Diels-Alder Reaction on Free C68Fullerene and Endohedral Sc3N@C68Fullerene Violating the Isolated Pentagon Rule: Importance of Pentagon Adjacency

The reaction mechanism and regioselectivity of the Diels-Alder re- actions of C68 and Sc3N@C68, which vio- late 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 Sc3N@C68 is reduced remarkably owing to charge transfer from the cluster to the fullerene cage. The (5,5) bond be- comes the most reactive site thermody- namically and kinetically. The bonds around the pentagon adjacency show the highest chemical reactivity, which demonstrates the importance of penta- gon adjacency. Furthermore, the viabil- ity of Diels-Alder cycloadditions of several dienes and Sc3N@C68 was ex- amined theoretically. o-Quinodime- thane is predicted to react with Sc3N@C68 easily, which implies the pos- sibility of using Diels-Alder cycloaddi- tion to functionalize Sc3N@C68.