Umpolung of the Allylpalladium Reactivity: Mechanism and Regioselectivity of the Electrophilic Attack on Bis-Allylpalladium Complexes Formed in Palladium-Catalyzed Transformations

The structure and reactivity of various bis-allylpalladium complexes occurring as catalytic intermediates in important synthetic transformations have been studied by applying density functional theory at the B3PW91(DZ+P) level. It was found that η1,η3 coordinated bis-allylpalladium complexes are readily formed from the corresponding η3,η3 complexes, especially in the presence of π-acceptor phosphine ligands. The theoretical calculations indicate dσ→π* type hyperconjugative interactions occurring in the η1-coordinated allyl moiety of the η1,η3 coordinated complexes. These hyperconjugative interactions influence the structure of the complexes and dramatically increase the reactivity of the double bond in the η1-moiety. The DFT results indicate a remarkably low activation barrier for the electrophilic attack on the η1-allyl functionality. In bridged η1,η3 complexes, the electrophilic attack occurs with a very high regioselectivity, which can be explained on the basis of d–π type hyperconjugative interactions.