Direct catalytic C–H arylation of imidazo[1,2-a]pyridine with aryl bromides using magnetically recyclable Pd–Fe3O4 nanoparticles

Synthesis and site selective C-H functionalization of imidazo-[1,2-a]pyridines

Imidazo[1,2-a]pyridine has attracted much interest in drug development because of its potent medicinal properties, therefore the discovery of novel methods for its synthesis and functionalization continues to be an exciting area of research. Although transition metal catalysis has fuelled the most significant developments, extremely beneficial metal-free approaches have also been identified. Even though pertinent reviews focused on imidazo[1,2-a]pyridine synthesis, properties (physicochemical and medicinal), and functionalization at the C3 position have been published, none of these reviews has focused on the outcomes obtained in the field of global ring functionalization. We wish here to describe a brief synthesis and an overview of all the functionalization reactions at each carbon atom, viz, C2, C3, C5, C6, C7 and C8 of this scaffold, divided into sections based on site-selectivity and the type of functionalization methods used.

From Structure to Function: Understanding Synthetic Conditions in Relation to Magnetic Properties of Hybrid Pd/Fe-Oxide Nanoparticles

Heterostructured magnetic nanoparticles show great potential for numerous applications in biomedicine due to their ability to express multiple functionalities in a single structure. Magnetic properties are generally determined by the morphological characteristics of nanoparticles, such as the size/shape, and composition of the nanocrystals. These in turn are highly dependent on the synthetic conditions applied. Additionally, incorporation of a non-magnetic heterometal influences the final magnetic behavior. Therefore, construction of multifunctional hybrid nanoparticles with preserved magnetic properties represents a certain nanotechnological challenge. Here, we focus on palladium/iron oxide nanoparticles designed for combined brachytherapy, the internal form of radiotherapy, and MRI-guided hyperthermia of tumors. The choice of palladium forming the nanoparticle core is envisioned for the eventual radiolabeling with ¹⁰³Pd to enable the combination of hyperthermia with brachytherapy, the latter being beyond the scope of the present study. At this stage, we investigated the synthetic mechanisms and their effects on the final magnetic properties of the hybrid nanoparticles. Thermal decomposition was applied for the synthesis of Pd/Fe-oxide nanoparticles via both, one-pot and seed-mediated processes. The latter method was found to provide better control over morphology of the nanoparticles and was therefore examined closely by varying reaction conditions. This resulted in several batches of Pd/Fe-oxide nanoparticles, whose magnetic properties were evaluated, revealing the most relevant synthetic parameters leading to promising performance in hyperthermia and MRI.

Magnetically recoverable nickel-palladium alloy nanocatalysts for direct C-H arylation reactions

Novel magnetically recoverable nanocatalyst comprising nickel-palladium (NiPd) alloy nanoparticles (NPs) supported on reduced graphene oxide (rGO) modified with cobalt ferrite (CoFe₂O₄) NPs was fabricated for the direct C-H arylation of imidazopyridine, imidazole, indolizine and furan with aryl halides. To prepare the presented catalyst, rGO nanosheets were first modified with as-synthesized CoFe₂O₄ NPs and then the obtained CoFe₂O₄-rGO nanocomposites served as a support material for the synthesis of bimetallic NiPd alloy NPs at various compositions. The obtained CoFe₂O₄-rGO/NiPd nanocatalysts were characterized by many advanced analytical techniques including TEM, STEM-EDS, XRD, XPS, and ICP-MS. Next, to optimize the reaction conditions, CoFe₂O₄-rGO/NiPd nanocatalysts with different alloy compositions and their monometallic counterparts (CoFe₂O₄-rGO/Ni and CoFe₂O₄-rGO/Pd) were initially tested in the direct C-H arylation of imidazopyridine with bromobenzene. Among all tested nanocatalysts under the optimum reaction conditions, CoFe₂O₄-rGO/Ni₂₀Pd₈₀ showed the best catalytic activity in terms of the isolated product yields. The C-H arylation reactions were studied over a broad substrate scope (35 examples from 36 substrates) and gave the related biaryl products in good to excellent yields. Besides a broad substrate scope, the late-stage C-H arylation of zolimidine, a gastroprotective drug, was realized under the optimized reaction conditions. Moreover, the CoFe₂O₄-rGO/Ni₂₀Pd₈₀ nanocatalysts were recovered from the reaction medium using a simple magnet and reused in the C-H arylation reactions up to five consecutive runs without a significant drop in the product yield. This study shows that magnetically recoverable Pd nanoalloys are promising heterogeneous catalysts to be used in sustainable C-H functionalization reactions.

