Imidazo[1,2-a]pyridine and Imidazo[1,5-a]pyridine: Electron Donor Groups in the Design of D-π-A Dyes

This work investigates the electron-donating capabilities of two 10-π electron nitrogen bridgehead bicyclic [5,6]-fused ring systems, imidazo[1,2-a]pyridine and imidazo[1,5-a]pyridine rings. Eight compounds with varying positions of electron-withdrawing moieties (TCF or DCI) coupled to the imidazopyridine ring were synthesized and studied. DCI-containing compounds (Ib-IVb) exhibited a purely dipolar nature with broad absorption bands, weak fluorescence, large Stokes shifts, and strong solvatochromism. In contrast, TCF-containing compounds (Ia-IVa) demonstrated diverse properties. Imidazo[1,2-a]pyridine derivatives Ia and IIa were purely dipolar, while imidazo[1,5-a]pyridine derivatives IIIa and IVa displayed a cyanine-like character with intense absorption and higher quantum yields of emission. The observed gradual red shift in optical properties with changing electron-donor groups (IIb < Ib < IIIb < IVb) and (IIa < Ia < IIIa < IVa) underscores the stronger electron-donor character of imidazo[1,5-a]pyridine compared to that of imidazo[1,2-a]pyridine. Furthermore, crystalline powders of imidazo[1,2-a]pyridine derivatives exhibited fluorescence despite minimal emission in solution. Two compounds (Ib and IVa) were successfully formulated into nanoparticles for potential in vivo imaging applications in zebrafish embryos.

Imidazopyridine Family: Versatile and Promising Heterocyclic Skeletons for Different Applications

In recent years, there has been increasing attention focused on various products belonging to the imidazopyridine family; this class of heterocyclic compounds shows unique chemical structure, versatile optical properties, and diverse biological attributes. The broad family of imidazopyridines encompasses different heterocycles, each with its own specific properties and distinct characteristics, making all of them promising for various application fields. In general, this useful category of aromatic heterocycles holds significant promise across various research domains, spanning from material science to pharmaceuticals. The various cores belonging to the imidazopyridine family exhibit unique properties, such as serving as emitters in imaging, ligands for transition metals, showing reversible electrochemical properties, and demonstrating biological activity. Recently, numerous noteworthy advancements have emerged in different technological fields, including optoelectronic devices, sensors, energy conversion, medical applications, and shining emitters for imaging and microscopy. This review intends to provide a state-of-the-art overview of this framework from 1955 to the present day, unveiling different aspects of various applications. This extensive literature survey may guide chemists and researchers in the quest for novel imidazopyridine compounds with enhanced properties and efficiency in different uses.

One-Dimensional and Two-Dimensional Zn(II) Coordination Polymers with Ditopic Imidazo[1,5-a]pyridine: A Structural and Computational Study

Zn(II) coordination polymers are being increasingly studied for their stability and properties. Similarly, there is a growing interest in imidazo[1,5-a]pyridine derivatives, which show great potential in luminescence and pharmaceutical applications. In this work, we successfully synthesized and crystallized three new coordination polymers, using Zn(II) as the metallic node, dicarboxylic acids of different length and nature as linkers, and a linear ditopic imidazo[1,5-a]pyridine derivative, to explore the role of this molecule as a propagator of the dimensionality of the structure or as an ancillary ligand. Our work demonstrates the structural capability of imidazo[1,5-a]pyridines in an unexplored domain for this family of ligands. Notably, we observed a pronounced ability of this heterocyclic scaffold to establish π···π interactions in the solid state. The supramolecular π-stacked assemblies were theoretically analyzed using DFT calculations based on model structures.

Construction of Imidazole-Fused-Ring Systems by Iron-Catalyzed C(sp3)-H Amination-Cyclization under Aerobic Conditions

An iron-catalyzed efficient C-H amination for the construction of imidazole-fused-ring systems was developed under aerobic conditions. Compared to previous studies, this work exhibited green features. The reaction was conducted in the green solvent anisole, with water as the only byproduct. Four C(sp3)-H bonds were cleaved and three C-N bonds were formed in this transformation. Imidazo[1,5-a]pyridine-, imidazo[5,1-b]oxazole-, imidazo[5,1-b]thiazole-, imidazo[1,5-a]pyrazine-, and imidazo[1,5-a]imidazole-related N-heterocycles were obtained in acceptable-to-excellent yield.

