Notably Enhanced Reactivity of the Fulleroid anti-Bredt Double Bond in Diels-Alder Reactions

A kinetic study of Diels–Alder (DA) reactions of some fullerene dienophiles, i.e. C60, [R1, R2]‐substituted fulleroids 1 (a: [H, H], b: [H, CN], c: [H, CO2Et]; d: [H, Ph], e: [Ph, Ph]) and [H, H]‐substituted methanofullerene (2), with various dienes [cyclopentadiene (3a), 9‐methylanthracene (3b), 1,3‐cyclohexadiene (3c), 2,3‐dimethylbutadiene (3d) and 2‐methylbutadiene (3e)] has been carried out in toluene at 30 °C. Notably, the relative rate ratio k/k0 for each dienophile (vs. C60) was remarkably dependent on the diene used; the less hindered H/CN fulleroids 1a and 1b became 25–104 times more reactive than C60 with conformationally flexible dienes such as 3d and 3e. These fulleroids, however, showed similar or rather diminished reactivity with rigid 3a and 3b. In contrast, slightly hindered 1c exhibited a three to seven times reduced rate of reaction for 3d–e, and more hindered 1d–e and the 58‐π‐conjugated 2 displayed reduced reactivity with almost all the dienes. Based on product analysis, the enhanced reactivity was ascribed to the exclusive addition at the highly strained bridgehead double bond of the transannular [5,6] open ring orifice. It was concluded that the twisted anti‐Bredt double bond can perform a topologically suitable [π2s + π4s] orbital interaction with the less congested, labile dienes. Such an orbital interaction is inhibited for both the congested fulleroids and the rigid dienes, which displayed alternative addition at a less twisted double bond. A kinetic study of Diels–Alder reactions of fulleroids with various dienes indicates that regioselective addition occurs at the bridgehead olefin. The regioselectivity depends on the flexibility of the dienes as well as the steric hindrance of the fulleroids.