Diazadiene-Controlled C-C Coupling Reactions on Palladium and Iron

Transition metal centers can act as electrophiles or nucleophiles and also transfer electrons to or accept electrons from substrates. In addition the metals can act as simple templates and thus impose geometrical factors on two or more substrates. All of these features can be monitored by additional ligands. While for stoichiometric metal centered reactions drastic conditions and changes in the energy content of the metal compound are possible the energetic hypersurface for a good catalytic reaction must be rather smooth relative to the temperature. Since many substrates such as olefines, carbon monoxide or hydrogen are kinetically inert it seems unprobable that stable metal compounds are catalysts per se. Many of the activation steps in homogeneous catalysis therefore correspond to raising the total system to the ground level of the catalytic energy hypersurface. For a catalytic reaction in the mechanistic sense such a surface must not necessarily have a descend (almost degenerate catalyes such as olefin metathesis, H/D-scrambling, some olefin isomerizations), for a productive catalysis there should of course be a declining slope with the reaction coordinate. In the following paragraphs reactions involving the metal center activation, substrate binding and transformation, and product release will be discussed with many new examples from the catalytic chemistry of palladium and iron systems, in which diazadienes (DAD) constitute the control ligands [1].