How Does Spin Play with the Cycloaddition to Paramagnetic Endohedral Metallofullerenes? The Curious Case of TiSc₂N@C₈₀

Diels-Alder cycloaddition is one of the most important reactions for fullerenes, providing a powerful means for exohedral cage functionalization. When it comes to endohedral metallofullerenes (EMFs), however, it is well accepted that they are much less reactive toward Diels-Alder addition than empty fullerenes because of the charge transfer from the encapsulated metal cluster to the carbon cage. Herein, using density functional theory calculations, we report that the paramagnetic EMF, TiSc2N@C80, exhibits a considerably enhanced reactivity toward the cycloaddition with s-cis-1,3-butadiene (BD), with quite different regioselectivity from that for the empty C80 and Sc3N@C80. Most interestingly, the lowest-barrier pathways follow a [4 + 3]-like stepwise mechanism, in stark contrast with the conventional [4 + 2] concerted mechanism. Such a drastic mechanistic modification can be understood by the fact that the spin on Ti in TiSc2N@C80 transfers to BD upon formation of the intermediate and returns to Ti after forming the cycloadduct. Accordingly, by attaching π-withdrawing substituents to BD, the intermediate can be further stabilized through delocalization of the radical in BD and may thus remarkably improve the addition reactivity. These findings showcased by TiSc2N@C80 might widen our knowledge of how spin could profoundly change the chemical picture of paramagnetic EMFs.