Bright,Fluorescent Dyes Based on Imidazo[1,2‐a]pyridines that are Capable of Two‐Photon Absorption

A library of imidazo[1,2‐a]pyridines was synthesized by using the Gevorgyan method and their linear and non‐linear optical properties were studied. Derivatives that contained both electron‐donating and electron‐withdrawing groups at the 2 position were comprehensively investigated. Their emission quantum yield ranged between 0.2–0.7 and it was shown to depend on the substitution pattern, most notably that on the phenyl ring. Electron‐donating substituents improved the luminescence performance of these compounds, whereas electron‐withdrawing substituents led to a more erratic behavior. Substitution on the six‐membered ring had less effect on the fluorescence properties. Extension of the delocalization increased the luminescence quantum yield. A new quadrupolar system was designed that contained two imidazo[1,2‐a]pyridine units on its periphery and a 1,4‐dicyanobenzene unit at its center. This system exhibited a large Stokes‐shifted luminescence that was affected by the polarity and rigidity of the solvent, which was ascribed to emission from an excited state with strong charge‐transfer character. This quadrupolar feature also led to an acceptable two‐photon absorption response in the NIR region.     

Hypervalent Iodine(III) Sulfonate Mediated Synthesis of Imidazo[1,2‐a]pyridines

A direct and efficient method for the conversion of alkyl aryl ketones to imidazo[1,2‐α]pyridines has been developed based on initial formation of α‐organosulfonyloxy ketones and their subsequent cyclocondensation by 2‐aminopyridines in one‐pot conditions.     

Regioselective Metal‐ and Reagent‐Free Arylation of Benzothiophenes by Dehydrogenative Electrosynthesis

A novel strategy for the synthesis of biaryls consisting of a benzothiophene and a phenol moiety is reported. These heterobiaryls are of utmost interest for pharmaceutical, biological, and high‐performance optoelectronic applications. The metal‐ and reagent‐free, electrosynthetic, and highly efficient method enables the generation of 2‐ and 3‐(hydroxyphenyl)benzo[b]thiophenes in a regioselective fashion. The described one‐step synthesis is easy to conduct, scalable, and inherently safe. The products are afforded in high yields of up to 88 % and with exquisite selectivity. The reaction also features a broad scope and tolerates a large variety of functional groups.     

Highly Emissive Luminogens Based on Imidazo[1,2‐a]pyridine for Electroluminescent Applications

A search for novel organic luminogens led us to design and synthesize some N‐fused imidazole derivatives based on imidazo[1,2‐a]pyridine as the core and arylamine and imidazole as the peripheral groups. The fluorophores were synthesized through a multicomponent cascade reaction (A³ coupling) of a heterocyclic azine with an aldehyde and alkyne, followed by Suzuki coupling and a multicomponent cyclization reaction. All of the compounds exhibited interesting photophysical responses, especially arylamine‐containing derivatives, which displayed strong positive solvatochromism in the emission spectra that indicated a more polar excited state owing to an efficient charge migration from the donor arylamine to the imidazo[1,2‐a]pyridine acceptor. The quantum yields ranged from 0.2 to 0.7 and depended on the substitution pattern, most notably that based on the donor group at the C2 position. Moreover, the influence of general and specific solvent effects on the photophysical properties of the fluorophores was discussed with four‐parameter Catalán and Kamlet–Taft solvent scales. The excellent thermal, electrochemical, and morphological stability of the compounds was explored by cyclic voltammetry, thermogravimetric analysis, and AFM methods. Furthermore, to understand the structure, bonding, and band gap of the molecules, DFT calculations were performed. The performance of the electroluminescence behavior of the imidazo[1,2‐a]pyridine derivative was investigated by fabricating a multilayer organic light‐emitting diode with a configuration of ITO/NPB (60 nm)/EML (40 nm)/BCP (15 nm)/Alq₃ (20 nm)/LiF (0.5 nm)/Al(100 nm) (ITO=indium tin oxide, EML=emissive layer, BCP=2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline, Alq₃=tris(8‐hydroxyquinolinato)aluminum), which exhibited white emission with a turn‐on voltage of 8 V and a brightness of 22 cd m^?2.     

