Mechanism of the η³−η¹−η³Isomerization in Allylpalladium Complexes:  Solvent Coordination, Ligand, and Substituent Effects

The mechanism of the η 3 → η 1 → η 3 isomerization of (η 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 (η 1 -allyl)palladium intermediates. In these intermediates a solvent molecule or another ancillary ligand coordinates to palladium. The stability of the (η 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 σ → π* type hyperconjugative interaction occurring in the η 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 (η 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.