Regioselectivity in Diels-Alder Cycloadditions of ^#6094C₆₈ Fullerene with a Triplet Ground State

To achieve full control on the regioselectivity of chemical additions to fullerenes is a major goal in the field of reactivity of carbon nanostructures. In this work, we computationally analyze the regioselectivity of the Diels-Alder (DA) reaction of cyclopentadiene to the hollow nonisolated pentagon rule (IPR) ^#6094C₆₈ fullerene, which possesses a triplet ground state. Our aim is to check whether the typically favored [6,6]-addition in fullerenes can be shifted to the [5,6] bonds in ^#6094C₆₈ due to the change in the ground state. Our results show that the [5,5] adduct is the thermodynamic reaction product, whereas the kinetic product is the [5,6] bond of type F, adjacent to a pentalene unit. As compared to the singlet state, in the triplet state, the Gibbs barrier for the attack to the [5,6] bond of ^#6094C₆₈ is reduced by about 5 kcal·mol^-1, the DA cycloaddition becoming more regioselective. Our energy decomposition analysis shows that the change of regioselectivity in the DA reaction of hollow fullerenes from the usual [6,6] bond to the [5,6] bond in ^#6094C₆₈ is driven by higher stabilizing orbital interactions in the latter bond favored by the spin density accumulation around the two pentalene units of the cage. The findings of this investigation complement those of earlier studies on regioselectivity of IPR fullerenes and endohedral metallofullerenes.