Copper-Catalyzed Denitrogenative Transannulation Reaction of Pyridotriazoles: Synthesis of Imidazo[1,5-a]pyridines with Amines and Amino Acids

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.

Cu(I)-Catalyzed Three-Component Annulation for the Synthesis of 3-Acyl Imidazo[1, 5-a]Pyridines from 2-Pyridinyl-Substituted p-Quinone Methides, Terminal Alkynes, and TsN3 Using O2 as the Oxygen Source

Currently, most conventional methods to achieve imidazo[1,5-a]pyridines have limitations for the synthesis of 3-acyl imidazo[1,5-a]pyridines. Herein, a novel and efficient Cu(I)-catalyzed three-component annulation method for the synthesis of valuable 3-acyl imidazo[1,5-a]pyridines by the reaction of 2-pyridinyl-substituted p-QMs, terminal alkynes, and TsN3 in the presence of O2 under mild conditions have successfully been developed. The investigation indicated that molecular oxygen (O2) and TsN3, respectively, serving as oxygen and nitrogen sources, were essential for the successful completion of the reaction system.

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.

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.

Synthesis of N-vinyl isothiocyanates and carbamates by the cleavage of NH-1,2,3-triazoles with one-carbon electrophiles

Metal-free cascade reaction of NH-1,2,3-triazoles with one-carbon electrophiles, such as thiophosgene and triphosgene, led to N-vinylated ring cleavage products. Using this approach the synthesis of N-vinylisothiocyanates from NH-triazoles and thiophosgene was achieved. A variety of multifunctional compounds, such as N-vinylcarbamates, unsymmetrical vinylureas, carbamothioates, etc. was prepared by a one-pot method from NH-triazoles, triphosgene and nucleophiles.

Transition-Metal-Catalyzed Denitrogenative Annulation to Access High-Valued N-Heterocycles

Over the past few years, the development of efficient methods to construct high-valued N-heterocyclic molecules have received massive attention owing to their extensive application in the areas of medicinal chemistry, drug discovery, natural product synthesis and so on. To access those high-valued N-heterocycles, many methods have been developed. In this context, transition-metal-catalyzed denitrogenative annulation of 1,2,3-triazoles and 1,2,3,4-tetrazoles has appeared as a powerful synthetic tool because it offers a step- and atom-economical route for the preparation of the nitrogen-rich molecules. Compared with the denitrogenative annulation of various 1,2,3-triazole frameworks, annulation of 1,2,3,4-tetrazole remains more challenging due to the inertness of the tetrazole moiety. This Review summarizes the significant achievements made in the field of denitrogenative annulation of various 1,2,3-triazoles and 1,2,3,4-tetrazoles including some pioneering examples in this area of research. We anticipate that this denitrogenative annulation reaction will find broad applications in the pharmaceutical industry, drug discovery and other fields of medicinal chemistry.

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.

Annulation-Triggered Denitrogenative Transformations of 2-(5-Iodo-1,2,3-triazolyl)benzoic Acids

The ability of [1,2,3]triazolobenzoxazinones to act as a source of "hidden" diazo group was discovered. These diazo precursors can be easily prepared by the intramolecular cyclization of 2-(5-iodo-1,2,3-triazolyl)benzoic acids. The Cu-catalyzed capture of the hidden diazo group allows for further functionalization through the denitrogenative pathway. The transformations proceed via the formation of either diazoimine or diazoamide intermediates. Novel routes to various anthranilamides as well as thiolated benzoxazinones were developed using the one-pot cyclization/diazo capture procedure.

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.

Rhodium-catalyzed denitrogenative gem-difunctionalization of pyridotriazoles with thioesters: formal carbene insertion into C(O)-S bonds

A formal carbene insertion into C(O)-S bonds to access α-quaternary pyridines was achieved via a rhodium(II)-catalyzed in situ formation of sulfonium ylides from pyridotriazoles with thioesters followed by acyl group migration. This protocol has enabled an efficient denitrogenative gem-acylthiolation of pyridotriazoles to incorporate an acyl, pyridyl, and sulfur-substituted quaternary carbon center with high selectivity, broad substrate scope, and good functional group tolerance.

