Reaction of cyclopropenylium ions with the tert-butyl-C-60 anion: carbocation-carbanion coordination vs salt formation

The reaction of the tert-butyl-C-60 anion (t-BuC60-) with tricyclopropylcyc lopropenylium ion and 1,2-dicyclopropyl-3-(4-methylphenyl)cyclopropenylium ion underwent C-C covalent bond formation to give 1-tert-butyl-4-(2-cyclopr open-1-yl)-1,4-dihydro[60]-fullerenes. The newly formed C-C bond of these h ydrocarbons was found to undergo reversible heterolysis in polar solvents t o give the original ions. A more highly stabilized carbocation tris[1-(5-is opropyl-3,8-dimethylazulenyl)]cyclopropenylium ion did not react with t-BuC 60-, and a hydrocarbon with a salt structure was isolated as a solid. Tri-t ert-butylcyclopropenylium ion, although less stable than the other cations, underwent no C-C bond formation with t-BuC60-, indicating that steric hind rance effectively suppresses carbocation-carbanion coordination. These resu lts provide a demonstration of a change in reactivity between t-BuC60- and substituted cyclopropenylium ions toward C-C coordination, depending on the thermodynamic stability and steric crowding of the cation. (C) 2001 Elsevi er Science Ltd. All rights reserved.