Theoretical Study on the Transformation of Bis(acetylene)cobalt to Cobaltacyclopentadiene and the Regioselectivity in this Transformation

Abstract The transformation of bis(acetylene)cobalt complex to cobaltacyclopentadiene complex was studied using a hybrid density functional theory method. B3LYP calculations showed that the reaction of an unsubstituted system, bis(η2-acetylene)cobalt complex, on a singlet potential energy surface is an easy reaction with a small activation energy of 11.2 kcal/mol and an exothermicity of −19.2 kcal/mol. The low activation barrier was as expected for a symmetry-allowed reaction. Because the product of cobaltacyclopentadiene has a low-lying unoccupied orbital, the two Co–Cα bonds are different in distance due to the second-order Jahn–Teller effect, and the triplet cobaltacyclopentadiene is more stable than the singlet cobaltacyclopentadiene, different from the reactant and transition state. In addition, we performed calculations for the reactions of acetylenes substituted by methyl and/or methoxycarbonyl groups, in order to investigate the factors that control the regioselectivity observed in this type of reaction. The calculations for the mono- and disubstituted reactions showed that these substituents prefer α-carbon to β-carbon. We analyzed the origin of this regioselectivity based on the relative stability of the products, to find that it is closely related to the site preference in the substituted butadienes. This suggests that the site preference of substituents is an important factor of regioselectivity.