Gold-Catalyzed Synthesis of 3-Acylimidazo[1,2-a]pyridinesviaCarbene Oxidation

Synthesis of N-, O-, and S-heterocycles from aryl/alkyl alkynyl aldehydes

In the field of heterocyclic synthesis, alkynyl aldehydes serve as privileged reagents for cyclization reactions with other organic compounds to construct a broad spectrum of N-, O-, and S-heterocycles. Due to the immense application of heterocyclic molecules in pharmaceuticals, natural products, and material chemistry, the synthesis of such scaffolds has received wide attention. The transformations occurred under metal-catalyzed, metal-free-promoted, and visible-light-mediated systems. The present review article highlights the progress made in this field over the past two decades.

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.

1,2-Aminofunctionalization Reactions of Pyridino-Alkynes via Carbophilic Activation

Transition metal-catalyzed 1,2-difunctionalization reactions of alkynes have emerged as a powerful tool to forge carbon-carbon and carbon-heteroatom bonds for the rapid synthesis of polyfunctionalized molecular scaffolds. In this regard, our group has persistently been developing transition metal-mediated 1,2-aminofunctionalization reactions of alkynes through a rationally designed pyridino-alkyne core by utilizing the carbophilic activation strategy. In this account, we present an array of such 1,2-aminofunctionalization reactions which have been successfully executed on this core to afford important polycyclic frameworks such as functionalized quinalizinones, pyridinium oxazole dyads (PODs), N-doped polycyclic aromatic hydrocarbons (PAHs), N-doped spiro-PAHs. Additionally, the synthesis of phosphine ligated gold complexes bearing pyrido-isoquinoline scaffold from the pyridino-alkynes will be discussed briefly.

I2-Catalyzed Three-Component Consecutive Reaction for the Synthesis of 3-Aroylimidazo[1,2-a]-N-Heterocycles

A convenient one-pot, three-component reaction has been developed for the synthesis of 3-aroylimidazo[1,2-a]-N-heterocycles from aryl ketones and 2-amino-N-heterocycles using dimethyl sulfoxide as a methylene donor. The reaction proceeds smoothly catalyzed by I₂ in the presence of K₂S₂O₈ and affords the desired products in moderate to good yields. This protocol offers significant superiority in accessing biologically active 3-aroylimidazo[1,2-a]-N-heterocycles with various substitution patterns.

Iodine Promoted Efficient Synthesis of 2-Arylimidazo[1,2-a]pyridines in Aqueous Media: A Comparative Study between Micellar Catalysis and an "On-Water" Platform

In a new and environmentally sustainable approach, a series of 2-arylimidazo[1,2-a]pyridine derivatives were synthesized in aqueous media in the presence of iodine as a catalyst. The reaction proceeded by condensation of various aryl methyl ketones with 2-aminopyridines to afford 2-arylimidazo[1,2-a]pyridines in good overall yields. Although several of the reactions were efficiently performed "on water", the addition of a surfactant, namely, sodium dodecyl sulphate , was found effective in terms of substrate scope and yield enhancement. Both methods were successfully used for the gram-scale synthesis of a marketed drug, zolimidine. The simple experimental setup, water as "green" media, and inexpensive catalyst are some of the merits of this protocol.

Aminooxygenation of Ynamides with N-Hydroxybenzotriazoles: Synthesis of α-Benzotriazolyl Carbonyl Compounds

Addition of N-hydroxybenzotriazoles to ynamides causes spontaneous rearrangement, resulting in α-benzotriazolyl imides. The transformation proceeded at rt in the absence of any catalyst but could be efficiently catalyzed by Zn(OTf)₂. Crossover experiments confirmed that the rearrangement is an intramolecular process, most likely via a concerted mechanism. However, heating the mixture above 110 °C resulted in isomerization of N2 into N1 product, via heterolytic C-N bond dissociation. This tandem addition-rearrangement sequence provides an efficient and atom-economical synthetic route for the synthesis of α-benzotriazolyl carbonyl compounds.

