Reaction of C-60 with sultines: Synthesis, electrochemistry, and theoretical calculations of organofullerene acceptors

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 C-60. The cycloadducts formed (21a-d) were further oxidized to the respective p-benzoquinone-containing fullerenes 23a-c. The temperature dependent H-1 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 C-60 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.