Less Expected Substitutions of the Azulene Nucleus

While electrophilic substitution is used to attack positions 1(3) and possibly 2 of azulenes, nucleophilic substitution is often used to obtain azulenes with substituents at positions 6 or 4(8). The electrical charge at positions 2, 5, or 7 makes them unsuitable for electrophilic or nucleophilic substitution. Azulenes bearing substituents at these positions have been synthesized mainly by the building of the azulene skeleton or especially resorting to reactions catalyzed by metallic compounds. In this context, the proposed mini review will focus on some of the cases in which compounds with substituents in these positions are obtained by substitution without the intervention of a catalyst and are therefore advantageous from both an ecological and economic point of view.