Azulene, substitution

We have reported the synthesis of a series of azulene-substituted methyl cations, i.e., tri(l-azulenyl)methyl, di(l-azulenyl)phenylmethyl, and (1-azul-enyl)diphenylmethyl cations (2a+, 3a+, and 4a+) (Figure 6) (6). In order to examine substituent effects on the azulene rings and to enhance their stabilities, a series of the cations (2b-d+, 3b-d, and 4b-d+) bearing tert-buty groups on each azulene ring were also synthesized (7). 

Azulene-substituted methyl cations were prepared as illustrated in Figure 7. The hydro derivatives (7a-d) became good precursors for the methyl cations. These derivatives were readily obtained by the reaction of azulenes 6a-d with 1-formyl compounds 5a-d under acidic conditions. The synthesis of the tri(l-... 

Figure 7. Preparation of the azulene-substituted methyl cation salts 2a-it PFf ...

A maximum yield of 70% was obtained if ring-closure was carried out in a high-boiling basic organic solvent such as benzidine, and by a continuous process in which only small amounts of the intermediate fulvene were present at any time. Alkylcyclopentadienes react similarly to give 1-substituted azulenes, and substituted pyridines miy also be used to give azulenes substituted in the seven-membered ring. 

Azulene itself was obtained in this way in 68% yield [30]. A number Of azulenes substituted in the seven-membered ring have been prepared in the same way [32],... 

The Danheiser annulation has also been reported to be useful for the preparation of substituted azulene products." For example, treatment of silylallene 58 with tropylium cation 59 at 23 °C produces intermediate cyclopentene dihydroazulene 60, which is not isolated but rather undergoes in situ dehydrogenation with a second equivalent of tropylium cation 59 to provide azulene 61. The use of the tert-butyldimethylsilyl group as opposed to the trimethylsilyl group is necessary since the trimethylsilyl group tends to undergo premature desilylation rather than cyclization to the azulene. Substitution at the 3-position of the allene was also found to significantly... 

A number of highly stable azulene-substituted cations, (17) and several others, have been prepared and studied some of them have pA R+ values of 14 and above." The amazingly stable species (18) is also reported with = morpholino the p.STr+ value is 21.5, and with R = Me it is 24.3, which means that these species remain as cations even in superbase media." While en route to these species, it was found that 1-azulenecarbaldehydes can be decarbonylated under fairly mild conditions by treating them with pyrrole in acetic acid." ... 

The [3-1-2] and [4-1-2] cycloaddition reactions of IV-sulfinylamines are also well investigated. High pressure is used to assist the cycloaddition reactions of azulene-substituted A -sulfinylamines . The [4-1-2] cycloaddition reaction of MeCONSO with pentacene (yield >90 %) is used to synthesize a solution-processable organic thin film transistor. ... 

Proton Resonances in Azulene, Substituted Azulenes, and their n Complexes... 

Ito, S., lida, T., Kawakami, J., Okujima, T. and Morita, N. (2009) Towards the preparation of electrochromic materials with strong absorption in the near-infrared region synthesis and redox behavior of azulene-substituted enediyne scaffolds connected by a 9,10-anthracenediyl spacer. European Journal of Organic Chemistry, (31), 5355-5364. 

Formylation of Aromatic Rings. The Vilsmeier reagent attacks electron-rich aromatic systems to form aryl-methyleneiminium ions which liberate a formylated aromatic compound upon hydrolysis (eq 2). Thio- and selenoaldehydes can be prepared by hydrolysis in the presence of Sodium Hydrogen Sulfide or Sodium Hydrogen Selenide. A wide range of aromatic systems can be formylated in this fashion, including benzene derivatives, polyaromatic hydrocarbons (eq 3), and azulene. Substitution occurs at relatively electron-rich positions. 

---