Diels-alder reaction on free C₆₈ fullerene and endohedral Sc₃N@C₆₈ fullerene violating the isolated pentagon rule: Importance of pentagon adjacency

The reaction mechanism and regioselectivity of the Diels-Alder reactions of C₆₈ and Sc₃N@C₆₈, which violate the isolated pentagon rule, were studied with density functional theory calculations. For C₆₈, the [5,5] bond is the most favored thermodynamically, whereas the cycloaddition on the [5,6] bond has the lowest activation energy. Upon encapsulation of the metallic cluster, the exohedral reactivity of Sc ₃N@C₆₈ is reduced remarkably owing to charge transfer from the cluster to the fullerene cage. The [5,5] bond becomes the most reactive site thermodynamically and kinetically. The bonds around the pentagon adjacency show the highest chemical reactivity, which demonstrates the importance of pentagon adjacency. Furthermore, the viability of Diels-Alder cycloadditions of several dienes and Sc₃N@C₆₈ was examined theoretically. o-Quinodimethane is predicted to react with Sc₃N@C₆₈ easily, which implies the possibility of using Diels-Alder cycloaddition to functionalize Sc₃N@C₆₈.