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

Abstract

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.

Mass Spectrometry Studies of the Retro-Cycloaddition Reaction of Pyrrolidino and 2-Pyrazolinofullerene Derivatives Under Negative ESI Conditions

“…In contrast, the N-alkylpyrrolidine 11 ( Figures S19 and S20) undergoes a direct cycloreversion process. These results are in total agreement with the reported data on the thermal retro-cycloaddition reactions of substituted fulleropyrrolidines [46]. …”

“…Thus, Compounds 4 and 6 only undergo a direct retro-cycloaddition reaction, while 5 fragments in a different pathway due to the extended rings fused to the fullerene sphere which avoid partially the elimination of a suitable 1,3-dipole and, in consequence, a loss of methanimine takes place forming the intermediate fragment 12. The reported data on the thermal retrocycloaddition process for Compound 6 indicate that the process takes place easily even in the absence of a dipolarophile [46]. The stabilization in the eliminated azomethine ylide, specially produced by the electronwithdrawing character of the methoxycarbonyl group on the C2 position of the pyrrolidine ring (Compound 6), accelerates the thermal reaction.…”

“…Surprisingly, Compound 8 with a similar structure but with a double bond attached at the carbonyl group of the amide moiety does not undergo neither the CO extrusion nor the retro-cycloaddition process ( Figures S11 and S12). It is important to note that the thermal retro-cycloaddition reactions of N-acylpyrrolidinofullerenes do not take place even in the presence of strong dipolarophiles and copper triflate [46]. Theoretical calculations have shown that the benzoyl substituent destabilizes the azomethine ylide, thus preventing the reaction.…”

“…Compounds 9 and 10, which cannot undergo retrocycloaddition thermal reaction [46], behave differently under mass spectrometry conditions, presenting a very important cycloreversion process. However, this process does not take place directly from the molecular ion, being necessary a previous elimination of the acyl group in order to form an appropriate pyrrolidine derivative for the cycloreversion process.…”

Microwave‐Assisted Functionalization of Carbon Nanostructures in Ionic Liquids

“…[17] No other additive is required, as the IL solvates the incipient 1,3-dipole by electrostatic interactions, while the MW-induced dielectric heating speeds up the thermal cycloreversion with unprecedented rates. [17,18] As a matter of fact, in pure IL media, cycloreversion turned out to be the dominating process, because [60]fullerene, which is hardly soluble in such a polar environment, precipitates, thus hampering the thermodynamic equilibration of the reagent/product distribution. In pure IL, functionalization of [60]fullerene failed to yield any cycloadduct, regardless of the IL type, applied MW power, and irradiation time (entry 1, Table 1).…”