Azulenium ion

The azulenium ion (15), while possessing the characteristic tropylium moiety, by merit of conjugation of the aromatic nucleus with a double bond, might be expected to undergo significantly different photochemical. reactions from 1 (van Tamelen et cU., 1968, 1971). In fact, when 15 is... 

A theoretical ab initio study of the interconversion of isobutonium ions has been carried out. The 1,1-trimethylene-l//-azulenium ion (202) has been prepared and its chemical behaviour has been shown to be different from that of its three-membered ring homologue. The solvolysis of l-[trans-2-(m- or / -substituted phenyl)... 

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. 

The long-wave bands of 182a and 183a (which are probably due, as in the azulenium ion,129 for example, to electron transfer from the phenyl group to the 1,2-dithiolium or tropylium ion) are very similar in position and maximum extinction they suffer bathochromic shifts of similar magnitudes when identical donor substituents such as OCHs or N(CHS)2 are introduced into the p-positions of the phenyl groups, with a consequent increase in the probability of electron transfer. 

In the presence of strong acids (e.g., 60% sulfuric acid), azulene suffers reversible protonation at the 1- or 3-position to yield the azulenium ion... 

Tills also explains the solubility of azulenes in strong acids to form azulenium ions which do not have the characteristic blue colour of azulene ... 

A special example of a proton-transfer reaction is shown by the azulenes.( > Hence, the protonized azulenium ion is reduced at... 

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)]. 

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