Efficient preparation of 2-azulenylboronate and Miyaura-Suzuki cross-coupling reaction with aryl bromides for easy access to poly(2-azulenyl)benzenes

Dancing with Azulene

It seems interesting to adopt the idea of dance in the context of the arrangement of molecular blocks in the building of molecular systems. Just as various dances can create various feelings, the nature and arrangement of molecular blocks in the generated molecular system can induce different properties. We consider obtaining such “dancing” systems in which the still little-known azulene moieties are involved. The dark blue nonbenzenoid aromatic azulene has one less axis of symmetry relative to the two axes of its isomer, i.e., the fully benzenoid naphthalene, acquiring valuable properties as a result that can be used successfully in technical applications. In a dancing system, the azulene moieties can be connected directly, or a more or less complex spacer can be inserted between them. Several azulene moieties can form a linear oligomer or a polymer and the involvement of azulene moieties in nonlinear molecules, such as crown ethers, calixarenes, azuliporphyrins, or azulenophane, is a relatively new and intensely studied topic. Some aspects are covered in this review, which are mainly related to obtaining the mentioned azulene compounds and less to their characterization or physico-chemical properties.

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.

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.

Azulene Functionalization by Iron-Mediated Addition to a Cyclohexadiene Scaffold

The functionalization of azulenes via reaction with cationic η5-iron carbonyl diene complexes under mild reaction conditions is demonstrated. A range of azulenes, including derivatives of naturally occurring guaiazulene, were investigated in reactions with three electrophilic iron complexes of varying electronic properties, affording the desired coupling products in 43-98% yield. The products were examined with UV-vis/fluorescence spectroscopy and showed interesting halochromic properties. Decomplexation and further derivatization of the products provide access to several different classes of 1-substituted azulenes, including a conjugated ketone and a fused tetracycle.

The Effect of Base and Nucleophile on the Nucleophilic Substitution of Methoxytropone Derivatives: An Easy Access to 4- and 5-Substituted Multifunctional Azulenes

The nucleophilic substitution on 3-substituted 2-methoxytropones to form azulenes is dependent on the nucleophile and base employed. With bulkier nucleophiles (ethyl/methyl cyanoacetate), the reaction proceeds with the abnormal nucleophilic substitution irrespective of the base and with smaller nucleophiles (malononitrile), the reaction follows base-dependent normal and abnormal nucleophilic substitution. Thus, the methodologies are developed to selectively obtain 4- and 5-substituted azulenes based on the nature of bases and nucleophiles employed.

Abnormal Nucleophilic Substitution on Methoxytropone Derivatives: Steric Strategy to Synthesize 5-Substituted Azulenes

Azulene is a non-alternant non-benzenoid aromatic system, and in turn, it possesses unusual photophysical properties. Azulene-based conjugated systems have received increasing interest in recent years as optoelectronic materials. Despite the routes available for the preparation of substituted azulene derivatives, there remain few methods that allow regioselective substitution on the seven-membered ring of azulenes due to the subtle reactivity difference among the various positions. This report explores the reactivity of substituted tropolones as the azulene precursors and also provides a new method to create 5-substituted azulenes. The reaction of cyanoacetate enolate with unsubstituted 2-methoxytropone affords azulene through the attack of the nucleophile on the C-2 center (normal pathway). We have observed that 3-substituted 2-methoxytropones undergo steric-guided nucleophilic addition at the C-7 center (abnormal pathway) to afford 5-substituted azulene derivatives. Based on this observation and DFT calculation, a new synthetic strategy is devised for the regioselective synthesis of 5-substituted multifunctional azulenes, which cannot be accessed by any other method.

Azulenes with aryl substituents bearing pentafluorosulfanyl groups: synthesis, spectroscopic and halochromic properties

Azulenes with SF₅-containing substituents gave significantly different spectroscopic responses to protonation depending on the regioisomer in question.

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.

Linear 6,6'-biazulenyl framework featuring isocyanide termini: synthesis, structure, redox behavior, complexation, and self-assembly on Au(111)

The key step in accessing the title species (5), the first nonbenzenoid diisocyanobiaryl, involved an unexpected homocoupling of a 6-bromoazulene derivative. The reversible 2e(-) reduction of 5 was addressed electrochemically and computationally. The shifts in energies of the S(0)→S(1) and S(0)→S(2) transitions for a series of related 6,6'-biazulenyl derivatives correlate with the e(-)-donating/-withdrawing strength of their 2,2'-substituents but follow opposite trends. Species 5 adsorbs end-on (η(1)) to the Au(111) surface via one of its -NC groups to form a 2-nm-thick film. In addition, bimetallic coordination of 5's -NC termini can be readily achieved.