Lewis Acid-Catalyzed Intermolecular Annulation: Three-Component Reaction toward Imidazo[1,2-a]pyridine Thiones

A Lewis acid-catalyzed three-component annulation reaction of 2-aminopyridines and ynals with elemental sulfur was established. A series of imidazo[1,2-a]pyridine thiones was obtained in moderate to excellent yields. The merits of this transformation include easily available starting materials, multiple C-heteroatom bond formation in one pot, good functional group tolerance, elemental sulfur as S source, operational simplicity, etc.

Direct Observation of Aryl Gold(I) Carbenes that Undergo Cyclopropanation, C-H Insertion, and Dimerization Reactions

Mesityl gold(I) carbenes lacking heteroatom stabilization or shielding ancillary ligands have been generated and spectroscopically characterized from chloro(mesityl)methylgold(I) carbenoids bearing JohnPhos-type ligands by chloride abstraction with GaCl3 . The aryl carbenes react with PPh3 and alkenes to give stable phosphonium ylides and cyclopropanes, respectively. Oxidation with pyridine N-oxide and intermolecular C-H insertion to cyclohexane have also been observed. In the absence of nucleophiles, a bimolecular reaction, similar to that observed for other metal carbenes, leads to a symmetrical alkene.

Chemodivergent synthesis of N-(pyridin-2-yl)amides and 3-bromoimidazo[1,2-a]pyridines from α-bromoketones and 2-aminopyridines

N-(Pyridin-2-yl)amides and 3-bromoimidazo[1,2-a]pyridines were synthesized respectively from α-bromoketones and 2-aminopyridine under different reaction conditions.

I2-catalyzed intramolecular oxidative amination of C(sp3)–H bond: efficient access to 3-acylimidazo[1,2-a]pyridines under neat condition

An efficient and “green” protocol for the synthesis of 3-acylimidazo[1,2-a]pyridines through intramolecular oxidative α-amination of carbonyl compounds has been developed. The reaction proceeds smoothly utilizing I₂ as a catalyst and H₂O₂ as an oxidant under neat condition with broad substrate scope. Several complex nitrogen-containing fused rings are conveniently constructed, which are not easy to access by traditional methods.

An efficient and green protocol for the synthesis of 3-acylimidazo[1,2-a]pyridines utilizing I₂ as a catalyst and H₂O₂ as an oxidant under neat condition or in water was developed.

Metal-free synthesis of 1,N6-ethenoadenines from N6-propargyl-adenines via NIS mediated radical cascade reaction

In the present paper, an efficient approach for the construction of 1,N⁶-ethenoadenines from conveniently prepared N⁶-propargyl-adenines is developed. This reaction merges N-iodosuccinimide radical initiation and aerobic aminooxygenation in dioxane. This mild, 5-exo-dig, and metal-free cascade reaction could be applied to a wide substrate scope to provide 1,N⁶-ethenoadenines in moderate to good yields. The reaction mechanism was proposed and tested using radical inhibitor (butylated hydroxytoluene) and isotopic labelling (¹⁸O₂) experiments.

An efficient approach for the construction of 1,N⁶-ethenoadenines is developed through the metal-free mediated radical cascade cyclization of conveniently prepared N⁶-propargyl-adenines.

Heterogeneous Gold(I)-Catalysed Annulation between 2-Aminopyridines and Propiolaldehydes Leading to 3-Acylimidazo[1,2-A]Pyridines

The heterogeneous annulation between 2-aminopyridines and propiolaldehydes was achieved in CH2Cl2 at 25 °C in the presence of 3 mol% of MCM-41-immobilised phosphine gold(I) complex (MCM-41-PPh3-AuCl) and AgSbF6 under air, yielding a variety of 3-acylimidazo[1,2-a] pyridines in good yields. This heterogeneous gold(I) catalyst can be easily prepared by a simple procedure, recovered by filtration of the reaction solution and recycled up to seven times without significant loss of activity.

