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.