Regioselective and Chemodivergent Synthesis of Azulenolactones and Azulenolactams from Rhodium(III)‐Catalyzed Reactions of Azulenecarboxamides with Sulfoxonium Ylides

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.

Base-controlled regio-divergent C-H bond functionalization

The choice of appropriate directing groups, catalysts, ligands, bases, and solvents to give different regioisomers from the same precursors can improve the efficiency and atom economy in synthetic organic chemistry, while minimizing costs and expanding know chemical space. Base-controlled regio-divergent C-H bond functionalization is an important control strategy in organic synthesis, with the advantages of being operationally simple, and using cheap and available bases with low-toxicity that are easily separable. This highlight is classified into three sections: base-controlled, base-controlled transition metal-catalysed and base-controlled photocatalytic regio-divergent C-H bond functionalization. By summarizing the C-H bond activation model, the mechanisms underlying the regio-divergence and the influencing factors, this highlight aims to deepen the understanding of selective C-H bond functionalization at a molecular level. We expect that this highlight article will provide valuable information for understanding the mechanism of C-H bond functionalization, whilst providing a reference for developing green, efficient C-H bond transformation strategies.

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.

α-Carbonyl sulfoxonium ylides in transition metal-catalyzed C-H activation: a safe carbene precursor and a weak directing group

Transition metal-catalyzed cross-coupling of sp² C-H bonds with diazo compounds via carbene migratory insertion represents an efficient strategy for the construction of C-C and C-heteroatom bonds in organic synthesis. Despite the popularity of diazo compounds as coupling partners in C-H activation, they pose serious safety and stability issues due to potential exothermic reactions linked with the release of N₂ gas. However, compared with diazo compounds, sulfoxonium ylides are generally crystalline solids, more stable, widely used in industrial scales, and easier/safer to prepare. Therefore, recent years have witnessed an upsurge in employing α-carbonyl sulfoxonium ylides as an alternative carbene surrogate in transition metal-catalyzed C-H activation. Unlike diazo compounds, α-carbonyl sulfoxonium ylides contain inherent potential to serve as a coupling partner as well as a weak directing group. This review will summarize the progress made in both categories of reactions.

Iridium(III)-Catalyzed B(4)-Acylmethylation and B(3,5)-Diacylmethylation from o-Carboranes and Sulfoxonium Ylides

An iridium(III)-catalyzed regioselective acylmethylation of the cage B(4)-H bond in o-carborane acids with sulfoxonium ylides is demonstrated through B(4)-H activation in ethanol under very mild conditions, affording a number of B(4)-acylmethylated o-carboranes. Additionally, the selective sequential B(4)- and B(6)-acylmethylation reactions finally gave B(3,5)-diacylmethylated o-carboranes in one pot.

Photo-Thermo-Mechanochemical Approach to Synthesize Quinolines via Addition/Cyclization of Sulfoxonium Ylides with 2-Vinylanilines Catalyzed by Iron(II) Phthalocyanine

A novel photo-thermo-mechanochemical approach to assembling quinolines catalyzed by iron(II) phthalocyanine has been realized for the first time. This transformation features a cost-efficient catalytic system and operational simplicity, is free of solvent, and shows good substrate tolerance, providing a green alternative to existing thermal approaches. Mechanistic experiments demonstrate that the in-situ-formed secondary amine may be the key intermediate for the further cyclization/aromatization process.

Ruthenium(ii)-catalyzed synthesis of CF3-isoquinolinones via C–H activation/annulation of benzoic acids and CF3-imidoyl sulfoxonium ylides

The synthesis of 3-trifluoromethylisoquinolinones by a ruthenium(ii)-catalyzed C–H activation/annulation reaction of benzoic acids and CF3-imidoyl sulfoxonium ylides has been achieved.

Annulation of CF3-Imidoyl Sulfoxonium Ylides with 1,3-Dicarbonyl Compounds: Access to 1,2,3-Trisubstituted 5-Trifluoromethylpyrroles

A lithium-bromide-promoted nucleophilic substitution/annulation cascade reaction between CF₃-imidoyl sulfoxonium ylides and 1,3-dicarbonyl compounds has been established, and the corresponding 1,2,3-trisubstituted 5-trifluoromethylpyrroles have been obtained in 27-78% yield. This reaction features a broad substrate scope and generates dimethyl sulfoxide and H₂O as byproducts.

Synthesis and Halochromic Properties of 1,2,6-Tri- and 1,2,3,6-Tetra-aryl Azulenes

A series of novel 2,6-functionalized azulene molecules Azu1-3 with varied fluorene substituents at the 1- and 3-positions of azulene as well as at the 5'-position of 2-thiophene group were synthesized. Their electronic absorption and emission spectra at neutral and protonated states were examined. It was found that after functionalization with fluorenyl groups, Azu1-3 exhibited absorption maxima at 445, 451 to 468 nm, respectively. In contrast, their corresponding protonated species showed much redshifted absorption maxima at 560, 582 to 643 nm, respectively, mainly due to the extension of conjugation length and the large dipole moment along the C_2v axis of 2,6-substituted azulene molecules. Azu1-3 are non-fluorescent in their neutral forms, but became emissive in their protonated states. Analysis of absorption and emission spectra shows that substitution of the 1- or 3-position of azulene led to decrease in response to trifluoroacetic acid.

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.

Hydroxyl-Directed Ruthenium-Catalyzed peri-Selective C-H Acylmethylation and Annulation of Naphthols with Sulfoxonium Ylides

Herein, we report a highly efficient ruthenium-catalyzed peri-selective C(sp²)-H acylmethylation of 1-naphthols with α-carbonyl sulfoxonium ylides by utilizing hydroxyl as a weakly coordinating directing group. This new method imparts good reactivity, excellent chemo- and regioselectivity, and broad functional group tolerance and involves mild reaction conditions. The C-H acylmethylated products can be readily cyclized into fluorescent annulated pyrans by a one-pot process.

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.