Transition-metal-free regioselective C-H halogenation of imidazo[1,2-a]pyridines: sodium chlorite/bromite as the halogen source

A facile transition-metal-free regioselective halogenation of imidazo[1,2-a]pyridines using sodium chlorite/bromite as the halogen source is presented. The reaction has provided an efficient method for the formation of C-Cl or C-Br bonds to synthesize 3-chloro or 3-bromo-imidazo[1,2-a]pyridines which were then efficiently transformed into imidazo[1,2-a]pyridine core π-systems by Suzuki-Miyaura reactions.

I2-Catalyzed intramolecular dehydrogenative aminooxygenation of alkynes to acylated imidazo[1,2-a]pyridines and indolizines

An efficient and green protocol for the construction of acylated imidazo[1,2-a]pyridines and indolizines via intramolecular dehydrogenative aminooxygenation of alkynes has been developed.

Access to sulfonylated furans or imidazo[1,2-a]pyridines via a metal-free three-component, domino reaction

An efficient three-component reaction for the synthesis of sulfonylated furans or imidazo[1,2-a]pyridines has been developed.

Synthesis of Imidazopyridines via Copper-Catalyzed, Formal Aza-[3 + 2] Cycloaddition Reaction of Pyridine Derivatives with α-Diazo Oxime Ethers

The Cu-catalyzed, formal aza-[3 + 2] cycloaddition reaction of pyridine derivatives with α-diazo oxime ethers in trifluoroethanol was used to synthesize imidazopyridines via the release of molecular nitrogen and elimination of alcohol. These methods enabled modular synthesis of a wide range of N-heterobicyclic compounds such as imidazopyridazines, imidazopyrimidines, and imidazopyrazines with an α-imino Cu-carbenoid generated from the α-diazo oxime ethers and copper.

Synthesis and structures of gold and copper carbene intermediates in catalytic amination of alkynes

Metal carbenes are often proposed as reactive intermediates in the late transition metal-catalysed transformations of alkynes. Owing to their high reactivity, however, isolation and structural characterization of metal carbene intermediates in these transformations has remained unknown. Herein, we report the isolation of two acyclic gold and copper carbene intermediates in either Au(I)- or Cu(I)-catalysed cyclization of N-alkynyl formamidines through five-exo-dig cyclization. X-ray diffraction, ¹³C NMR spectra data and computational analyses provide evidence for the formation of a gold carbene intermediate with a carbocation-like electronic character. Using the intrinsic bond orbital (IBO) approach, we also evaluate the π-stabilizing effects of organic substituents at the carbene carbon atom in the gold carbene intermediate. Another rare six-membered copper carbene complex is also obtained through 6-endo-dig cyclization. These metal carbenes have proven reactive toward oxidation. The metal-promoted cyclization of N-alkynyl formamidine provides a facile approach to synthesize metal carbene species.

Metal carbenes are typically highly reactive and unstable species, proposed as intermediates in numerous catalytic processes. Here the authors report the isolation and characterization of gold and copper carbene intermediates involved in catalytic cyclization reactions.

An efficient and catalyst-free synthesis of N-arylidene-2-arylimidazo[1,2-a]pyridine-3-ylamine derivatives via Strecker reaction under controlled microwave heating

An efficient one-pot multicomponent reaction of 2-aminopyridine with aromatic aldehydes and either benzoyl cyanide or cyanamide in pyridine under controlled microwave heating afforded N-aryldene-2-arylimidazo[1,2-a]-pyridine-3-ylamine derivatives. The reaction is catalyst free and is of high atom economy.

Silver-catalyzed [3 + 2] domino reaction: an efficient strategy to synthesize imidazole-5-carbaldehydes

An unprecedented regioselective silver-catalyzed [3 + 2] domino reaction of amidines and ynals for the formation of C–N bonds has been developed.

Copper catalyzed aerobic oxidative cyclization and ketonization: one pot synthesis of benzoimidazo[1,2-a]imidazolones

A highly efficient synthesis of benzoimidazo[1,2-a]imidazolone through a novel oxidative 5-exo-dig cyclization-ketonization cascade of 2-aminobenzimidazole, aldehyde and terminal alkyne has been explored under aerobic conditions.

One-step synthesis of imidazo[1,2-a]pyridines in water

A novel efficient and convenient synthesis of imidazo[1,2-a]pyridines in water has been developed.