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 ac cesible from the commercially available rongalite and smoothly generat e o-quinodimethanes, by extrusion of sulfur dioxide, which are efficie ntly trapped by the active dienophile C-60. The cycloadducts formed (2 1a-d) were further oxidized to the respective p-benzoquinone-containin g fullerenes 23a-c. The temperature dependent H-1 NMR spectra show a d ynamic process of the methylene protons. The activation free energy de termined for the boat-to-boat inversion (11.3-11.6 kcal/mol) is remark ably lower than that obtained for other related carbocyclic or heteroc yclic analogues. Semiempirical PM3 calculations show that the geometri cal features and not the electronic properties of the organic addend i n 23 are responsible for the low activation energy barriers. A linear correlation is found between the activation energy barriers and the le ngth of the C62-C63 bond. The electrochemical properties of 23a-c have been rationalized on the basis of DFT-B3P86/3-21G calculations. The a ttachment of the first electron in the reduction process takes place i n either the C-60 cage or the organic addend depending upon the nature of the substituents on the p-benzoquinone ring, which controls the re lative energy of the LUMO of the p-benzoquinone moiety. A full agreeme nt between the theoretical predictions and the electrochemical measure ments is found.