Mechanism of the η3-η1-η3 isomerization in allylpalladium complexes:: Solvent coordination, ligand, and substituent effects

The mechanism of the eta (3) --> eta (1) --> eta (3) isomerization of (eta (3)-allyl)palladium complexes occurring as catalytic intermediates in important synthetic transformations has been studied by applying density functional theory at the B3PW91(DZ+P) level. It was found that under catalytic conditions, in the condensed phase, the isomerization process involves tetracoordinated (eta (1)-allyl)palladium intermediates. In these intermediates a solvent molecule or another ancillary ligand coordinates to palladium. The stability of the (eta (1)-allyl)palladium intermediates critically depends on the electronic effects and on the coordination ability of the solvent molecules and the ancillary ligands. The theoretical calculations indicate a d(sigma) --> pi* type hyperconjugative interaction occurring in the eta (1)-allyl moiety of the intermediary complexes. These hyperconjugative interactions influence the structure of the complexes and the activation barrier to rotation through the C1-C2 bond. Alkyl substitution of the metalated carbon leads to destabilization of the (eta (1)-allyl)palladium complexes, which increases the activation energy of the syn/anti isomerization process. This substituent effect arises from a dual steric and electronic destabilizing interaction between the methyl substituent and the metal atom.