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 occurr ing as catalytic intermediates in important synthetic transformations have been studied by applying density functional theory at the B3PW91(DZ+P) leve l. It was found that eta (1),eta (3) coordinated bis-allylpalladium complex es are readily formed from the corresponding eta (3),eta (3) complexes, esp ecially in the presence of pi -acceptor phosphine ligands. The theoretical calculations indicate d(sigma) --> pi* type hyperconjugative interactions o ccurring in the eta (1)-coordinated allyl moiety of the eta (1),eta (3) coo rdinated complexes. These hyperconjugative interactions influence the struc ture of the complexes and dramatically increase the reactivity of the doubl e bond in the eta (1)-moiety. The DFT results indicate a remarkably low act ivation barrier for the electrophilic attack on the eta (1)-allyl functiona lity. In bridged eta (1),eta (3) complexes, the electrophilic attack occurs with a very high regioselectivity, which can be explained on the basis of d-pi type hyperconjugative interactions.