Regioselective Synthesis of [6,6]‐Open and [5,6]‐Closed C 70 (CF 3 ) 8 [CH 2 ] Methanofullerenes with Rapid [6,6]‐to‐[5,6] Phototransformation

Fullerene derivatives with >CH 2 addends in [6,6]‐open or [5,6]‐closed configuration are uncommon of fullerene derivatives, but they are readily accessible via treatment of C s ‐C 70 (CF 3 ) 8 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 C s ‐C 70 (CF 3 ) 8 to give a thermally labile pyrazoline intermediate. Thermal extrusion of N 2 from the latter is a kinetically controlled process with orbital symmetry controlled Woodward–Hoffmann‐allowed mechanism. It quantitatively yields the less thermodynamically favorable [6,6]‐open isomer of C 70 (CF 3 ) 8 [CH 2 ] homofullerene, but the latter turns out to be capable of unexpectedly rapid quantitative phototransformation into the thermodynamically preferable [5,6]‐closed methanofullerene isomer. The transformation involves the manifold of the triplet states that facilitate the required cleavage of the C cage –CH 2 bonds.