Reaction of cyclopropenylium ions with the tert-butyl-C60 anion: carbocation–carbanion coordination vs salt formation

The reaction of the tert-butyl-C60 anion (t-BuC60−) with tricyclopropylcyclopropenylium ion and 1,2-dicyclopropyl-3-(4-methylphenyl)cyclopropenylium ion underwent C–C covalent bond formation to give 1-tert-butyl-4-(2-cyclopropen-1-yl)-1,4-dihydro[60]fullerenes. The newly formed C–C bond of these hydrocarbons was found to undergo reversible heterolysis in polar solvents to give the original ions. A more highly stabilized carbocation tris[1-(5-isopropyl-3,8-dimethylazulenyl)]cyclopropenylium ion did not react with t-BuC60−, and a hydrocarbon with a salt structure was isolated as a solid. Tri-tert-butylcyclopropenylium ion, although less stable than the other cations, underwent no C–C bond formation with t-BuC60−, indicating that steric hindrance effectively suppresses carbocation–carbanion coordination. These results 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.