Synthesis, Stability, and X-Ray Crystallographic Structure Analysis of Spiro[1H-azulenium-1,1′-cycloalkane] Ions

Direct cycloalkylation of 1,6-dihydroazulene and subsequent hydride abstraction with a trityl salt gave spiro[1H-azulenium-1,1′-cyclopentane and -1,1′-cyclohexane] ions (3 and 4). On the other hand, spiro[1H-azulenium-1,1′-cycloheptane] ion (5) was synthesized from 1-acetylcyclohepta-1,3,5-triene by a sequence involving the Mukaiyama aldol reaction, the Nazarov cyclization, the Shapiro reaction, and hydride abstraction. These cations showed greater kinetic stability than the three- and four-membered ring homologues. Their pKR+ values are far greater compared with those of the known disubstituted tropylium cations and are in the order of the number of carbon atoms at the 1 position, indicating that an inductive effect of the spiroalkyl groups, besides the π-conjugative and σ-conjugative effects, governs the thermodynamic stability. X-Ray crystallographic structure analysis of these cations was also described.