Abstract
Studies in our laboratory on the syntheses of hydroxyazulenes, azulene thiols, polyaminoazulenes, and azulene analogue of carixarene, and on the synthesis, stabilities, and dynamic stereochemistries of extremely stable carbocations containing azulene were reviewed. 2- And 6- hydroxyazulenes were synthesized as stable crystals, which were found to exist as their keto- and enol-forms depend on the solvent used. 1-Hydroxyazulene and 3-hydroxyguaiazulene were synthesized starting from the corresponding benzoyloxyazulenes as extremely unstable compounds, and the latter was found to isomerize to a mixture of its keto form and oxidative dimers. 1-, 2-, and 6-azulene thiols, and 1, 2-diamino- and 1, 2, 3-triaminoazulene derivatives were synthesized as stable compounds. A series of azulene analogues of triphenylmethyl cation; tri (1-azulenyl) methyl, di (1-azulenyl) phenylmethyl, and (1-azulenyl) diphenylmethyl hexafluorophosphates and their various derivatives were synthesized by hydride abstraction of the corresponding methane derivatives with DDQ. Their pKR+values showed that the stabilities of these cations dramatically increase with the number of azulene rings. Tris (3, 6-di-t-butyl-1-azulenyl) methyl cation showed the highest pKR+ value (14.3) ever observed. The dynamic stereochemistries of these cations were also studied based on the temperature-dependent NMR spectra, which were analyzed by a flip mechanism. First example of one-ring flip mechanism was found for these molecular propellers. Extremely stable di- and trications were also synthesized.