Oxidative N-Formylation of Secondary Amines Catalyzed by Reusable Bimetallic AuPd-Fe3O4 Nanoparticles

Bimetallic catalysts are gaining attention due to their characteristics of promoting reactivity and selectivity in catalyzed reactions. Herein, a new catalytic N-formylation of secondary amines using AuPd–Fe₃O₄ at room temperature is reported. Methanol was utilized as the formyl source and 1.0 atm of O₂ gas served as an external oxidant. The bimetallic catalyst, consisting of Au and Pd, makes the reaction more efficient than that using each metal separately. In addition, the catalyst can be effectively recycled owing to the Fe₃O₄ support.

Metal Nanoparticles as Sustainable Tools for C-N Bond Formation via C-H Activation

The design of highly active metal nanoparticles to be employed as efficient heterogeneous catalysts is a key tool for the construction of complex organic molecules and the minimization of their environmental costs. The formation of novel C-N bonds via C-H activation is an effective atom-economical strategy to access high value materials in pharmaceuticals, polymers, and natural product production. In this contribution, the literature of the last ten years on the use of metal nanoparticles in the processes involving direct C-N bond formation will be discussed. Where possible, a discussion on the role and influence of the support used for the immobilization and/or the metal chosen is reported. Particular attention was given to the description of the experiments performed to elucidate the active mechanism.

Diversity-Oriented Synthesis toward Aryl- and Phosphoryl-Functionalized Imidazo[1,2-a]pyridines

We report herein an efficient synthesis of diversely polysubstituted imidazo[1,2-a]pyridines, a family of aza-heterocycles endowed with numerous biological properties, through a sequence involving two consecutive palladium-catalyzed cross-coupling reactions. First, we demonstrated that a Hirao coupling occurred straightforwardly in high yields at positions 3, 5, and 6 of imidazopyridine derivatives, giving access to a wide variety of substituted phosphonates, phosphinates, and phosphine oxides. In a second step, direct CH-arylation of phosphorylimidazopyridines with aryl halides was found to be effective and fully selective, leading to 3-aryl-substituted imidazopyridines in moderate to high yields depending on steric hindrance.

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.

Selective, C-3 Friedel-Crafts acylation to generate functionally diverse, acetylated Imidazo[1,2-a]pyridine derivatives

Carbon-carbon bonds are integral for pharmaceutical discovery and development. Frequently, C-C bond reactions utilize expensive catalyst/ligand combinations and/or are low yielding, which can increase time and expenditures in pharmaceutical development. To enhance C-C bond formation protocols, we developed a highly efficient, selective, and combinatorially applicable Friedel-Crafts acylation to acetylate the C-3 position of imidazo[1,2-a]pyridines. The reaction, catalyzed by aluminum chloride, is both cost effective and more combinatorial friendly compared to acetylation reactions requiring multiple, stoichiometric equivalents of AlCl3. The protocol has broad application in the construction of acetylated imidazo[1,2-a]pyridines with an extensive substrate scope. All starting materials are common and the reaction requires inexpensive, conventional heating methods for adaptation in any laboratory. Further, the synthesized compounds are predicted to possess GABA activity through a validated, GABA binding model. The developed method serves as a superior route to generate C-3 acetylated imidazo[1,2-a]pyridine building-blocks for combinatorial synthetic efforts.

Green synthesis of the Ag/ZnO nanocomposite using Valeriana officinalis L. root extract: application as a reusable catalyst for the reduction of organic dyes in a very short time

A facile and green synthesis of the Ag/ZnO nanocomposite by extract of Valeriana officinalis L. root in the absence of any stabiliser or surfactant has been reported in this work. The green synthesised Ag/ZnO nanocomposite was characterised by Field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy dispersive X‐ray spectroscopy (EDS), elemental mapping, Fourier‐Transform infrared (FT‐IR), X‐ray diffraction analysis (XRD) and UV‐Vis spectroscopy. According to SEM and TEM images, the Ag and ZnO particles are spherical with diameters of less than 20 and 40–50 nm, respectively. The Ag NPs/ZnO nanocomposite proved to be an effective catalyst in the reduction of various dyes including methyl orange (MO), Congo red (CR) and methylene blue (MB) in the presence of NaBH4 in aqueous media at ambient temperature. A maximum degradation (100%) of dyes was performed using Ag/ZnO nanocomposite. The extraordinary performance of the prepared Ag/ZnO nanocomposite is attributed to the synergetic effect induced by both ZnO and Ag NPs in the catalytic degradation of organic dyes. The catalyst could be reused and recovered several times with no significant loss of catalytic activity.

N-Heterocyclic carbene–palladium(ii)–1-methylimidazole complex catalyzed direct C–H bond arylation of imidazo[1,2-a]pyridines with aryl chlorides

NHC–Pd(ii) complex catalyzed C–H bond arylation of imidazo[1,2-a]pyridines with aryl chlorides was reported.

