The Bingel–Hirsch (BH) reaction is a key strategy for functionalizing fullerenes and endohedral metallofullerenes, typically affording cycloadducts with reduced reactivity toward metallofullerenes. Unexpectedly, a recent experiment on TiSc2N@C80 revealed single-bond BH adducts with markedly enhanced reactivity. However, the detailed mechanism and precise adduct structures remain unclear due to limited experimental evidence. Here, we report a comprehensive density functional theory investigation of the BH reaction between TiSc2N@C80 and diethyl bromomalonate. We show that single-bond addition pathways have substantially lower free-energy barriers than cycloaddition pathways, supporting experimental observations. Moreover, single-bond addition at pentagon–hexagon–hexagon (566) junctions is both thermodynamically and kinetically more favorable than at hexagon–hexagon–hexagon (666) sites, suggesting that the experimental products correspond to 566 adducts. This regioselectivity originates from the higher relative energies of 566 intermediates, which render them more reactive toward subsequent transformations. Remarkably, the single-bond adducts feature the most reduced fullerene cage (oxidation state −7) synthesized to date. These findings clarify the origin of the BH reactivity and selectivity of TiSc2N@C80 and offer valuable guidance for future functionalization of other paramagnetic metallofullerenes.
Lu et al. (Thu,) studied this question.