Synthesis, Structural, and Optical Properties of Azuleno[1,2-c]pyran-1-ones: BroÌ·nsted 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 BroÌ·nsted 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.

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.

Construction of a 2,2'-biazulene framework via Brønsted acid-promoted annulation of 2,3-di(1-azulenyl)benzofurans.

Dibenzofurans featuring a 2,2'-biazulene framework were prepared in good yields by Brønsted acid-promoted annulation of 2,3-di(1-azulenyl)benzofurans in 100% H3PO4. NMR, UV-Vis, and fluorescence spectroscopies were used to investigate the structural and optical properties of the products prepared. Remarkably, the annulated products exhibited fluorescence, with the longest wavelength of azulene derivatives reported to date, which extended into the near-infrared region under acidic conditions.

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.

Molecular Transformation to Pyrroles, Pentafulvenes, and Pyrrolopyridines by [2+2] Cycloaddition of Propargylamines with Tetracyanoethylene.

In this paper, we describe an efficient and atom-economical synthesis of highly functionalized pyrroles, pentafulvenes, and pyrrolopyridines by [2+2] cycloaddition-retroelectrocyclization of N-substituted propargylamines with tetracyanoethylene, followed by the treatment of the resulting tetracyanobutadiene derivatives with silica gel. In this reaction, silica gel plays an important role to promote the intramolecular cyclization to afford the heterocyclic products from the tetracyanobutadiene intermediates. The products were obtained selectively depending on the substituent on the nitrogen atom of the starting propargylamines.

Preparation of a large-sized highly flexible carbon nanohoop.

The preparation of large-sized carbon nanohoops 1 and 2 was acheieved by Pt-mediated macrocyclization of the diborylated hexa-peri-hexabenzocoronene (HBC) derivative containing four mesityl substituents at the periphery. The temperature-dependent NMR measurements revealed the highly flexible nature of the carbon nanohoop 1. Structural features of 1 and 2 were examined by comparison of photophysical and electrochemical measurements with those of the reference non-strained HBC derivative. Theoretical calculations clarified the possible molecular structure of the carbon nanohoops 1 and 2 and the structures involved in the dynamics.

Synthesis of 6-Amino- and 6-Arylazoazulenes via Nucleophilic Aromatic Substitution and Their Reactivity and Properties.

The nucleophilic aromatic substitution (SNAr) reaction of diethyl 6-bromoazulene-1,3-dicarboxylate (1) with a variety of amines afforded the corresponding 6-aminoazulene derivatives 2a-2j in good-to-excellent yields. 6-Aminoazulene derivatives 3a-3f without the 1,3-diethoxycarbonyl functions were obtained by the deesterification of 2a-2f with 100% H3PO4. The reactivity of 6-aminoazulenes toward the bromination, SNAr, and palladium-catalyzed cross-coupling reactions was also clarified. 6-Arylazoazulenes 13a-13c were also prepared via the SNAr reaction of 1 with arylhydrazines, followed by oxidation with Pb(OAc)4 in the presence of N2H4. The structural, optical, and electrochemical properties of the 6-amino- and 6-arylazoazulenes were revealed by single-crystal X-ray structure analysis, UV/vis spectroscopy, voltammetry analysis, spectroelectrochemistry, and theoretical calculations.

Synthesis of 2-Methyl-1-azulenyl Tetracyanobutadienes and Dicyanoquinodimethanes: Substituent Effect of 2-Methyl Moiety on the Azulene Ring toward the Optical and Electrochemical Properties.

We describe the comparative study of optical and electrochemical properties of tetracyanobutadienes (TCBDs) and dicyanoquinodimethanes (DCNQs) with a 2-methyl-1-azulenyl group and their derivatives with a 1-azulenyl substituent examined under the same conditions. TCBDs and DCNQs with a 2-methyl-1-azulenyl substituent have been prepared by the Sonogashira-Hagihara alkynylation of the 2-methyl-1-iodoazulene with arylalkyne derivatives, followed by the formal [2+2] cycloaddition-retroelectrocyclization (CA-RE) reaction with tetracyanoethylene and 7,7,8,8-tetracyanoquinodimethane. The optical properties of the TCBDs and DCNQs with a 2-methyl-1-azulenyl group were investigated through the comparison with those of TCBDs and DCNQs with a 1-azulenyl substituent by employing the UV/vis spectroscopy and theoretical calculations. The electrochemical properties of the TCBD and DCNQ derivatives were also examined by cyclic voltammetry and differential pulse voltammetry experiments, which elucidated their multistep redox properties. Furthermore, noticeable spectral changes of these chromophores were identified by the spectroelectrochemical measurements.

Synthesis of azulene-substituted benzofurans and isocoumarins via intramolecular cyclization of 1-ethynylazulenes, and their structural and optical properties.

