Mechanism of the eta(3)-eta(1)-eta(3) 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 impor tant synthetic transformations has been studied by applying density functio nal theory at the B3PW91(DZ+P) level. It was found that under catalytic con ditions, in the condensed phase, the isomerization process involves tetraco ordinated (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 mo lecules 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 interac tions influence the structure of the complexes and the activation barrier t o rotation through the C1-C2 bond. Alkyl substitution of the metalated carb on leads to destabilization of the (eta (1)-allyl)palladium complexes, whic h increases the activation energy of the syn/anti isomerization process. Th is substituent effect arises from a dual steric and electronic destabilizin g interaction between the methyl substituent and the metal atom.