Azulene, Reactivity, and Scientific Interest Inversely Proportional to Ring Size; Part 2: The Seven-Membered Ring

Abstract

The second part of the article Azulene, Reactivity, and Scientific Interest Inversely Proportional to Ring Size deals with the chemical behavior of the seven-atom azulenic ring. As the title states, the ability of this system to react is lesser compared to that of the five-atom ring; despite this, a large number of syntheses contain it as a participant in the molecules of starting compounds. This review is focused on certain more frequent syntheses such as nucleophilic substitution of the seven-atom ring or its substituents, vicarious nucleophilic substitutions, substitutions of azulene metallic compounds, or reactions catalyzed by complexes of certain transition metals. The syntheses of tricyclic compounds, porphyrinogenic systems, or azulenocyanines containing an azulenyl moiety are also presented. The adopted presentation is mainly based on reaction schemes that include the reaction conditions, as well as the yields of the products formed.