Pd-catalyzed annulation of imidazo[1,2-a]pyridines with coumarins and indoles: synthesis of benzofuran and indole fused heterocycles

Palladium-catalyzed annulation of imidazo[1,2-a]pyridines with coumarins provided benzofuran fused transannulated products in good to excellent yields via a decarbonylative approach, while imidazo[1,2-a]pyridines with N-methyl indoles in the presence of palladium and base yielded conjugated imidazopyridine fused indole derivatives. Additional experiments revealed that the presence of the phenyl ring at the C-2 position of imidazo[1,2-a]pyridines is essential for the annulation than the alkyl groups. Both transformations follow the ionic mechanism by Pd-catalyzed double C-H bond activation. All the annulated products were shown to exhibit high fluorescence characteristics and their photophysical properties were evaluated.

Access to 2-arylquinazolines via catabolism/reconstruction of amino acids with the insertion of dimethyl sulfoxide

Quinazoline skeletons are synthesized by amino acid catabolism/reconstruction combined with the insertion/cyclization of dimethyl sulfoxide for the first time. The amino acid acts as a carbon and nitrogen source through HI-mediated catabolism and is then reconstructed using aromatic amines and dimethyl sulfoxide (DMSO) as a one-carbon synthon. This protocol is of great significance for the further study of the conversion of amino acids.

Merging Annulation with Ring Deconstruction: Synthesis of (E)-3-(2-Acyl-1H-benzo[d]imidazol-4-yl)acrylaldehyde Derivatives via I2/FeCl3-Promoted Dual C(sp3)-H Amination/C-N Bond Cleavage

An unprecedented I₂/FeCl₃-promoted cascade reaction of aryl methyl ketones with 8-aminoquinolines for the convenient synthesis of (E)-3-(2-acyl-1H-benzo[d]imidazol-4-yl)acrylaldehydes was developed by merging annulation with ring deconstruction. This novel strategy unlocked the new reactivity of 8-aminoquinolines and provided an attractive platform for the ring opening of unactivated N-heteroaromatic compounds. Preliminary mechanistic investigation suggested that dual C(sp³)-H amination/C-N bond cleavage were key reaction steps. Furthermore, late-stage modification of the obtained products successfully delivered pyrazole and isoxazole derivatives, increasing the practicability and application potential of this methodology in organic synthesis.

BF3·Et2O catalyzed transannulation of pyridotriazoles with isothiocyanates: synthesis of thiazolo[3,4-a]pyridin-3-imines

The synthesis of thiazolo[3,4-a]pyridin-3-imines, through denitrogenative transannulation of pyridotriazoles, under transition metal-free conditions is described.

Csp–Csp bond cleavage and fragment coupling: a transition metal-free “extrusion and recombination” approach towards synthesis of 1,2-diketones

A metal-free strategy for C–C bond activation of 1,3-diynes has been established via an “extrusion and recombination” approach to derive structurally important 1,2-diketones in good yields with excellent selectivity.

Imidazo[1,5-a]pyridine derivatives: useful, luminescent and versatile scaffolds for different applications

In the last few years, imidazo[1,5-a]pyridine nuclei and derivatives have attracted growing attention due to their unique chemical structure and versatility, optical behaviours, and biological properties.

Rh(ii)-Catalyzed denitrogenative 1-sulfonyl-1,2,3-triazole-1-alkyl-1,2,3-triazole cross-coupling as a route to 3-sulfonamido-1H-pyrroles and 1,2,3-triazol-3-ium ylides

The reaction of 1-alkyl-1H-1,2,3-triazoles with rhodium(ii) azavinyl carbenes, generated from 1-sulfonyl-1H-1,2,3-triazoles, was utilized to prepare 3-sulfonamido-1H-pyrroles and 1,2,3-triazol-3-ium ylides in good yields.

