilitate the heterolytic fission of the C-Si bon d. It was also concluded that regioselection of the nucleophilic attack is enhanced in the presence of strong beta-silyl effects.