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This review focuses on obtaining embedded azulene polycyclic molecules treated according to their particular structure. The division of material into the azulene unit grafted only on one or two bonds and poly-fused azulene compounds was suggested with the intention of facilitating the presentation and assimilation of information. The similarity of some structural features in the compounds included in different analyzed classes results in the presence of the same synthesis protocol in several places. Obtaining benz[a]azulenes, azulene-fused acenes, and helicenes or azulene-embedded nanographene, along with other compounds, is presented.

The target of this review is to familiarize researchers from the field, especially for those who want to find out information about the synthesis pathways of azulenes. The properties of the azulene system are very different from those of its isomer, naphthalene. This difference is a consequence of the monoaxial symmetry of azulene geometry and its non-alternating aromatic structure as opposed to the biaxial symmetry and benzenoid structure of naphthalene. The processing of the information from the literature on azulene synthesis is based on the raw materials from which the procedures start. In many cases, instead of the description of reaction pathways, presentation in the form of schemes was preferred, in which the steps and reaction conditions chosen for them, as well as the yields obtained, were highlighted.

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.

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.

The title compound, i.e., (Z)-4-(azulen-1-ylmethylene)-2-phenyloxazol-5(4H)-one (3), has previously been synthesized in a good yield (56%) using the Erlenmeyer–Plöchl procedure. The crystal structure of 3 is described herein along with the results of a Hirshfeld surface analysis. Crystals of compound 3 were obtained through the slow evaporation of a mixture of petroleum ether/dichloromethane = 1:1 (vol.) at room temperature. This compound has a monoclinic system: a P21/n space group at room temperature. Its crystal packing is driven by π-π interactions between neighboring molecules in the range of 3.60–3.71 Å and by hydrogen bonds (D-H···A = 2.58 Å).

The lack of azulene symmetry with respect to the axis perpendicular to a molecule creates an asymmetry of the electronic system, increasing the charge density of the five-atom ring and favoring its electrophilic substitutions. The increased reactivity of this ring has contributed to ongoing interest about the syntheses in which it is involved. The aim of this review is to present briefly and mainly in the form of reaction schemes the behavior of this system. After a short chapter that includes the research until 1984, subsequent research is presented as generally accepted chapters and subchapters to describe the behavior of the azulene system: metal free catalyst reactions; reactions catalyzed by metals; various azulene five-ring substitutions. The author insists on reaction yields, and in some cases considers it useful to present the proposed reaction mechanisms.

The non-alternant aromatic azulene, an isomer of alternant naphthalene, differs from the latter in peculiar properties. The large polarization of the π-electron system over the seven and five rings gives to azulene electrophile property a pronounced tendency to donate electrons to an acceptor, substituted at azulene 1 position. This paper presents cases in which azulene transfers electrons to a suitable acceptor as methylium ions, positive charged heteroaromatics and examples of neutral molecules that can accept electrons. The proposed product synthesis was outlined and the expected electron transfer was highlighted by analyzing the NMR, UV-Vis spectra and the pKR+ values.

It seems interesting to adopt the idea of dance in the context of the arrangement of molecular blocks in the building of molecular systems. Just as various dances can create various feelings, the nature and arrangement of molecular blocks in the generated molecular system can induce different properties. We consider obtaining such “dancing” systems in which the still little-known azulene moieties are involved. The dark blue nonbenzenoid aromatic azulene has one less axis of symmetry relative to the two axes of its isomer, i.e., the fully benzenoid naphthalene, acquiring valuable properties as a result that can be used successfully in technical applications. In a dancing system, the azulene moieties can be connected directly, or a more or less complex spacer can be inserted between them. Several azulene moieties can form a linear oligomer or a polymer and the involvement of azulene moieties in nonlinear molecules, such as crown ethers, calixarenes, azuliporphyrins, or azulenophane, is a relatively new and intensely studied topic. Some aspects are covered in this review, which are mainly related to obtaining the mentioned azulene compounds and less to their characterization or physico-chemical properties.

The review proposes an overview of the important general routes for the achievement of 1-heteroarylazulenes using both azulene substitution by heterocycles and azulenic cyclic construction pathways. With strong electrophilic heterocyclic derivatives, the electrophilic substitution takes place while concerted route is adopted for medium and low reactive heterocyclic substrates. Particular attention has been paid to the heterocycles construction from the functional groups attached to azulenic 1-position. Graphical abstract

Exothermal decomposition of six (E)-(substituted azulen-1-yldiazenyl-1,2,5-oxadiazoles) was investigated by differential scanning calorimetry (DSC) under non-isothermal conditions, in inert atmosphere. From the DSC curves recorded either at various heating rates for three compounds, or at 10 K min−1 for the rest, several physical properties such as the melting temperature, melting heat, temperature and heat of decomposition were evaluated. The activation parameters of the decomposition processes were determined by analyzing their multiple curves, measured at different heating rates, using both the Friedman and Flynn–Wall–Ozawa isoconversional methods and model-fitting methods. The linear dependence of the activation energy on conversion supports the idea of the single-step nature of the decomposition processes for all examined compounds. The decomposition steps were identified from the experimental DSC curves, and their kinetic parameters were evaluated using linear regression methods. New insight into the relationship between melting and structural properties was gained if the equilibrium free energy of melting was splitted into its energetic and entropic components. The resulted thermodynamic and kinetic parameters are used to predict the hazards associated with these compounds.