Co-Catalyzed Transannulation of Pyridotriazoles with Isothiocyanates and Xanthate Esters

An efficient radical transannulation reaction of pyridotriazoles with isothiocyanates and xanthate esters was developed. This method features conversion of pyridotriazoles into two N-fused heterocyclic aromatic systems-imino-thiazolopyridines and oxo-thiazolopyridine derivatives-via one-step Co(II)-catalyzed transannulation reaction proceeding via a radical mechanism. The synthetic usefulness of the developed method was illustrated in the synthesis of amino acid derivatives and further transformations of obtained reaction products.

Silver-Catalyzed Activation of Pyridotriazoles for Formal Intramolecular Carbene Insertion into Vinylic C(sp2)-H Bonds

A silver-catalyzed intramolecular denitrogenative annulation of pyridotriazole with alkene was reported to achieve the challenging carbene insertion into the vinylic C(sp²)-H bond. This protocol has enabled the construction of functionalized 1H-indenes with high efficiency and excellent functional group tolerance. Experimental and computational studies suggest a stepwise mechanism involving a water-promoted hydrogen atom transfer with the aid of a silver catalyst.

Recent Advances in Denitrogenative Reactions of Pyridotriazoles

Diazo compounds display versatile reactivity and therefore are widely used in organic synthesis. Diazo compounds bearing a 2-pyridyl or a related azine moiety on the diazo carbon exist in the form of fused 1,2,3-triazoles. In this short review, we summarize the recent advances in denitrogenative reactions of [1,2,3]triazolo[1,5-a]pyridines (‘pyridotriazoles’) and related fused 1,2,3-triazoles. Over the past decade, there has been a surge of activity in this field, with novel denitrogenative reactions of pyridotriazoles induced by metal compounds, light, and Brønsted and Lewis acids having been devised. As a result, heterocyclic compounds and functionalized α-picolines as well as bio­active molecules have been synthesized. In the review, emphasis is also placed on the mechanisms of the new reactions.

1 Introduction

2 Ring-Chain Isomerization of Pyridotriazoles

3 Metal-Catalyzed Reactions

3.1 Rh(II) Catalysis

3.2 Rh(III) Catalysis

3.3 Cu Catalysis

3.4 Pd Catalysis

3.5 Catalysis by Other Metals

4 Metal-Free Reactions

5 Conclusion

Denitrogenative Transformations of Pyridotriazoles and Related Compounds: Synthesis of N-Containing Heterocyclic Compounds and Beyond

The high demand for new and efficient routes toward synthesis of nitrogen-containing heterocyclic scaffolds has inspired organic chemists to discover several methodologies over recent years. This Perspective highlights one standout approach, which involves the use of pyridotriazoles and related compounds in denitrogenative transformations. Readily available pyridotriazoles undergo ring-chain isomerization to produce uniquely reactive α-diazoimines. Such reactivity, enabled by metal catalysts, additives, or visible-light irradiation, can be applied in transannulation, insertion, cyclopropanation, and many other transformations.

Mg3N2-assisted one-pot synthesis of 1,3-disubstituted imidazo[1,5-a]pyridine

A novel Mg₃N₂-assisted one-pot annulation strategy has been developed via cyclo-condensation reaction of 2-pyridyl ketones with alkyl glyoxylates or aldehydes, allowing the formation of imidazo[1,5-a]pyridines exclusively with an exellent yield.

Annulation of imidazo[1,2-a]pyridines under metal-free conditions

Synthesis of benzo[a]imidazo[5,1,2-cd]indolizines from 2-arylimidazo[1,2-a]pyridines and benzyne precursors under metal-free conditions has been described.

Light-induced metal-free transformations of unactivated pyridotriazoles

A highly efficient and practical method for incorporation of the arylmethylpyridyl moiety into diverse molecules has been developed. This method features the transition metal-free light-induced room temperature transformation of pyridotriazoles into pyridyl carbenes, which are capable of smooth arylation, X-H insertion, and cyclopropanation reactions. The synthetic usefulness of the developed method was illustrated in a facile synthesis of biologically active molecules.

