[4+2] Cycloaddition reactions of six-membered heterocyclic o-quinodimethanes, generated “in situ” from pyrazine derivatives, to [60]fullerene, either under thermal or microwave irradiation are described. Other microwave assisted cycloadditions involving o-quinodimethanes derived from thiophene were also performed. A comparative study of the activation energy for the boat-to-boat conformational inversion has been carried out by dynamic NMR experiments, the ΔG ⧧ values being highly dependent on the nature of the covalently attached heterocyclic systems. Theoretical calculations predict a more planar cyclohexene ring for the five member containing cycloadducts. The cycloaddition process is controlled by the HOMO of the heterocyclic o-quinodimethanes showing a LUMO(C60)-HOMO(diene) energy differences typical for favoured cycloadditions. The redox properties of the novel organofullerenes have been determined by cyclic voltammetry in solution, showing a cathodically shifted first reduction potential values, related to [60]fullerene. Compound 15c bearing two cyano groups exhibited an opposite trend which was accounted for by the lower LUMO energy determined by semiempirical calculations.
The [4 + 2] cycloaddition reaction of o-quinodimethanes, generated in situ from 4,5-benzo-3,6-dihydro-1,2-oxathiin 2-oxides (10a,b, 13, and 19) (sultines), to [60]fullerene is described. Sultines are readily accesible from the commercially available rongalite and smoothly generate o-quinodimethanes, by extrusion of sulfur dioxide, which are efficiently trapped by the active dienophile C60. The cycloadducts formed (21a−d) were further oxidized to the respective p-benzoquinone-containing fullerenes 23a−c. The temperature dependent 1H NMR spectra show a dynamic process of the methylene protons. The activation free energy determined for the boat-to-boat inversion (11.3−11.6 kcal/mol) is remarkably lower than that obtained for other related carbocyclic or heterocyclic analogues. Semiempirical PM3 calculations show that the geometrical features and not the electronic properties of the organic addend in 23 are responsible for the low activation energy barriers. A linear correlation is found between the activation energy barriers and the length of the C62−C63 bond. The electrochemical properties of 23a−c have been rationalized on the basis of DFT-B3P86/3-21G calculations. The attachment of the first electron in the reduction process takes place in either the C60 cage or the organic addend depending upon the nature of the substituents on the p-benzoquinone ring, which controls the relative energy of the LUMO of the p-benzoquinone moiety. A full agreement between the theoretical predictions and the electrochemical measurements is found.
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