Theoretical exploration of structure–reactivity relationships in organometallic chemistry: butadiene insertion into the organyltransition-metal bond and conversion of the allyltransition-metal fragment in the [NiII(η5-Cp)(η1-phenyl)(η2-butadiene)] compl

We have theoretically examined the reaction course of the butadiene insertion into the arylNiII bond in the [NiII(η5-Cp)(η1-phenyl)(η2-butadiene)] complex (1), by employing a gradient-corrected DFT method. Critical elementary processes have been scrutinized, viz. monomer insertion, rotational allylic isomerization and allylic η1-σ→η3-π rearrangement. The first mechanism suggested by Lehmkuhl et al. was refined and supplemented with important details. The critical factors that determine the generation of anti-η3- and syn-η3-allyl isomers of the [NiII(η5-Cp)(1-benzyl-allyl)] product have been elucidated. This let us to rationalize the experimentally observed, almost exclusive formation of the anti-η3-allyl isomer. Butadiene preferably inserts in η2-mode into the η1-phenylNiII bond, initially giving rise to the η1(C3)-allyl product species, 3σ. The direct formation of the η3-allyl product species, 3π, along the alternative path for η4-butadiene insertion, however, is found to be almost entirely disabled kinetically. The thermodynamically favorable η2-trans form of 1 is also shown to be more reactive in accomplishing CC bond formation. Species 3σ is indicated to be a metastable intermediate, occurring in an appreciable stationary concentration. Its respective anti and syn isomeric forms are likely to be in equilibrium, due to the facile rotational isomerization. The subsequent allylic rearrangement into the thermodynamically strongly favorable η3-allylNiII coordination mode is shown to be the crucial elementary step that discriminates which of the isomeric η3-allyl forms is preferably generated. The higher reactivity of the anti isomer in this process decisively determines the almost exclusive formation of the anti-η3-allyl product species under kinetic control. The requirement of elevated temperatures for the anti-η3-allyl→syn-η3-allyl isomerization to occur, as revealed from experiment, is attributed to the pronounced thermodynamic stability of the η3-allylNiII coordination.