Recent advances on the transition-metal-catalyzed synthesis of imidazopyridines: an updated coverage

A comprehensive account of recent advances in the synthesis of imidazopyridines, assisted through transition-metal-catalyzed multicomponent reactions, C-H activation/functionalization and coupling reactions are highlighted in this review article. The basic illustration of this review comprises of schemes with concise account of explanatory text. The schemes depict the reaction conditions along with a quick look into the mechanism involved to render a deep understanding of the catalytic role. At some instances optimizations of certain features have been illustrated through tables, i.e., selectivity of catalyst, loading of the catalyst and percentage yield with different substrates. Each of the reported examples has been rigorously analyzed for reacting substrates, reaction conditions and transition metals used as the catalyst. This review will be helpful to the chemists in understanding the challenges associated with the reported methods as well as the future possibilities, both in the choice of substrates and catalysts. This review would be quite appealing to a wider range of organic chemists in academia and industrial R&D sectors working in the field of heterocyclic syntheses. In a nutshell, this review will be a guiding torch to envisage: (i) the role of various transition metals in the domain dedicated towards method development and (ii) for the modifications needed thereof in the R&D sector.

Metal-Free Denitrogenative C-C Couplings of Pyridotriazoles with Boronic Acids To Afford α-Secondary and α-Tertiary Pyridines

Pyridotriazoles are utilized as robust building blocks to access α-secondary and α-tertiary pyridines via the development of a simple yet practically useful metal-free denitrogenative C-C cross-coupling with boronic acids. The reaction shows a high level of functional tolerance, broad substrate scope, and facile scalability. The synthetic potential of the method is demonstrated by the strurctural modification of a bioactive molecule and concise synthesis of pheniramine analogs.

Metal-free synthesis of imidazo[1,5-a]pyridines via elemental sulfur mediated sequential dual oxidative Csp3-H amination

A simple and efficient approach has been developed for the synthesis of imidazo[1,5-a]pyridines using the elemental sulfur mediated sequential dual oxidative Csp3-H amination of 2-pyridyl acetates and amines under metal- and peroxide-free conditions. Broad substrate scope, operational simplicity and gram-scale ability make this chemistry very practical.

Design and Exploration of Catalytic Activity of Two-Dimensional Surface-Engineered Graphene Oxide Nanosheets in the Transannulation of N-Heterocyclic Aldehydes or Ketones with Alkylamines

In this work, pharmaceutically and biologically important compounds containing imidazo[1,5-a]pyridine nuclei have been synthesized via transannulation of N-heteroaryl aldehydes or ketones with alkylamines using a graphene oxide-supported copper catalyst. The nanocatalyst was fabricated by the covalent immobilization of 4-aminoantipyrine onto an amine-functionalized graphene oxide nanosupport followed by its metallation with copper acetate. Structural analysis by transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction demonstrates that the two-dimensional sheet-like structure of graphene oxide is maintained even after the chemical modifications, whereas XPS revealed crucial information related to elemental composition and surface electronic states of the metal present in the catalyst. Apart from this, Fourier transform infrared spectroscopy helped in identifying the degree of oxidation and the presence of oxygenated groups in graphene oxide nanocomposites. As a heterogeneous catalyst, this graphene oxide-supported copper complex showed moderate to good catalytic activity in the C(sp3)-H bond activation/amination of a variety of substrates. This superior catalytic performance originated from the unique 2-dimensional structure of graphene oxide-based material which provided space between graphitic overlayers due to appropriate positioning of metal on their basal planes, decreasing the diffusion resistances of reactant surfaces, thus making it function as a nanoreactor. More importantly, this nanomaterial could be recovered easily and reused repeatedly by simple washing without chemical treatment with no appreciable loss in its catalytic activity, showing good potential for increasing the overall turnover number of this synthetically useful catalyst.