Recent Progress in the Chemistry of Ring-Fused Azulenes: Synthesis, Reactivity and Properties

Azulene, a non-alternative aromatic hydrocarbon, has attracted significant attention due to its unique electronic properties, and potential applications in organic electronics and optoelectronics. This review highlights recent advances in the synthesis, reactivity, and functional properties of ring-fused azulene derivatives. The discussion encompasses classical synthetic routes, including the Ziegler-Hafner and Nozoe methods, as well as novel approaches such as transition metal-catalyzed cyclizations. Key advancements in the construction of benzo[a]azulenes, naphthoazulenes, and other polycyclic azulene frameworks are detailed, emphasizing their regioselective functionalization and enhanced stability. Moreover, the incorporation of azulene moieties into polycyclic aromatic hydrocarbons (PAHs) and heterocyclic systems is explored, highlighting their potential applications in organic light-emitting diodes (OLEDs), field-effect transistors (OFETs), and photovoltaic devices. Special attention is given to azulene-fused helicenes and nanographenes, which demonstrate promising chiroptical properties and extended π-conjugation. This review aims to provide a comprehensive overview of the synthetic strategies and emerging applications of azulene-based compounds, contributing to the development of advanced materials for future electronic and photonic technologies.

Synthesis of 2-Amino-4-arylazulenes from 8-Aryl-2H-cyclohepta[b]furan-2-ones and Transformation into 6-Aryl-7H-naphth[3,2,1-cd]azulen-7-ones

2-Amino-4-arylazulene derivatives were prepared from 8-aryl-2H-cyclohepta[b]furan-2-ones, which were converted into 6-aryl-7H-naphth[3,2,1-cd]azulen-7-ones through a several step process. The reaction of 8-aryl-2H-cyclohepta[b]furan-2-ones bearing an ester group at the 3-position with malononitrile in the presence of triethylamine afforded 2-amino-4-arylazulenes. The prepared 2-amino-4-arylazulenes were converted to the corresponding 2-chloro derivatives by the Sandmeyer reaction, which were subsequently transformed into 2,4-diarylazulenes by Suzuki-Miyaura coupling with various aryl boronic acids. 2,4-Diarylazulenes underwent intramolecular cyclization between aryl and cyano groups by Brønsted acid to give 6-aryl-7H-naphth[3,2,1-cd]azulen-7-ones. UV/vis spectral analysis revealed that 6-aryl-7H-naphth[3,2,1-cd]azulen-7-one with a N,N-dimethylaminophenyl group at the 6-position exhibited a broad and strong absorption band in the visible region due to intramolecular charge transfer. Furthermore, 6-aryl-7H-naphth[3,2,1-cd]azulen-7-ones exhibited halochromism in 30% CF3CO2H/CH2Cl2. Although fluorescence was not observed in solution, 8-aryl-2H-cyclohepta[b]furan-2-ones with an ester function were found to fluoresce in the solid state. 6-Aryl-7H-naphth[3,2,1-cd]azulen-7-ones also displayed spectral changes with good reversibility under the electrochemical redox conditions.

Reactions of alkynes with C-S bond formation: recent developments

Alkynes are important in organic synthesis. This review mainly focuses on recent advances (2013-2023) on alkynes with C-S bond formation, based on more than 30 types of sulfur reagents. The reactions of alkynes with various sulfur-containing compounds including RSSR (disulfides), RSH (thiols), S8 (elemental sulphur), alkynyl thioethers, RSCN, AgSCF3, K2S, Na2S, dithiane, RSCl, NFSI, RNCS, EtOCS2K, thiocarbamate, RSONH2, thiourea, sulfoxide, RSO2N3, CS2, RSO2NH2, RSO2NHNH2, RSO2Cl, RSO2Oar, RSO2SR', DABCO·(SO2)2, Na2S2O5, K2S2O5, RSO2H, RSO2Na and related compounds are discussed. Diverse mechanisms such as radical, electrophilic/nucleophilic addition, rearrangement, C-C bond cleavage, and CuAAC are discussed. The content is organized by substrates and reactivity patterns. We hope it will help in future research in this area.

Gold(I)-Catalyzed Intramolecular 7-endo-dig Cyclization of Triene-Yne Systems: New Access towards Azulenothiophenes

We report the direct synthesis of new azulene derivatives through gold-catalyzed cyclization reactions. A five-membered ring as backbone in the applied triene-yne substrates turned out to be crucial to induce the 7-endo-dig cyclization mode necessary to trigger azulene formation. The obtained targets are of high interest due to their potential applications in different fields, like organic materials, medicine or cosmetics. UV/Vis spectra and cyclic voltammetry were measured, based on these the electronic properties were determined. Short two or three step sequences towards the applied starting materials make this approach synthetically highly attractive.

