Sodium azulene derivatives

Trimethylpyrylium perchlorate is a very versatile and useful starting material. Thus its reaction with cyclopentadienyl-sodium has made 4,6,8-trimethylazulene 12 easily available for general studies of the properties of azulenes 18 and for the synthesis of related compounds.14 In addition, pyrylium salts are readily converted to a variety of pyridine derivatives 9 15 as well as to derivatives of nitrobenzene16 and phenol.9 17,18 It is clear that its value as a starting material is such that it is receiving wide use. 

The reactivity of azulene towards electrophiles resembles that of activated benzene derivates in that it couples with dCazo-comoounds [47,83,91,92]. Reduction of 1-arylazoazulenes with sodium bisulphite gives 1-aminoazulenes which can only be isolated as salts or as derivatives such as acetyl derivatives [93]. 

Azulenes are halocienated by jy-bromosuccinimide or iV-chlorosuccinimide [42,47], or by copper(II) bromide or chloride [94], to give mixtures of 1- and 1,3-substituted azulenes. Pyridinium perbromide produces the dibromo derivative [95]. Mono- and di-iodo derivatives have been obtained by reaction with iodine monochloride [95], T -iodosuccinimide [42] or iodine in the presence of copper(II) acetate [94]. Iodine in the presence of sodium iodide reacts with azulene at room temperature to give a good yield of 1-iodoazulene the kinetics of this reaction are comparable to those of a reactive benzene derivative such as aniline [96]. 

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. 

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