BENZYNE ADDS ACROSS A CLOSED 5-6-RING FUSION IN C-70 - EVIDENCE FOR BOND DELOCALIZATION IN FULLERENES

Addition of benzyne to C-70 results in four isomeric monoadducts (comp ounds 1a-d) in a 42:35: 13:10 ratio as determined by H-1 NMR. These co mpounds were separated by repeated passes through HPLC columns. The ma jor,isomer (1a) resulted from addition to the highly pyramidalized C1- C2 bond as shown by C-13 NMR and UV/vis spectroscopy. The structure of the C1-C2 adduct (1a) was confirmed by X-ray crystallography. The sec ond isomer (1b) was assigned as an adduct to the C5-C6 bond on the bas is of C-13 NMR and UV/vis spectroscopy. Compound 1c exhibited a C-13 s pectrum consistent with an adduct to the C7 and C8 positions. The pres ence of sp(3) C-13 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.