Synthesis, Structural, and Optical Properties of Azuleno[1,2-c]pyran-1-ones: Bro̷nsted Acid-Mediated Cyclization of 2-Azulenylalkynes

Pyrones and their aromatic ring-fused derivatives have gained significant attention due to their diverse biological activities and potential as foundational frameworks for advanced materials. In this paper, we describe a proficient approach for the preparation of azuleno[1,2-c]pyran-1-ones, which are difficult to produce by using conventional methods. The synthesis was achieved through Bro̷nsted acid-mediated cyclization of 2-azulenylalkynes. The structural and optical properties of azuleno[1,2-c]pyran-1-ones were characterized by single-crystal X-ray analysis, NMR, UV/vis, and fluorescence spectroscopies. Under acidic conditions, these compounds displayed notable spectral alterations and emission, distinct from their spectra in neutral medium. These results suggest that azuleno[1,2-c]pyran-1-ones hold great potential for applications in organic electronic materials and fluorescent pH sensors.

Iodine-catalyzed three-component annulation: access to highly fluorescent trisubstituted thiophenes

An efficient synthesis of highly fluorescent trisubstituted thiophenes was achieved via iodine-catalyzed, base-promoted annulation employing elemental sulfur as a sulfur source. The compounds exhibit excellent photophysical properties like solid-state fluorescence, high quantum yield and solvatochromism. As these thiophene derivatives have potential application in the development of optoelectronic devices, gram-scale synthesis of the desired heterocycles was demonstrated.

Synthesis of Azuleno[2,1-b]quinolones and Quinolines via Brønsted Acid-Catalyzed Cyclization of 2-Arylaminoazulenes

Quinolone and quinoline derivatives are frequently found as substructures in pharmaceutically active compounds. In this paper, we describe a procedure for the synthesis of azuleno[2,1-b]quinolones and quinolines from 2-arylaminoazulene derivatives, which are readily prepared via the aromatic nucleophilic substitution reaction of a 2-chloroazulene derivative with several arylamines. The synthesis of azuleno[2,1-b]quinolones was established by the Brønsted acid-catalyzed intramolecular cyclization of 2-arylaminoazulene derivatives bearing two ester groups at the five-membered ring. The halogenative aromatization of azuleno[2,1-b]quinolones with POCl3 yielded azuleno[2,1-b]quinolines with a chlorine substituent at the pyridine moiety. The aromatic nucleophilic substitution reaction of azuleno[2,1-b]quinolines bearing chlorine substituent with secondary amines was also investigated to afford the aminoquinoline derivatives. These synthetic methodologies reported in this paper should be valuable in the development of new pharmaceuticals based on the azulene skeleton.

Three-component selective synthesis of phenothiazines and bis-phenothiazines under metal-free conditions

An iodine-containing reagent promoted three-component method for the selective synthesis of phenothiazines and bis-phenothiazines has been developed. The present protocol starts from simple and easily available cyclohexanones, elemental sulfur, and inorganic ammonium salts, selectively producing phenothiazines and bis-phenothiazines in satisfactory yields under aerobic conditions. This method has the advantages of simple and readily available starting materials and metal-free conditions, affording a facile and practical approach for the preparation of phenothiazines and bis-phenothiazines.

Persistent azulene α-carbocations: synthesis from aldehydes, spectroscopic and crystallographic properties

The non-benzenoid aromatic system azulene is sufficiently nucleophilic at C1 that it can react with a protonated aldehyde to form an α-azulenyl alcohol. This in turn may be protonated and undergo loss of water to give an azulene α-carbocation. We report the isolation of such azulenyl cations as salts with non-coordinating anions. The salts have been characterised by NMR, UV/Vis absorption and (in certain cases) X-ray crystallography. Reduction of representative salts to afford azulenyl(aryl) methylenes has been demonstrated.

Access to Thienopyridine and Thienoquinoline Derivatives via Site-Selective C-H Bond Functionalization and Annulation

To develop of an effective synthetic methodology for biologically relevant thienopyridines, a concise and efficient protocol is described for the synthesis of a series of substituted thienopyridine and thienoquinoline derivatives with high selectivity using EtOCS₂K as the sulfur source. The reaction proceeds via metal-free, site-selective C-H bond thiolation and cyclization of the alkynylpyridine and alkynylquinoline substrates.