Relay Copper-Catalyzed Synthesis of Imidazo[1,5-a]pyridine Scaffolds from Phenylalanine and Halohydrocarbon

An efficient and straightforward strategy to synthesize imidazo[1,5-a]pyridine compounds from phenylalanine and halohydrocarbon has been successfully developed. The protocol features a relay copper-catalyzed reaction involving intermolecular C-O coupling and intramolecular C-N cyclization, providing an approach to access a diverse range of imidazo[1,5-a]pyridine derivatives with unique aza quaternary carbon centers.

Mono-, Bis-, and Tris-Chelate Zn(II) Complexes with Imidazo[1,5-a]pyridine: Luminescence and Structural Dependence

New mono-, bis-, and tris-chelate Zn(II) complexes have been synthesized starting from different Zn(II) salts and employing a fluorescent 1,3-substituted-imidazo[1,5-a]pyridine as a chelating ligand. The products have been characterized by single-crystal X-ray diffraction; mass spectrometry; and vibrational spectroscopy. The optical properties have been investigated to compare the performances of mono-, bis-, and tris-chelate forms. The collected data (in the solid state and in solution) elucidate an important modification of the ligand conformation upon metal coordination; which is responsible for a notable increase in the optical performance. An intense modification of the emission quantum yield along the series in the solid state is observed comparing mono-, bis-, and tris-chelate adducts; independently from the anionic ligand introduced by ionic exchange.

Facile iodine-promoted synthesis of bis(1-imidazo[1,5-a]pyridyl)arylmethanes and exploration of applications

A practical strategy for the iodine-promoted synthesis of bis(1-imidazo[1,5-a]pyridyl)arylmethane and its derivatives has been developed. These compounds exhibit high cytotoxicity toward various cancer cell lines and moreover they are promising ligands for the Cu-catalysed synthesis of quinolines.

Coordination of Distal Carboxylate Anion Alters Metal Ion Specific Binding in Imidazo[1,2-a]pyridine Congeners

Imidazo[1,2-a]pyridine derivatives have excellent potential for chelation with transition metal ions. Two new imidazo[1,2-a]pyridine-8-carboxylates were synthesized and characterized by ¹H NMR, ¹³C NMR, HRMS, and single crystal-XRD techniques. Methyl carboxylate (probe 1) turns on fluorescence upon coordination with Zn²⁺, while sodium carboxylate (probe 2) turns off its fluorescence upon coordination with Co²⁺ or Cu²⁺ ions present in aqueous acetonitrile medium. ¹³C NMR study revealed that the change in metal ion specific binding was due to the involvement of carboxylate anion in complex formation with Co²⁺ or Cu²⁺ ions. The carboxylate anion at 8-position also enhanced the sensitivity of detection of probe 2 by an order of magnitude (detection limits: 3.804 × 10^-7 M, probe 1/Zn²⁺; 0.420 × 10^-7 M, probe 2/Co²⁺ and 0.304 × 10^-7 M, probe 2/Cu²⁺). The detection limits of probes 1 and 2 comply well with the World Health Organization (WHO) and US Environmental Protection Agency (US-EPA) guidelines for detection of heavy metal ions present in drinking water and ground water. Both the probes form a 1:1 complex with Zn²⁺, Co²⁺ or Cu²⁺, and the stoichiometry was verified by Job plot and ESI-mass analysis. The sensing mechanism is explained using ¹³C NMR experiments, ESI-mass analytical data and theoretical DFT calculations. The suitability of probes 1 and 2 for on-site detection and quantitative determination of Zn²⁺, Co²⁺ and Cu²⁺ ions present in biological, environmental and industrial samples is demonstrated. In addition, both 1 and 2 are used for detection of intracellular contamination of Zn²⁺, Co²⁺ or Cu²⁺ ions in onion epidermal cells.

