An MO theoretical characterization of organic reaction mechanisms—V : Transition metal catalyzed (2+2) reactions☆

Abstract The concept of the triplet instability for the conventional Hartree-Fock molecular orbitals has been applied to metal-catalyzed cyclodimerizations to olefin and acetylene as well as hydrogenation of olefin. It is found that the rectangular approach of reactants should be accompanied by the appearance of a significant biradical character. Hence, the concerted (one-step) reactions cannot necessarily be energetically favorable processes, even though transition metal catalyst can formally remove the orbital-symmetry restriction. It is emphasized that correlation diagrams which are usually drawn only on the basis of the knowledge of reactants and products are incapable of providing an adequate description of metal catalyzed reactions.