Regioselective Diels–Alder Reactions of C70 Directed by Endohedral Ca and Th Atoms

Diels–Alder cycloaddition reactions of fullerenes are frequently utilized to produce fullerene derivatives with fascinating physical properties. To achieve desirable properties, it is critical to control and tune the exohedral reactivity of the fullerene surface. For the fullerene C70, which is one of the most prominent members of the fullerene family, the Diels–Alder reactions usually take place at the hexagon–hexagon carbon ring junctions ([6,6] type C–C bonds), whereas the pentagon–hexagon ring junctions ([5,6] type C–C bonds) are highly inert. Although a large number of [6,6] adducts of C70 have been synthesized, activation of the [5,6] bond remains challenging. This work aims to develop a strategy for activation of the nonreactive [5,6] bond through embedding metal atoms in C70. By performing density functional theory calculations, we investigated the effect of endohedral Ca and Th atoms on the reactivity of C70 toward DA addition reactions. We find that encapsulation of a Ca or Th atom in C70 can greatly enhance the reactivity of the [5,6] bond to be higher than that of the [6,6] bond, which is explained by metal-induced changes in the electronic structure and aromaticity of the fullerene cage. Activation-strain analysis on the transition states of the reactions demonstrates that favorable DA addition to the [5,6] bond of Ca@C70 and Th@C70 can be ascribed to the small strain energy of the reactants. Furthermore, there exists a distinction in the sites displaying the utmost reactivity between Ca@C70 and Th@C70. This study offers useful guidance for the activation of the unreactive bond of fullerenes to obtain new derivatives.