Regioselective synthesis of [6,6]-open and [5,6]-closed C70(CF3)8[CH2] methanofullerenes with rapid [6,6]-to-[5,6] phototransformation

Fullerene derivatives with >CH2 addends in [6,6]-open or [5,6]-closed configuration are uncommon of fullerene derivatives, but they are readily accessible via treatment of Cs-C70(CF3)8 (1) with diazomethane followed by thermolysis or photolysis. Both thermodynamic and kinetic factors favor regioselective addition of diazomethane at the near-equatorial [5,6]-double bond of Cs-C70(CF3)8 to give thermally labile pyrazoline intermediate 2. Thermal extrusion of N2 from 2 is a kinetically controlled process with an orbital symmetry controlled Wodward-Hoffmann-allowed mechanism. It quantitatively yields less thermodynamically favorable [6,6]-open isomer of C70(CF3)8[CH2] homofullerene (3), but the latter turns out to be capable of unexpectedly rapid quantitative phototransformation into the thermodynamically preferable [5,6]-closed methanofullerene isomer 4. The transformation involves the manifold of the triplet states that facilitate the required cleavage of the Ccage-CH2 bonds

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