Benzyne Adds Across a Closed 5−6 Ring Fusion in C70: Evidence for Bond Delocalization in Fullerenes

Addition of benzyne to C 70 results in four isomeric monoadducts (compounds 1a − d ) in a 42:35:13:10 ratio as determined by 1 H NMR. These compounds were separated by repeated passes through HPLC columns. The major isomer ( 1a ) resulted from addition to the highly pyramidalized C1−C2 bond as shown by 13 C NMR and UV/vis spectroscopy. The structure of the C1−C2 adduct ( 1a ) was confirmed by X-ray crystallography. The second isomer ( 1b ) was assigned as an adduct to the C5−C6 bond on the basis of 13 C NMR and UV/vis spectroscopy. Compound 1c exhibited a 13 C spectrum consistent with an adduct to the C7 and C8 positions. The presence of sp 3 13 C NMR resonances proved that the C7−C8 bond is still intact, making this the first identification of direct addition to a 5−6 ring fusion in a fullerene, and the first example of an adduct to a 5−6 ring fusion where the ring fusion bond remains intact. The fourth isomer ( 1d ) displayed spectral data consistent with a compound with C 1 symmetry and is assigned as an adduct to the C7−C21 bond.