Synthesis, Reactivity, and Properties of Benz[a]azulenes via the [8 + 2] Cycloaddition of 2H-Cyclohepta[b]furan-2-ones with an Enamine

Starting with the reaction of 2H-cyclohepta[b]furan-2-ones with an enamine, which was prepared from 4-tert-butylcyclohexanone and pyrrolidine, benz[a]azulenes having both formyl and tert-butyl groups were obtained in the three-step sequence. Subsequently, both the formyl and tert-butyl groups were eliminated by heating the benz[a]azulene derivatives in 100% H₃PO₄ to give benz[a]azulenes without these substituents in high yields. In terms of product yield, this method is the best one ever reported for the synthesis of the parent benz[a]azulene so far. The conversion of the benz[a]azulene derivatives with a formyl group into cyclohept[a]acenaphthylen-3-one derivatives was also investigated via Knoevenagel condensation with dimethyl malonate, followed by Brønsted acid-mediated intramolecular cyclization. The structural features including the bond alternation in the benz[a]azulene derivatives were revealed by NMR studies, NICS calculations, and a single-crystal X-ray structural analysis. The optical and electrochemical properties of a series of benz[a]azulene derivatives were evaluated by UV/Vis, fluorescence spectroscopy, and voltammetry experiments. As a result, we found that some benz[a]azulene derivatives showed remarkable luminescence in acidic media. In addition, the benz[a]azulene derivatives with the electron-withdrawing group and cyclohept[a]acenaphthylen-3-one derivative displayed good reversibility in the spectral changes under the electrochemical redox conditions.

Synthesis of Azulene Derivatives from 2H-Cyclohepta[b]furan-2-ones as Starting Materials: Their Reactivity and Properties

A variety of synthetic methods have been developed for azulene derivatives due to their potential applications in pharmaceuticals and organic materials. Particularly, 2H-cyclohepta[b]furan-2-one and its derivatives have been frequently used as promising precursors for the synthesis of azulenes. In this review, we describe the development of the synthesis of azulenes by the reaction of 2H-cyclohepta[b]furan-2-ones with olefins, active methylenes, enamines, and silyl enol ethers as well as their reactivity and properties.

Azulene-Substituted Donor-Acceptor Polymethines and 1,6'-Bi-, 1,6';3,6''-Ter-, and Quinqueazulenes via Zincke Salts: Synthesis, and Structural, Optical, and Electrochemical Properties

Azulene-substituted donor-acceptor polymethines, bi-, ter-, and quinqueazulenes composed of the 1,6'-biazulene unit have been successfully prepared from corresponding Zincke salts. The synthesis of polymethines through the reaction of Zincke salts with several amines, followed by a Knoevenagel reaction with malononitrile, was accomplished in moderate to high yields (40-92 %). Meanwhile, the reaction of Zincke salts with secondary amines and the subsequent sequential condensation-cyclization with cyclopentadienide ions, so-called Ziegler-Hafner method, produced the corresponding 1,6'-biazulenes, 1,6';3,6''-terazulenes, and quinqueazulene, respectively. The structural, optical, and electrochemical properties of the azulene-substituted donor-acceptor polymethines, bi-, ter-, and quinqueazulenes were revealed by single-crystal X-ray structure analysis, UV/vis spectroscopy, voltammetry analysis, spectroelectrochemistry, and theoretical calculations. These results suggested that the substituents on the azulene ring and their substitution positions directly affect their reactivities, optical and electrochemical properties.

Azulene-A Bright Core for Sensing and Imaging

Azulene is a hydrocarbon isomer of naphthalene known for its unusual colour and fluorescence properties. Through the harnessing of these properties, the literature has been enriched with a series of chemical sensors and dosimeters with distinct colorimetric and fluorescence responses. This review focuses specifically on the latter of these phenomena. The review is subdivided into two sections. Section one discusses turn-on fluorescent sensors employing azulene, for which the literature is dominated by examples of the unusual phenomenon of azulene protonation-dependent fluorescence. Section two focuses on fluorescent azulenes that have been used in the context of biological sensing and imaging. To aid the reader, the azulene skeleton is highlighted in blue in each compound.

