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.

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.

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.

Synthesis of 2- and 6-thienylazulenes by palladium-catalyzed direct arylation of 2- and 6-haloazulenes with thiophene derivatives

Preparation of 2- and 6-thienylazulene derivatives was established by the palladium-catalyzed direct cross-coupling reaction of 2- and 6-haloazulenes with the corresponding thiophene derivatives in good yield.