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

Addition of benzyne to C₇₀ results in four isomeric monoadducts (compounds 1a−d) in a 42:35:13:10 ratio as determined by ¹H 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 ¹³C 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 ¹³C NMR and UV/vis spectroscopy. Compound 1c exhibited a ¹³C 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 C₁ symmetry and is assigned as an adduct to the C7−C21 bond.