Metal‐Free Cross‐Dehydrogenative Coupling (CDC): Molecular Iodine as a Versatile Catalyst/Reagent for CDC Reactions

The development of ecofriendly methods for carbon–carbon (C?C) and carbon–heteroatom (C?Het) bond formation is of great significance in modern‐day research. Metal‐free cross‐dehydrogenative coupling (CDC) has emerged as an important tool for organic and medicinal chemists as a means to form C?C and C?Het bonds, as it is atom economical and more efficient and greener than transition‐metal catalyzed CDC reactions. Molecular iodine (I₂) is recognized as an inexpensive, environmentally benign, and easy‐to‐handle catalyst or reagent to pursue CDCs under mild reaction conditions, with good regioselectivities and broad substrate compatibility. This review presents the recent developments of I₂‐catalyzed C?C, C?N, C?O, and C?S/C?Se bond‐forming reactions for the synthesis of various important organic molecules by cross‐dehydrogenative coupling.     

2,7‐Di‐tert‐butylnaphtho[1,8‐cd][1,2]dithiole 1,2‐dioxides: Thermally Stable,Photochemically Active vic‐Disulfoxides

Bulking up: The thermal barrier to rearrangement of a vic‐disulfoxide is significantly increased through steric buttressing about the (O)S? S(O) bond. Whereas the title compounds represent the most thermally stable vic‐disulfoxides known to date, they also undergo a novel photomediated epimerization at room temperature (see scheme).

     

Environmentally‐Safe Conditions for a Palladium‐Catalyzed Direct C3‐Arylation with High Turn Over Frequency of Imidazo[1,2‐b]pyridazines Using Aryl Bromides and Chlorides

Pd(OAc)₂ was found to catalyze very efficiently the direct arylation of imidazo[1,2‐b]pyridazine at C3‐position under a very low catalyst loading and phosphine‐free conditions. The reaction can be performed in very high TOFs and TONs employing as little as 0.1–0.05 mol % catalyst using a wide range of aryl bromides. In addition, some electron‐deficient aryl chlorides were also found to be suitable substrates. Moreover, 31 examples of the cross couplings were reported using green, safe, and renewable solvents, such as pentan‐1‐ol, diethylcarbonate or cyclopentyl methyl ether, without loss of efficiency.     

Dibenzothienopyrrolo[3,2‐b]pyrrole: The Missing Member of the Thienoacene Family

Dibenzothienopyrrolo[3,2‐b]pyrrole and the corresponding bis(S,S‐dioxide) were synthesized by using a concise synthetic strategy. Despite the presence of six fused aromatic rings, π‐expanded pyrrolo[3,2‐b]pyrroles of this type absorb and emit at relatively short wavelengths, which reflects inefficient π conjugation due to the angular arrangement of the aromatic rings. They exhibit interesting and complex electrochemical behavior, which highlights their potential in organic electronics. Both heteroacenes undergo two‐stage oxidation while retaining the independence of each 1‐phenyl‐1H‐[1]benzothieno[3,2‐b]pyrrole, which was proved by in situ electron spin resonance measurements. Interestingly, electrochemically generated dicationdiradicals are not only distributed over the pyrrolo[3,2‐b]pyrrole scaffold, but also over the phenyl substituents located on nitrogen atoms.     

Metal‐Free Intramolecular Carbocyanation of Alkenes: Catalytic Stereoselective Construction of Pyrrolo[2,1‐a]isoquinolines with Multiple Substituents

The first catalytic stereoselective approach to prepare multiple substituted cyclic nitriles by means of a novel metal‐free intramolecular carbocyanation strategy has been developed, which provided an efficient protocol to construct densely functionalized diastereoenriched pyrrolo[2,1‐a]isoquinoline derivates under mild reaction conditions (up to 20/1 d.r., 98 % yield).     

