Synthesis and properties of novel azulenic pi-electronic compounds

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 pK(R)(+) 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 pK(R)(+) 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.