Approaches to open fullerenes: synthesis and thermal stability of cis-1 bis(isobenzofuran) Diels-Alder adducts of C60.

In a quest to form wider openings within the cage of the fullerene C60 through controlled bond-breaking reactions, we have examined the double saturation of adjacent C=C bonds within a six-membered ring of C60. We have investigated the double Diels-Alder cycloaddition of two tethered isobenzofurans to the fullerene C60. We obtained cis-1 adducts in good yields after reacting the methylene- or quinoxaline-tethered bis(isobenzofuran) precursors 2a-k with parent 3,6-dihydro-1,2,4,5-tetrazine (3b). The X-ray structure of the methylene-tethered bis(isobenzofuran)-C60 adduct 4b has been obtained; four-eclipsed substituents are held rigidly by the bicyclic addends. The cis-1 bis(isobenzofuran) bisadducts 4b and 4e-j are kinetically far more stable toward thermal retro-Diels-Alder fragmentation than are mono(isobenzofuran) adducts of C60, in solution and in the solid state as determined by 1H NMR spectroscopy or thermogravimetric analysis. A methodology for the reversible solubilization of other fullerene derivatives based on this work is also presented.