Density functional theory studies of ruthenium-catalyzed bis-Diels-Alder cycloaddition of 1,5-cyclooctadiene with alkynes

Density functional theory calculations at the B3LYP level have been performed to study the reaction mechanism of the Ru-catalyzed cycloaddition of 1,5-cyclooctadiene (COD) with alkynes. Our calculations point toward the proposed mechanism that the cycloaddition reaction occurs via an intermediate formed by the active species [CpRu(COD)](+) and an alkyne substrate molecule. The first C-C coupling step was found to be the rate-determining step. The active species [CpRu(COD)]+, which is crucial for the catalytic reaction, can be obtained through the ionization of Cl- from [CpRu(COD)Cl] under the polar solvent of MeOH. The extra pi(perpendicular to) bond of alkynes in comparison to alkenes has been found to play a key role in stabilizing the relevant reaction intermediate as well as lowering the reaction barriers. In the olefin case, the absence of the extra pi(perpendicular to) bond leads to the instability of the corresponding intermediate as well as the high reaction barriers. Reaction mechanisms that are deemed possible on the basis of known fundamental organometallic reactions, such as oxidative coupling, ligand isomerization, and Cp ring slippage, were found to have higher reaction barriers.