On the Mechanism of the Thermal Retrocycloaddition of Pyrrolidinofullerenes (Retro-Prato Reaction)

In contrast to N‐methyl or N‐unsubstituted pyrrolidinofullerenes, which efficiently undergo the retrocycloaddition reaction to quantitatively afford pristine fullerene, N‐benzoyl derivatives do not give this reaction under the same experimental conditions. To unravel the mechanism of the retrocycloaddition process, trapping experiments of the in‐situ thermally generated azomethine ylides, with an efficient dipolarophile were conducted. These experiments afforded the respective cycloadducts as an endo/exo isomeric mixture. Theoretical calculations carried out at the DFT level and by using the two‐layered ONIOM (our own n‐layered integrated molecular orbital and molecular mechanics) approach underpin the experimental findings and predict that the presence of the dienophile is not a basic requirement for the azomethine ylide to be able to leave the fullerene surface under thermal conditions. Once the 1,3‐dipole is generated in the reaction medium, it is efficiently trapped by the dipolarophile (maleic anhydride or N‐phenylmaleimide). However, for N‐unsubstituted pyrrolidinofullerenes, the participation of the dipolarophile in assisting the 1,3‐dipole to leave the fullerene surface throughout the whole reaction pathway is also a plausible mechanism that cannot be ruled out. N‐Substitution decides: In contrast to N‐methyl and unsubstituted (NH) pyrrolidinofullerenes, N‐benzoyl derivatives do not undergo a retrocycloaddition reaction (see scheme). In a combined study, involving experimental trapping experiments and theoretical calculations, the mechanism for the retrocycloaddition process of the well‐known pyrrolidinofullerenes has been unravelled.