Azulenium cations

Alkylazuelenes are protonated at C( 1) or C(3) position to form the stable azulenium cations 220,464 which can be viewed as vinyl-substituted tropylium ions [Eq. (3.52)]. 

Azulenes are basic compounds (see Section 4.1.1). The formation of carbocyclic azulenium cations in the presence of strong adds is a transformation to tropylium salts that is accompanied by a large hypsochromic shift and the color changes to yellow (55FCF(3)334, p. 380 66CZ691). Azulenes fused to heterocyclic nitrogen bases, which are primarily N-protonated, do not behave uniformly. 

Fischer and Breitschaft 49a) have prepared complexes which contain the azulenium cation, C10H9+, by displacement of benzene fromw-CsHsCr-w-CeHj as shown in the scheme... 

In practice, extrapolations of p fR in water have usually used the older acidity function based method, for example, for trityl,61,62 benzhydryl,63 or cyclopropenyl (6) cations.66,67 These older data include studies of protonation of aromatic molecules, such as pKSi = —1.70 for the azulenium ion 3,59 and Kresge s extensive measurements of the protonation of hydroxy- and methoxy-substituted benzenes.68 Some of these data have been replotted as p fR or pKa against XQ with only minor changes in values.25,52 However, for more unstable carbocations such as 2,4,6-trimethylbenzyl, there is a long extrapolation from concentrated acid solutions to water and the discrepancy is greater use of an acidity function in this case gives pA 2° = —17.5,61 compared with —16.3 (and m = 1.8) based on X0. Indeed because of limitations to the acidity of concentrated solutions of perchloric or sulfuric acid pICs of more weakly nucleophilic carbocations are not accessible from equilibrium measurements in these media. 

Azulenium salts. Ethereal fluoroboric acid added dropwise with stirring and ice-cooling under anhydrous conditions to a soln. of 4,6,8-trimethylazulene in abs. ether, benzaldehyde added to the resulting 4,6,8-trimethylazulenium fluoro-borate, and the product isolated after ca. 4 hrs. l-benzylidene-4,6,8-trimethyl-azulenium fluoroborate. Y 94%.— Azulenes can be converted into azulenium salts, which are derivatives of the tropenium cation, as well as into azuleniate salts, which are derivatives of ihe cyclopentadienyl anion. F. e. and reactions s. K. Hafner, H. Pelster, and J. Schneider, A. 650, 62 (1961) syntheses via azuleniate salts s. A. 650, 80. 

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