Copper‐Promoted Annulation of Terminal Alkynes with 2‐Aminopyridines to Assemble 2‐Halogenated Imidazo[1,2‐a]pyridines

Copper‐promoted annulation reactions of terminal alkynes with 2‐aminopyridines have been developed for the synthesis of 2‐halogenated imidazo[1,2‐a]pyridines using copper halide as the halogen source. A variety of substrates survived under the reaction conditions and gave the desired products in good yields. This reaction features advantages such as easily available starting materials, broad substrate scope, and mild reaction conditions.     

Synthesis of 1,2,4‐Triazolo[4,3‐a]pyridines and Related Heterocycles by Sequential Condensation and Iodine‐Mediated Oxidative Cyclization

A facile and efficient approach to access 1,2,4‐triazolo[4,3‐a]pyridines and related heterocycles has been accomplished through condensation of readily available aryl hydrazines with corresponding aldehydes followed by iodine‐mediated oxidative cyclization. This transition‐metal‐free synthetic process is broadly applicable to a variety of aromatic, aliphatic, and α,β‐unsaturated aldehydes, and can be conveniently conducted on the gram scale.     

Vertically π‐Expanded Imidazo[1,2‐a]pyridine: The Missing Link of the Puzzle

The dehydrogenative coupling of imidazo[1,2‐a]pyridine derivative has been achieved for the first time. In cases in which the most‐electron‐rich position of the electron‐excessive heterocycle was blocked by a naphthalen‐1‐yl substituent, neither oxidative aromatic coupling nor reaction under Scholl conditions enabled the fusion of the rings. The only method that converted the substrate into the corresponding imidazo[5,1,2‐de]naphtho[1,8‐ab]quinolizine was coupling in the presence of potassium in anhydrous toluene. Moreover, we discovered new, excellent conditions for this anion‐radical coupling reaction, which employed dry O₂ from the start in the reaction mixture. This method afforded vertically fused imidazo[1,2‐a]pyridine in 63 % yield. Interestingly, whereas the fluorescence quantum yield (Φ_fl) of compound 3 , despite the freedom of rotation, was close to 50 %, the Φ_fl value of flat naphthalene‐imidazo[1,2‐a]pyridine was only 5 %. Detailed analysis of this compound by using DFT calculations and a low‐temperature Shpol′skii matrix revealed phosphorescence emission, thus indicating that efficient intersystem‐crossing from the lowest‐excited S₁ level to the triplet manifold was the competing process with fluorescence.     

Synthesis of Novel Imidazo[1,2‐a]pyridin‐2‐amines from Arylamines and Nitriles via Sequential Addition and I2/KI‐Mediated Oxidative Cyclization

A novel and practical strategy for the construction of imidazo[1,2‐a]pyridin‐2‐amine frameworks has been developed. The present sequential approach involves addition of arylamines to nitriles and I₂/KI‐mediated oxidative C?N bond formation without purification of the intermediate amidines. This operationally simple synthetic process provides a facile access to a variety of new 2‐amino substituted imidazo[1,2‐a]pyridines and related heterocyclic compounds in an efficient and scalable fashion.     

An Efficient One‐Pot Four‐Component Synthesis of Functionalized Imidazo[1,2‐a]pyridines

An efficient one‐pot four‐component protocol for the synthesis of imidazo[1,2‐a]pyridines was developed by condensing ethane‐1,2‐diamine ( 2 ), 1,1‐bis(methylthio)‐2‐nitroethene ( 1 ), aldehydes 3 , and activated methylene compounds in EtOH under reflux conditions (Tables 1–3). The features of this procedure are operational simplicity, good yields of products, in situ preparation of heterocyclic ketene aminals (HKA), and catalyst‐free conditions.