Expansion of Azulenes as Nonbenzenoid Aromatic Compounds for C–H Activation: Rhodium- and Iridium-Catalyzed Oxidative Cyclization of Azulene Carboxylic Acids with Alkynes for the Synthesis of Azulenolactones and Benzoazulenes

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.

Recent advances in carboxyl-directed dimerizations and cascade annulations via C-H activations

C-H functionalization provides an efficient route to construct complex organic molecules. The introduction of directing groups enhances the site-selectivity of the reaction. Carboxyl as a directing group can be easily transformed into other functional groups afterwards. Due to its good reactivity, it can undergo cascade annulation reactions to build valuable heterocycle skeletons in one pot. Moreover, carboxyl can easily be removed via decarboxylation, which allows it to serve as a unique traceless directing group in C-H functionalization. These characteristics make carboxyl a promising directing group, which is superior to nitrogen-containing compounds with strong coordination ability to a certain extent. This feature article reviews the applications of carboxyl as a classical directing group and a unique traceless-directing group in cascade annulation reactions to access diverse carbocycles and heterocycles.

Palladium-Catalyzed Double Decarboxylative [3 + 2] Annulation of Naphthalic Anhydrides with Internal Alkynes

A palladium-catalyzed [3 + 2] annulation of naphthalic anhydrides with internal alkynes has been developed. The present protocol offers an efficient and convenient route to access a series of 1,2-disubstituted acenaphthylenes with excellent functional group compatibility. The reaction is proposed to proceed through a double decarboxylation sequence. The reported synthetic protocols can be extended to napthalene- and perylenedicarboximide-containing substrates. The molecular structures, photophysical properties, and frontier molecular orbitals of the obtained adducts were investigated by X-ray crystallography, UV-vis and fluorescence spectroscopy, and DFT calculations.

Efficient Synthesis and Structural Analysis of Chiral 4,4'-Biazulene

4,4'-Biazulene is a potentially attractive key component of an axially chiral biaryl compound, however, its structure and properties have not been clarified owing to the lack of its efficient synthesis. We report a breakthrough in the reliable synthesis of 4,4'-biazulene, which is achieved by the access to azulen-4-ylboronic acid pinacol ester and 4-iodoazulene as novel key synthetic intermediates for the Suzuki-Miyaura cross-coupling reaction. The X-ray crystallographic analysis of 4,4'-biazulene confirmed its axial chirality. The enantiomers of 4,4'-biazulene were successfully resolved by HPLC on the chiral stationary phase column. The kinetic experiments and DFT calculations indicate that the racemization energy barrier of 4,4'-biazulene is comparable to that of 1,1'-binaphthyl.

Selective ring expansion and C-H functionalization of azulenes

We report a transition metal-catalyzed ring expansion of azulene that can be contrasted with C-H functionalization. This study represents the first example of the successful ring expansion of azulene using Cu(hfacac)2 (hfacac: hexafluoroacetylacetonate) with a diazo reagent. This result is notable for extending the Buchner reaction, previously limited to benzenoid aromatics, to nonbenzenoid compounds. The chemoselectivity of the reaction can be directed towards C-H functionalization by substituting the Cu catalyst with AgOTf. This approach does not require the addition of phosphine, NHC, or related ligands, and prefunctionalization of azulenes is unnecessary. Furthermore, the method exhibits excellent functional group tolerance, allowing for the synthesis of a wide range of 6,7-bicyclic compounds and C-H functionalized azulenes. We also present a theoretical study that explains the experimental observations, explaining why copper afford the ring expansion product while silver forms the C-H alkylation product.

Rhodaelectro-Catalyzed Decarboxylative Cross-Dehydrogenative Coupling of Indole-3-carboxylic Acids and Olefins via Weakly Coordinating Carboxyl Groups

A rhodaelectro-catalyzed C2-H selectively decarboxylative alkenylation of 3-carboxy-1H-indoles employing electricity as the traceless terminal oxidant has been accomplished. The weakly coordinating carboxyl group serves as the traceless directing groups. External oxidant-free in an undivided cell with constant current in aqueous solution ensures the decarboxylative C-H alkenylation to be viable and sustainable.

Synthesis and Properties of Carbo- and Heterocyclic Benz[a]azulenes

A new and convenient synthesis of aryl-substituted naphtho[2,1-a]azulenes by the combination of Suzuki-Miyaura, Sonogashira, and cycloisomerization reactions is reported. The methodology was applied to the synthesis of hitherto unknown azuleno[1,2-h]quinolines, cyclohepta[1,2]indeno[4,5-b]thiophenes, and cyclohepta[1,2]indeno[4,5-c]thiophenes. The impact of different fused-heterocyclic rings on the photophysical and electrochemical properties of these azulene derivatives was studied by experimental and theoretical methods and hence provides a rationale for the preparation of novel azulene derivatives with improved properties for application as organic materials.

Tandem C-H Annulation Reaction of Benzaldehydes and Aminobenzoic Acids with Two Equivalents of Alkyne toward Isocoumarin-Conjugated Isoquinolinium Salts: A Family of Organic Luminophores

A novel rhodium-catalyzed tandem C-H annulation of commercially available benzaldehydes and aminobenzoic acids with 2 equiv of alkyne is reported for the construction of isocoumarin-conjugated isoquinolinium salts that demonstrate diverse outstanding photoactivity. Depending on the substituents in the isoquinolinium moiety, they display either highly efficient fluorescence (up to 99% of quantum yield) or strong fluorescence quenching, which is provided by the transfer of the HOMO from the isoquinolinium to the isocoumarin moiety. Importantly, the functional groups in the benzaldehyde coupling partner also strongly affect the reaction selectivity, shifting the pathway to the formation of the photoinactive isocoumarin-substituted indenone imines and indenyl amines. Selective formation of the latter can be achieved by using a reduced amount of the oxidizing additive.

