Chemistry of the Higher Fullerenes: Preparative isolation of C76 by HPLC and synthesis, separation, and characterization of Diels-Alder monoadducts of C70 and C76

HPLC Separation of higher fullerenes was compared on two different stationary phases, and the preparative isolation of pure C76 is described. Higher-fullerene derivatives 1 and 2 were prepared by Diels-Alder reaction of C70 and C76 with an ortho-quinodimethane intermediate generated in situ. Three out of four possible isomeric C70 monoadducts, i.e.1a–c, and, for the first time, one isomeric C76 monoadduct, i.e.2c, could be isolated in pure form and characterized by 1H-NMR, 13C-NMR, UV/VIS, and mass spectrometry. Five other C76 isomers i.e., 2a,b,d–f were obtained in partially separated product fractions. Coalescence temperatures and energy barriers were determined for the cyclohexene-ring inversion in two of the isomeric C70 derivatives. The structure of the C70 adducts could be deduced unambiguously from symmetry considerations based on high- and low-temperature 1H-NMR spectroscopy. A simple model for the qualitative evaluation of the local curvature of fullerene surfaces is presented and used for the prediction of addition sites in higher fullerenes. These predictions are compared to the experimental results mentioned above as well as to predictions resulting from π-bond-order considerations and from calculated pyramidalization angles.