Development of Heterocycle-Substituted and Fused Azulenes in the Last Decade (2010-2020)

Azulene derivatives with heterocyclic moieties in the molecule have been synthesized for applications in materials science by taking advantage of their unique properties. These derivatives have been prepared by various methods, involving electrophilic substitution, condensation, cyclization, and transition metal-catalyzed cross-coupling reactions. Herein, we present the development of the synthetic methods, reactivities, and physical properties for the heterocycle-substituted and heterocycle-fused azulenes reported in the last decade.

Recent Advances in Sulfur-Containing Heterocycle Formation via Direct C–H Sulfuration with Elemental Sulfur

The synthesis of sulfur heterocycles via the construction of C–S bonds has received considerable attention due to their biological value and extensive pharmaceutical application. While diverse sulfurating agents have been developed over the past few decades, in this regard, elemental sulfur, with advantages of low toxicity, odorless nature and chemical stability, has great potential for the construction of diverse sulfur heterocycles through its direct incorporation into the target molecules in a concise way. Direct functionalization of inert C–H bonds can shorten the number of reaction steps and minimize the amount of waste formed. Hence, heteroannulations via direct C–H sulfuration is considered to be an attractive strategy for the synthesis of sulfur heterocycles. In the last few years, a vast array of concise systems have been reported for the synthesis of some valuable sulfur heterocycles such as thiophenes, thienoindoles, thienothiazoles, thiazoles, benzothiazoles, and thiadiazoles through direct C–H sulfuration/annulations with elemental sulfur. These are discussed in detail in this review.

1 Introduction

2 Thiophenes

3 Thienoindoles

4 Thienothiazoles

5 Other Fused Thiophenes

6 Thiazoles

7 Benzothiazoles

8 Thiadiazoles

9 Others

10 Summary and Outlook

Synthesis of thiophene-fused heptalenes by cycloaddition of azulenothiophenes with dimethyl acetylenedicarboxylate

Heptalene has a fused structure of two cycloheptatrienes which is one of the non-aromatic bicyclic molecules with a 12π-electronic structure. We report herein the synthesis of thiophene-fused heptalene derivatives from the corresponding azulenothiophenes via cycloaddition reaction with dimethyl acetylenedicarboxylate. Their structure was clarified by single-crystal X-ray structural analysis. The electronic properties of the thiophene-fused heptalenes obtained by this study were characterized by UV/Vis and fluorescence spectroscopy measurements. The electrochemical features of these derivatives were also examined by voltammetry and spectroelectrochemical experiments.

Direct synthesis of 2-arylazulenes by [8+2] cycloaddition of 2H-cyclohepta[b]furan-2-ones with silyl enol ethers

We developed a procedure for the direct synthesis of 2-arylazulenes, which were obtained in moderate to excellent yields, by [8+2] cycloaddition of 2H-cyclohepta[b]furan-2-ones with aryl-substituted silyl enol ethers. The structures of some 2-arylazulenes were clarified by single-crystal X-ray analysis. The 2-phenylazulene derivatives obtained by this study showed noticeable fluorescence in acidic media.

Synthesis of phthalimides cross-conjugated with an azulene ring, and their structural, optical and electrochemical properties

The preparation of phthalimides cross-conjugated with an azulene ring was established by a one-pot Diels-Alder reaction of the corresponding 2-aminofuran derivatives with several maleimides, without the isolation of the intermediately formed [4 + 2] cycloadducts. The structure, optical and electrochemical properties of the novel phthalimide derivatives were clarified by single-crystal X-ray analysis, UV/Vis and fluorescence spectra, spectroelectrochemistry and voltammetry experiments, and theoretical calculations. These results indicated that the substituents on the azulene ring greatly affect the optical and electrochemical properties of the molecules.

Three-component synthesis of 1,4-benzothiazines via iodide-catalyzed aerobic C–H sulfuration with elemental sulfur

Five to Six: Beyond the well-established thiazole formation from elemental sulfur, this method provides the first access to the corresponding six-membered N,S-heterocyclic products via direct functionalization of multiple C–H bonds.

Three-component reaction of azulene, aryl glyoxal and 1,3-dicarbonyl compound for the synthesis of various azulene derivatives

A three-component reaction of an azulene, an aryl glyoxal and a 1,3-dicarbonyl compound has been elaborated to access a series of azulene derivatives. Some of these azulene-containing adducts were further subjected to post-MCR transformations to assemble azulene–heterocycle conjugates.

Three-component reaction of azulene, aryl glyoxal and 1,3-dicarbonyl compound and subsequent post-transformations provide access to three distinct types of azulene derivatives.