A Computational Investigation of the Mechanism of Hydrogen Activation by Heterolytic Addition

The mechanism of olein hydrogenation reactions catalyzed by Wilkinson\u27s catalyst, (PPh3)3RhCl, has been well-known to occur via a so-called dihydride mechanism, in which a metal-dihydride species is formed from an oxidative addition of H2 to the metal center. In contrast, the mechanism of the hydrogenation reactions catalyzed by complexes of the type (PPh3)3(CO)RuH and (PPh3)3RuHCl has been commonly regarded to proceed via a monohydride pathway. Despite experimental evidence of its existence, however, the metal-alkyl intermediate species has been rarely observed under catalytically viable conditions. The need for a better mechanistic understanding of the monohydride pathway has also been exemplified by recent developments in metalcatalyzed asymmetric hydrogenation reactions, wherein the monohydride mechanistic pathway has been suggested in some cases. The ruthenium- hydride complex (PCy3)z(CO)(Cl)(Ru)H was found to catalyze the hydrogenation of terminal and cyclic alkenes. For example, the reaction of 1-hexene with 4 atm of H2 in the presence of la produced the hydrogenation product. The treatment of la with excess ethylene led to the observation of both the ruthenium-ethyl and ruthenium-ethylene-hydride species, respectively. These results are consistent with a monohydride mechanistic pathway. In my project, the computational investigation was done to prove the plausible mechanism