Palladium nanoparticles supported on organofunctionalized kaolin as an efficient heterogeneous catalyst for directed C–H functionalization of arylpyrazoles

A heterogeneous catalyst system based on the immobilization of Pd⁰ nanoparticles onto organofunctionalized kaolin is reported with a view to introducing new synthetic routes of directed C–H functionalization of arylpyrazoles.

Heterogenized Bimetallic Pd-Pt-Fe3O4 Nanoflakes as Extremely Robust, Magnetically Recyclable Catalysts for Chemoselective Nitroarene Reduction

A very simple synthesis of bimetallic Pd-Pt-Fe3O4 nanoflake-shaped alloy nanoparticles (NPs) for cascade catalytic reactions such as dehydrogenation of ammonia-borane (AB) followed by the reduction of nitro compounds (R-NO2) to anilines or alkylamines (R-NH2) in methanol at ambient temperature is described. The Pd-Pt-Fe3O4 NPs were easily prepared via a solution phase hydrothermal method involving the simple one-pot coreduction of potassium tetrachloroplatinate (II) and palladium chloride (II) in polyvinylpyrrolidone with subsequent deposition on commercially available Fe3O4 NPs. The bimetallic Pd-Pt alloy NPs decorated on Fe3O4 NPs provide a unique synergistic effect for the catalysis of cascade dehydrogenation/reduction. Various nitroarene derivatives were reduced to anilines with very specific chemoselectivity in the presence of other reducible functional groups. The bimetallic Pd-Pt-Fe3O4 NPs provide a unique synergistic effect for the catalysis of cascade dehydrogenation/reduction. The nitro reduction proceeded in 5 min with nearly quantitative conversions and yields. Furthermore, the magnetically recyclable nanocatalysts were readily separated using an external magnet and reused up to 250 times without any loss of catalytic activity. A larger scale (10 mmol) reaction was also successfully performed with >99% yield. This efficient, recyclable Pd-Pt-Fe3O4 NPs system can therefore be repetitively utilized for the reduction of various nitro-containing compounds.

Catalytic C-H bond functionalisation chemistry: the case for quasi-heterogeneous catalysis

This feature article examines the potential of heterogeneous Pd species to mediate catalytic C-H bond functionalisation processes employing suitable substrates (e.g. aromatic/heteroaromatic compounds). A focus is placed on the reactivity of supported and non-supported Pd nanoparticle (PdNPs) catalysts, in addition to the re-appropriation of well-established heterogeneous Pd catalysts such as Pd/C. Where possible, reasonable comparisons are made between PdNPs and traditional 'homogeneous' Pd precatalyst sources (which form PdNPs). The involvement of higher order Pd species in traditional cross-coupling processes, such as Mizoroki-Heck, Sonogashira and Suzuki-Miyaura reactions, allows the exemplification of potential future topics for study in the area of catalytic C-H bond functionalisation processes.

An unprecedented Pd-catalyzed decarboxylative coupling reaction of aromatic carboxylic acids in aqueous medium under air: synthesis of 3-aryl-imidazo[1,2-a]pyridines from aryl chlorides

An efficient and practical protocol for palladium-catalyzed decarboxylative arylation of imidazo[1,2-a]pyridine-3-carboxylic acids with aryl chlorides has been developed. Note that the reaction could proceed smoothly without an additive in aqueous medium under an ambient atmosphere, and the addition of H2O could effectively promote the decarboxylative arylation. Particularly noteworthy is that these results represent the first examples of Pd-catalyzed decarboxylative coupling reactions of (hetero) aromatic carboxylic acids in aqueous medium under air, and the first successful examples of the synthesis of 3-aryl-imidazo[1,2-a]pyridines using cheap, diverse aryl chlorides and heteroaryl chlorides as the starting materials.

Oxidative organophotoredox catalysis: a regioselective synthesis of 2-nitro substituted imidazopyridines and 3-substituted indoles, initiated by visible light

Intramolecular C–N heterocyclization and C–C bond formation under visible light irradiation at room temperature was accomplished with a metal-free photoredox catalyst.

Double head-to-tail direct arylation as a viable strategy towards the synthesis of the aza-analog of dihydrocyclopenta[hi]aceanthrylene – an intriguing antiaromatic heterocycle

Straightforwardly prepared head-to-tail bis-imidazo[1,2-a]pyridine displays antiaromaticity and no fluorescence.

Ligand-free Pd-catalysed decarboxylative arylation of imidazo[1,2-a]pyridine-3-carboxylic acids with aryl bromides

A facile ligand-free method for Pd(OAc)₂ catalysed decarboxylative arylation of imidazo[1,2-a]pyridine-3-carboxylic acids with hetero(aryl) bromides has been developed.

