DFT Studies on the Reaction Mechanism of 1,3-Conjugated Dienes Isomerization Catalyzed by Ruthenium Hydride

The detailed reaction mechanism for the isomerization of 1,3-conjugated dienes catalyzed by the ruthenium hydride complex RuHCl­(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 that 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, the steric effect of the bulky N-heterocyclic carbene ligand in ruthenium hydride RuHCl­(CO)­(H2IMes)­(PCy3) was considered theoretically.