FeCl3-Catalyzed Synthesis of 6-Thioxo-hexahydroindeno[1',2':4,5]imidazo[1,5-a]pyridin-12(6H)-ones via an Interesting [1,2] Oxygen Shift Pathway

An iron-catalyst mediated one-pot multicomponent route for the synthesis of novel 6-thioxo-hexahydroindeno[1',2':4,5]imidazo[1,5-a]pyridin-12(6H)-one scaffolds has been developed using ninhydrin, l-proline, and aryl isothiocyanates in ethanol medium. This methodology offers an interesting [1,2] oxygen shift mechanism pathway via a number of ring-opening and ring-closing cascade steps to provide diverse substituted hexahydroindeno-imidazo[1,5-a]pyridinones in excellent to good yields. The stereochemistry of the proline ring is lost during the course of the reaction. This protocol is well acceptable toward both electron-accepting and electron-donating functionalities at the ortho-, meta-, and para-positions of the isothiocyanate moiety. Nonhazardous conditions, step economic, and easy operational process are the advantages of this methodology.

Imidazo[1,5-a]pyridine-Based Fluorescent Probes: A Photophysical Investigation in Liposome Models

Imidazo[1,5-a]pyridine is a stable scaffold, widely used for the development of emissive compounds in many application fields (e.g., optoelectronics, coordination chemistry, sensors, chemical biology). Their compact shape along with remarkable photophysical properties make them suitable candidates as cell membrane probes. The study of the membrane dynamics, hydration, and fluidity is of importance to monitor the cellular health and to explore crucial biochemical pathways. In this context, five imidazo[1,5-a]pyridine-based fluorophores were synthesized according to a one-pot cyclization between an aromatic ketone and benzaldehyde in the presence of ammonium acetate and acetic acid. The photophysical features of prepared compounds were investigated in several organic solvents and probes 2–4 exhibited the greatest solvatochromic behavior, resulting in a higher suitability as membrane probes. Their interaction with liposomes as artificial membrane model was tested showing a successful intercalation of the probes in the lipid bilayer. Kinetic experiments were carried out and the lipidic phase influence on the photophysical features was evaluated through temperature-dependent experiments. The results herein reported encourage further investigations on the use of imidazo[1,5-a]pyridine scaffold as fluorescent membrane probes.

Thioether-Assisted Cu-Catalyzed C5-H Arylation of Imidazo[1,5-a]pyridines

A Cu-catalyzed regioselective C5-H arylation of imidazo[1,5-a]pyridines with aryl iodides was achieved with the assistance of an ethylthio group at the C3 position. This directing group could be easily removed to furnish a range of 5-(hetero)arylimidazo[1,5-a]pyridine derivatives. The reaction tolerates a variety of functionalities and is compatible with sterically hindered substrates.

Cobalt-catalyzed tandem one-pot synthesis of polysubstituted imidazo[1,5-a]pyridines and imidazo[1,5-a]isoquinolines

An efficient cobalt-catalyzed tandem one-pot method has been developed for the synthesis of polysubstituted imidazo[1,5-a]-N-heteroaromatics. The method involves Knoevenagel condensation followed by cobalt-catalyzed direct alkenylation to give the desired polysubstituted imidazo[1,5-a]pyridines and imidazo[1,5-a]isoquinolines in a one-pot manner. This method exhibits a broad substrate scope, provides moderate to good (39-74%) yields and is amenable to scale-up to the gram scale.

Anomeric Stereoauxiliary Cleavage of the C−N Bond of d ‐Glucosamine for the Preparation of Imidazo[1,5‐a]pyridines

The targeted cleavage of the C−N bonds of alkyl primary amines in sustainable compounds of biomass according to a metal‐free pathway and the conjunction of nitrogen in the synthesis of imidazo[1,5‐a]pyridines are still highly challenging. Despite tremendous progress in the synthesis of imidazo[1,5‐a]pyridines over the past decade, many of them can still not be efficiently prepared. Herein, we report an anomeric stereoauxiliary approach for the synthesis of a wide range of imidazo[1,5‐a]pyridines after cleaving the C−N bond of d‐glucosamine (α‐2° amine) from biobased resources. This new approach expands the scope of readily accessible imidazo[1,5‐a]pyridines relative to existing state‐of‐the‐art methods. A key strategic advantage of this approach is that the α‐anomer of d‐glucosamine enables C−N bond cleavage via a seven‐membered ring transition state. By using this novel method, a series of imidazo[1,5‐a]pyridine derivatives (>80 examples) was synthesized from pyridine ketones (including para‐dipyridine ketone) and aldehydes (including para‐dialdehyde). Imidazo[1,5‐a]pyridine derivatives containing diverse important deuterated C(sp²)−H and C(sp³)−H bonds were also efficiently achieved.

