Chemistry of the Higher Fullerenes: Preparative isolation of C₇₆ by HPLC and synthesis, separation, and characterization of <i>Diels</i>‐<i>Alder</i> monoadducts of C₇₀ and C₇₆

Abstract

Abstract HPLC Separation of higher fullerenes was compared on two different stationary phases, and the preparative isolation of pure C 76 is described. Higher‐fullerene derivatives 1 and 2 were prepared by Diels ‐ Alder reaction of C 70 and C 76 with an ortho ‐quinodimethane intermediate generated in situ . Three out of four possible isomeric C 70 monoadducts, i.e. 1a – c , and, for the first time, one isomeric C 76 monoadduct, i.e. 2c , could be isolated in pure form and characterized by 1 H‐NMR, 13 C‐NMR, UV/VIS, and mass spectrometry. Five other C 76 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 C 70 derivatives. The structure of the C 70 adducts could be deduced unambiguously from symmetry considerations based on high‐ and low‐temperature 1 H‐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.