Ru(ii)-catalysed oxidative (4 + 2) annulation of chromene and coumarin carboxylic acids with alkynes/propargylic alcohols: isolation of Ru(0) complexes

Ru(ii)-catalysed oxidative (4 + 2) annulation of chromene and coumarin carboxylic acids with alkynes affords pyrano-chromones via vinylic C–H bond activation; use of methyl-tethered propargylic alcohols instead of alkynes gives Ru(0) complexes.

Copper-Catalyzed 3-Positional Amination of 2-Azulenols with O-Benzoylhydroxylamines

A copper-catalyzed ortho-selective amination of 2-azulenols with O-benzoylhydroxylamines (RR'N-OBz) to synthesize ortho-aminoazulenols is reported. A wide range of functional groups on amines are compatible, furnishing the corresponding amino-azulene derivatives in moderate to good yields. The further synthetic elaboration using 3-amino-2-azulenols as starting materials is demonstrated.

Doyle-Kirmse reaction using 3,3-difluoroallyl sulfide and N-sulfonyl-1,2,3-triazole: an efficient access to gem-difluoroallylated multifunctional quaternary carbon

A Doyle-Kirmse reaction of N-sulfonyl-1,2,3-triazole with 3,3-difluoroallyl sulfide through a Rh(ii)-catalyzed [2,3]-sigmatropic rearrangement has been developed, which provides an efficient access to multifunctional quaternary centers containing aryl, imino, thio, and brominated gem-difluoroallyl groups. The reaction features broad substrate scope with moderate to excellent yields. The applicability of the method is confirmed by gram-scale synthesis and further transformations.

Rhodium(III)-Catalyzed Sequential C-H Activation and Cyclization from N-Methoxyarylamides and 3-Diazooxindoles for the Synthesis of Isochromenoindolones

An efficient synthetic method for structurally various isochromenoindolones has been demonstrated through Rh(III)-catalyzed C-H activation followed by a cyclization reaction of N-methoxyarylamides with 3-diazooxindoles. The sequential reaction involves the streamlined formation of C-C and C-O bonds in one pot. The present method provides a broad range of isochromenoindolones as a new privileged scaffold in moderate to good yields with the release of methoxyamine and molecular nitrogen and has the benefits of a broad substrate scope and good functional group tolerance.

C4-aldehyde of guaiazulene: synthesis and derivatisation

Guaiazulene is an alkyl-substituted azulene available from natural sources and is a much lower cost starting material for the synthesis of azulene derivatives than azulene itself. Here we report an approach for the selective functionalisation of guaiazulene which takes advantage of the acidity of the protons on the guaiazulene C4 methyl group. The aldehyde produced by this approach constitutes a building block for the construction of azulenes substituted on the seven-membered ring. Derivatives of this aldehyde synthesised by alkenylation, reduction and condensation are reported, and the halochromic properties of a subset of these derivatives have been studied.

Palladium-Catalyzed Q-Tube-Assisted Protocol for Synthesizing Diaza-dibenzo[a,e]azulene and Diaza-benzo[a]fluorene Derivatives via O2 Acid-Promoted Cross-Dehydrogenative Coupling

An appropriate and efficient Q-tube-assisted palladium-catalyzed strategy for the synthesis of novel, unparalleled diaza-dibenzo[a,e]azulene and diaza-benzo[a]fluorene derivatives has been sophisticated, which includes oxygen and AcOH-induced oxidative C(sp³)-C(sp²) cross-dehydrogenative coupling reactions of 1-amino-2-imino-4-arylpyridine-3-carbonitriles with benzocyclic ketones such as benzosuberone, tetralone, thiochromone, and chromone, respectively. This Q-tube gas purging kit assisted-protocol features safe due to easy pressing and sealing, a wide substrate scope, easy workup and purifying phases, and the use of O₂ as a benign oxidant, in addition to being scalable and having a high atom economy. The suggested mechanistic pathway includes a formal dehydrative step followed by palladium AcOH-induced CH(sp³)-CH(sp²) oxidative cross-coupling. In this study, X-ray crystallographic analysis has been used to authenticate the targeted products.

NIR-Absorbing π-Extended Azulene: Non-Alternant Isomer of Terrylene Bisimide

The first planar π-extended azulene that retains aromaticity of odd-membered rings was synthesized by [3+3] peri-annulation of two naphthalene imides at both long-edge sides of azulene. Using bromination and subsequent nucleophilic substitution by methoxide and morpholine, selective functionalization of the π-extended azulene was achieved. Whilst these new azulenes can be regarded as isomers of terrylene bisimide they exhibit entirely different properties, which include very narrow optical and electrochemical gaps. DFT, TD-DFT, as well as nucleus-independent chemical shift calculations were applied to explain the structural and functional properties of these new π scaffolds. Furthermore, X-ray crystallography confirmed the planarity of the reported π-scaffolds and aromaticity of their azulene moiety.