Approaches to open fullerenes: Synthesis and kinetic stability of Diels-Alder adducts of substituted isobenzofurans and C₆₀

(Chemical Equation Presented) We have examined the reactions of 1,3-disubstituted isobenzofurans with the fullerene C₆₀ in the context of an approach to open a large orifice on the fullerene framework. A variety of substituted isobenzofurans (6a-h), generated from the reaction of 1,4-substituted 1,4-epoxynaphthalenes 3a-h with 3,6-bis(2-pyridyl)-1,2,4,5- tetrazine (4a) or 1,2,4,5-tetrazine (4b), were added to C₆₀ to afford the Diels-Alder adducts 7a-h. The thermal stability of these adducts toward retro-Diels-Alder fragmentation differs greatly in solution from that in the solid state. In solution, the relatively facile retro-Diels-Alder fragmentation of monoadducts 7a and 7c, to give C₆₀ and the free isobenzofurans 6a and 6c, have rate constants (and activation barriers) of k = 9.29 × 10^-5 s^-1 at 70°C (E_a = 32.6 kcal mol ^-1) and k = 1.36 × 10^-4 s^-1 at 40°C (E_a = 33.7 kcal mol^-1), respectively, indicating that the addition of isobenzofurans to C₆₀ is readily reversible at those temperatures. In the solid state, thermogravimetric analysis of adduct 7a indicates that its decomposition occurs only within the temperature range of 220-300°C. As a result, these compounds can be stored at room temperature in the solid state for several weeks without significant decomposition but have to be handled within several hours in solution.