1,1-Ethylene-1H-azulenium ion and its alkyl substituted derivatives: Synthesis, characterization, and some reactions thereof

1,1-Ethylene-1H-azulenium tetrafluoroborate (1b) and its alkyl substituted derivatives, 6-t-butyl (1c) and 4-isopropyl-3,8-dimethyl ones (Id), were synthesized starting from their corresponding azulenes by a three- step sequence which includes reduction, cyclopropanation and hydride abstraction reactions. The cation 1b and its 6-t-butyl derivative 1c, generated in deuterated acetonitrile at -20 °C, were characterized by low- temperature NMR spectroscopy. On the other hand, the cation Id was isolated os slightly unstable, greenish-yellow crystals. While the cation 1b in solution underwent expansion of the cyclopropane ring at elevated temperatures, 1c and 1d just decomposed. All the cations were found to react easily with nucleophiles to give thermodynamically controlled, stable addition products at their cyclopropane methylene carbons.