Synthesis, stability, and X-ray crystallographic structure analysis of Spiro[lH-azulenium-1,1 '-cycloalkane] ions

Direct cycloalkylation of 1,6-dihydroazulene and subsequent hydride abstrac tion 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'-cy cloheptane] 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 great er kinetic stability than the three- and four-membered ring homologues. The ir pK(R)+ values are far greater compared with those of the known disubstit uted tropylium cations and are in the order of the number of carbon atoms a t the 1 position, indicating that an inductive effect of the spiroalkyl gro ups, besides the pi -conjugative and sigma -conjugative effects, governs th e thermodynamic stability. X-Ray crystallographic structure analysis of the se cations was also described. (C) 2000 Elsevier Science Ltd. All rights re served.