Recent synthetic methodologies for imidazo[1,5-a]pyridines and related heterocycles

Imidazo[1,5-a]pyridine is a significant structural component of a large number of agrochemicals and pharmaceuticals. The synthesis of imidazo[1,5-a]pyridine has been a subject of intense research for numerous decades. A large number of transformations are now available to conveniently access imidazo[1,5-a]pyridine from readily available starting materials. This review details the recent development in imidazo[1,5-a]pyridine construction involving cyclocondensation, cycloaddition, oxidative cyclization, and transannulation reactions.

Understanding Cu(II)-based systems for C(sp3)-H bond functionalization: insights into the synthesis of aza-heterocycles

Herein we report the synthesis of imidazo[1,5-a]pyridine heterocycles via a Cu(II)-mediated functionalization of α'-C(sp³)-H bonds of pyridinylaldimines and subsequent cyclization. This strategy exploits the inherent directing ability of heteroleptic aldimine and pyridine groups in the substrate yielding the C-H functionalization of α'-methylene groups in a regioselective fashion over distant methyl or methylene groups in β or γ positions. The observed correlation between the nature of the anionic ligands (halide vs. carboxylate) bonded to copper and the chemoselectivity of the C(sp³)-H activation process points to a concerted metalation-deprotonation pathway prior to cyclization to furnish the corresponding imidazo[1,5-a]pyridine derivative. This copper-mediated C(sp³)-H bond functionalization reaction works for a variety of substrates incorporating linear alkyl chains (from 3 to 12 carbon atoms), and good functional group tolerance (aryl, ether and ester groups). Cu-Catalyzed C(sp²)-H cyanation on the imidazole ring can then take place selectively under oxidative conditions.

Dipyridylmethane Ethers as Ligands for Luminescent Ir Complexes

This work reports two new cationic heteroleptic cyclometalated iridium complexes, containing ether derivatives of di(pyridin-2-yl)methanol. The new ligands are based on dipyridin-2-ylmethane and are designed to obtain ether-based intermediates with extended electronic conjugation by insertion of π system such as phenyl, allyl and ethynyl. Different synthetic strategies were employed to introduce these units, as molecular wires, between the dipyridin-2-ylmethane chelating portion and the terminal N-containing functional group, such as amine and carbamide. The corresponding complexes show luminescence in the blue region of the spectrum, lifetimes between 0.6 and 2.1 μs, high quantum yield and good electrochemical behavior. The computational description (DFT) of the electronic structure highlights the key role of the conjugated π systems on optical and electrochemical properties of the final products.

Ortho C-H Functionalization of 2-Arylimidazo[1,2-a]pyridines

C-H activation and functionalization is quite promising in recent days as the strategy offers a go-to general method for different bond formations and hence grants synthetic versatility. At the same time, imidazopyridine, a fused bicycle of imidazole moiety with pyridine ring, has a profound impact due to its ubiquitous and prodigious application in medicinal as well as material chemistry. The presence of N-1 atom in 2-arylImidazo[1,2-a]pyridine facilitates the coordination with metal catalysts leading to the formation of ortho-substituted products. This review summarizes all the articles on ortho C-H functionalization of 2-arylImidazo[1,2-a]pyridines published till August 2021.

Microwave-Assisted Synthesis, Optical and Theoretical Characterization of Novel 2-(imidazo[1,5-a]pyridine-1-yl)pyridinium Salts

In the last few years, imidazo[1,5-a]pyridine scaffolds and derivatives have attracted growing attention due to their unique chemical structure and optical behaviors. In this work, a series of pyridylimidazo[1,5-a]pyridine derivatives and their corresponding pyridinium salts were synthesized and their optical properties investigated to evaluate the effect of the quaternization on the optical features both in solution and polymeric matrix. A critical analysis based on the spectroscopic data, chemical structures along with density functional theory calculation is reported to address the best strategies to prevent aggregation and optimize the photophysical properties. The obtained results describe the relationship between chemical structure and optical behaviors, highlighting the role of pendant pyridine. Finally, the presence of a positive charge is fundamental to avoid any possible aggregation process in polymeric films.

Methoxy-substituted copper complexes as possible redox mediators in dye-sensitized solar cells

Methoxy-substituted aromatic diimines and corresponding homoleptic copper(i) and copper(ii) complexes as possible redox mediators in dye-sensitized solar cells.