The preparation of azulene-substituted benzofurans and isocoumarins was established by two types of intramolecular cyclization reaction of 1-ethynylazulenes. 2-(1-Azulenyl)- and 2,3-bis(1-azulenyl)benzofurans were prepared by the palladium-catalyzed cross-coupling reaction of 1-iodoazulenes with 2-ethynylphenol and that of 1-ethynylazulenes with 2-iodophenol under Sonogashira-Hagihara reaction conditions following the intramolecular nucleophilic addition of the oxygen nucleophile to the presumed 1-arylethynylazulenes. In contrast, 1-(phenylethynyl)azulenes bearing an o-methoxycarbonyl function on the substituted phenyl moiety exhibited intramolecular cyclization either in the presence of trifluoroacetic acid or N-iodosuccinimide (NIS) to afford azulene-substituted isocoumarins and 4-iodoisocoumarins, and the structures were clarified by single-crystal X-ray analysis. The optical properties of these compounds were also investigated by UV/vis spectroscopy and theoretical calculations.

Synthesis of 2-amino- and 2-arylazoazulenes via nucleophilic aromatic substitution of 2-chloroazulenes with amines and arylhydrazines.

The SNAr reaction of 2-chloroazulene derivative 1 with ethoxycarbonyl groups at the 1,3-positions of the azulene ring with several amines afforded the corresponding 2-aminoazulenes 3-9 in excellent yields. 2-Chloroazulene (2) without the electron-withdrawing groups reacted with highly nucleophilic cyclic amines (i.e., morpholine, piperidine and pyrrolidine) under the high-temperature conditions in a sealed tube to produce the corresponding 2-aminoazulenes 10-12 in good yields. 2-Aminoazulenes 10-14 without the electron-withdrawing groups were also obtained in good yields by the treatment of compounds 3-7 with 100% H3PO4, but in the cases of the reaction of 8 and 9 with a secondary amine function, the decomposition of the products resulted. The synthesis of 2-arylazoazulenes 15-18 was also established via the SNAr reaction of 1 with arylhydrazines. The optical and electrochemical properties of the 2-arylazoazulene derivatives were examined by UV/Vis spectroscopy, theoretical calculations and voltammetric experiments.

Synthesis of 2-Aminofurans by Sequential [2+2] Cycloaddition-Nucleophilic Addition of 2-Propyn-1-ols with Tetracyanoethylene and Amine-Induced Transformation into 6-Aminopentafulvenes.

Synthesis of 2-aminofuran derivatives with an azulene or N,N-dimethylanilino substituent was established by the formal [2+2] cycloaddition-retroelectrocyclization of 3-(1-azulenyl or N,N-dimethylanilino)-2-propyn-1-ols with tetracyanoethylene, followed by intramolecular nucleophilic addition to the initially formed tetracyanobutadiene moiety of the internal hydroxyl group that come from 2-propyn-1-ol. The reaction proceeds under mild conditions with short reaction period. The products of the reaction are readily available through a simple purification procedure. 2-Aminofuran derivatives obtained by this reaction could be converted into 6-aminofulvene derivatives upon reaction with various amines. The structures of 2-aminofuran and 6-aminopentafulvene with a N,N-dimethylanilino substituent were confirmed by single-crystal X-ray structural analysis.

Synthesis of 2-Azulenyltetrathiafulvalenes by Palladium-Catalyzed Direct Arylation of 2-Chloroazulenes with Tetrathiafulvalene and Their Optical and Electrochemical Properties.

Tetrathiafulvalene (TTF) derivatives with 2-azulenyl substituents 5-11 were prepared by the palladium-catalyzed direct arylation reaction of 2-chloroazulenes with TTF in good yield. Photophysical properties of these compounds were investigated by UV-vis spectroscopy and theoretical calculations. Redox behavior of the novel azulene-substituted TTFs was examined by using cyclic voltammetry and differential pulse voltammetry, which revealed their multistep electrochemical oxidation and/or reduction properties. Moreover, these TTF derivatives showed significant spectral change in the visible region under the redox conditions.

Preparation of a cyclic polyphenylene array for a zigzag-type carbon nanotube segment.

Preparation of cyclic polyphenylene array 2, which corresponds to a complete carbon array of a zigzag-type CNT segment with (18,0)-structure, has been established by a Diels-Alder reaction of cyclic biphenylylene-acetylene derivative 1 with tetraphenylcyclopentadienone. The reaction of 2 with excess FeCl3 realized a presumed cyclodehydrogenation reaction and elimination of the alkyl chains that were introduced as a measure to counter the low solubility problem, but this resulted in the formation of a complicated mixture that included the mass region of a presumed zigzag-type CNT segment with (18,0)-structure. The rather efficient blue emission of cyclic compounds 1 and 2 was discussed utilizing fluorescence (FL) quantum efficiencies (Φ(FL)) and lifetimes (τ(FL)) in their crystalline state along with those in dichloromethane solution. Thermal analyses of these compounds revealed their characteristic phase transition behavior. The synthesis of a novel cyclic polyphenylene array by utilizing a Diels-Alder reaction of cyclic phenylene-acetylene compounds with tetraphenylcyclopentadienone should afford an attractive pathway to a novel belt-shaped CNT segment.