Direct arylation and heterogeneous catalysis; ever the twain shall meet

The formation of aryl-aryl bonds and heteroaryl analogues is one of the most important C-C bond forming processes in organic chemistry. Recently, a methodology termed Direct Arylation (DA) has emerged as an attractive alternative to traditional cross-coupling reactions (Suzuki-Miyaura, Stille, Negishi, etc.). A parallel focus of the pharmaceutical and other chemical industries has been on the use heterogeneous catalysis as a favourable substitute for its homogeneous counterpart in cross-coupling reactions. Only very recently has heterogeneous catalysis been proposed and applied, to DA reactions. In this perspective, we consider the terms 'heterogeneous' and 'homogeneous' and the problems associated with their delineation in transition-metal catalysed reactions. We highlight the reports at the interface of DA and heterogeneous catalysis and we comment briefly on the methods used which attempt to classify reaction types as homo- or heterogeneous. In future work we recommend an emphasis be placed on kinetic methods which provide an excellent platform for analysis. In addition two analytical techniques are described which if developed to run in situ with DA reactions would illuminate our understanding of the catalysis. Overall, we provide an entry point, and bring together the mature, yet poorly-understood, subject of heterogeneous catalysis with the rapidly expanding area of DA, with a view towards the acceleration of catalyst design and the understanding of catalyst behaviour.

Computer aided drug discovery of highly ligand efficient, low molecular weight imidazopyridine analogs as FLT3 inhibitors

The FLT3 kinase represents an attractive target to effectively treat AML. Unfortunately, no FLT3 targeted therapeutic is currently approved. In line with our continued interests in treating kinase related disease for anti-FLT3 mutant activity, we utilized pioneering synthetic methodology in combination with computer aided drug discovery and identified low molecular weight, highly ligand efficient, FLT3 kinase inhibitors. Compounds were analyzed for biochemical inhibition, their ability to selectively inhibit cell proliferation, for FLT3 mutant activity, and preliminary aqueous solubility. Validated hits were discovered that can serve as starting platforms for lead candidates.

Polycyclic imidazo[1,2-a]pyridine analogs – synthesis via oxidative intramolecular C–H amination and optical properties

Oxidative C–H amination has proved to be an efficient strategy to construct pyrido[2′,1′:2,3]imidazo[4,5-b]indoles and their π-expanded analogs.

Palladium(0)-catalyzed direct C–H hetero-arylation of 2-arylimidazo [1,2-a]pyridines with (E)-1-(5-bromothiophen-2-yl)-3-arylprop-2-en-1-ones and their anticancer activity

An efficient palladium(0)-catalyzed direct hetero-arylation of imidazo[1,2-a]pyridines at the C-3 position of the imidazole ring has been described.

Direct trifluoromethylation of imidazoheterocycles in a recyclable medium at room temperature

Regioselective C–H trifluoromethylation of imidazoheterocycles with Langlois' reagent in a recyclable mixed medium of 1-butyl-3-methylimidazoliumtetrafluoroborate ([Bmim]BF₄) and water at room temperature has been developed.

Advances in direct C–H arylation of 5,5- 6,5- and 6,6-fused-heterocycles containing heteroatoms (N, O, S)

This report aims to review the advances made in C–H arylation of 5,6, 6,6 and 5,5 fused-heterocyclic systems.

Vertically-expanded imidazo[1,2-a]pyridines and imidazo[1,5-a]pyridine via dehydrogenative coupling

The intramolecular dehydrogenative coupling mediated by potassium constitutes the general methodology leading to weakly emitting π-expanded heterocycles.

A magnetic supported iron complex for selective oxidation of sulfides to sulfoxides using 30% hydrogen peroxide at room temperature

Selective oxidation of sulfides to corresponding sulfoxides using H₂O₂ as a green oxidant at room temperature have been investigated by a magnetic supported iron (iron(ii) acetylacetonate) as an efficient and recyclable heterogeneous catalyst with excellent sulfide conversion and good sulfoxide selectivity.

Synthesis of benzil derivatives via oxidation of alkynes catalyzed by Pd–Fe3O4 heterodimer nanocrystals

An efficient, iterative, catalytic, Wacker-type oxidation of alkynes to 1,2-diketones using a Pd–Fe₃O₄ heterodimer nanocrystalline catalyst has been developed.

Ionic tagged DABCO grafted on magnetic nanoparticles: a water-compatible catalyst for the aqueous aza-Michael addition of amines to α,β-unsaturated amides

A water compatible DABCO grafted on magnetic nanoparticles with an ionic tag has been developed and used to catalyze the aza-Michael addition.