Theoretical Studies On The Reaction Mechanism Of Conjugated Dienes Isomerization Catalyzed By Ruthenium Hydride

Olefin isomerization, an atom-economical chemical reaction, plays an important role in many important chemical processes, such as petrochemical refining and polymerization reactions. In addition, olefin isomerization reaction has been highlighted as a very useful method to synthesize many desired targets. Many transition metals have been developed to facilitate olefin isomerization. Most such studies, however, focus on the migration of nonconjugated double bonds. Only a limited number of investigations consider the isomerization of conjugated dienes. In this paper, the detailed reaction mechanism for the isomerization of 1,3-conjugated dienes catalyzed by the ruthenium hydride complex Ru HCl(CO)(H2IMes)(PCy3) has been studied with the aid of density functional theory(DFT) calculations. Both cis and trans isomers of a 1,3-conjugated diene were considered as the reactants. For each isomer, two catalytic cycles were calculated, which(respectively) generate a 1,3-hydride shift product or a 1,5-hydride shift product. Both catalytic cycles proceed via alkene migratory insertion into the Ru-H bond, σ-allyl ruthenium isomerization, and β-H elimination steps. Our computational study shows the cis isomer of the model reactant reacts preferentially via the pathway leading to the 1,5-hydride shift product, consistent with the experimental results. The σ-allyl ruthenium isomerization step is found to be crucial for reaction regioselectivity. Strong binding of the C=C bond to Ru is involved in the generation of the 1,5-hydride shift product. In addition, we also performed some calculations on a small catalyst model to investigate the steric effect on the regioselectivity of the reaction. The results show that the bulky group in catalyst has a significant influence on reaction regioselectivity.