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Review

Recent Advances in Visible Light-Induced C-H Functionalization of Imidazo[1,2-a]pyridines

by
Juanjuan Gao
,
Xinlei Fu
,
Kai Yang
and
Zhaowen Liu
*
Jiangxi Province Key Laboratory of Pharmacology of Traditional Chinese Medicine, College of Pharmacy, Gannan Medical University, Ganzhou 341000, China
*
Author to whom correspondence should be addressed.
Molecules 2025, 30(3), 607; https://doi.org/10.3390/molecules30030607
Submission received: 16 January 2025 / Revised: 27 January 2025 / Accepted: 27 January 2025 / Published: 30 January 2025
(This article belongs to the Special Issue Heterocyclic Compounds: Synthesis, Application and Theoretical Study)
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Abstract

The imidazo[1,2-a]pyridine skeleton is widely present in many natural products and pharmaceutical agents. Due to its impressive and significant biological activities, such as analgesic, anti-tumor, antiosteoporosis, and anxiolytic properties, the derivatization of imidazo[1,2-a]pyridine skeleton has attracted widespread attention from chemists. In recent years, significant progress has been made in the derivatization of imidazo[1,2-a]pyridines through direct C-H functionalization, especially through visible light induction. This review highlights recent advances in visible light-induced C-H functionalization of imidazo[1,2-a]pyridines during the past ten years, and some reaction mechanisms are also discussed.

1. Introduction

Imidazo[1,2-a]pyridine is an important class of nitrogen-containing heterocyclic compounds and widely presents in natural products, pharmaceutical molecules, functional materials, and agrochemicals [1,2]. Imidazo[1,2-a]pyridine derivatives have important biological activities and pharmacological effects, including anticancer [3], antituberculosis [4], antibacterial [5], antiviral [6], anti-inflammatory [7], antidiabetic [8], and antiplasmodial [9] properties, etc. Moreover, some marketed drugs for clinical use, such as Miroprofen, Zolimidine, Olprinone, Minodronic acid, Alpidem, Necopidem, Soraprazan, Saripidem, and Zolpidem, are derived from the skeleton of imidazo[1,2-a]pyridine (Figure 1). Therefore, the development of the functionalization methods of imidazo[1,2-a]pyridine derivatives has important application value and practical significance [10,11].
So far, a series of synthetic methods have been developed to obtain diverse imidazo[1,2-a]pyridines. The strategy of C-H bond functionalization is considered a powerful approach for rapidly introducing functional groups into imidazo[1,2-a]pyridine skeleton, due to its relative straightforwardness, high atomic economy, and synthetic step economy [12,13,14,15]. However, this strategy still has some limitations, including harsh reaction conditions, strong oxidants, and expensive metal salt catalysts [16,17,18,19]. There is still a lot of room for developing green, simple, and sustainable synthesis methods. Photochemical synthesis is considered one of the most energy-efficient and eco-friendly methods, and can promote the activation of organic molecules through single electron transfer (SET) processes, thereby achieving various organic transformations [20,21,22,23]. In recent years, more and more chemical workers have achieved C-H functionalization of imidazo[1,2-a]pyridines through photochemical synthesis strategies. This article provides an overview of the relevant research in this field over the past ten years, with the aim of providing assistance for future research in this area.

2. C-C Bond Formation

2.1. C3-Fluoroalkylation of Imidazo[1,2-a]pyridines

Fluoroalkyl fragments play a crucial role in the fields of pharmaceuticals and materials science. The introduction of fluoroalkyl groups into heterocyclic frameworks can cause significant changes in the physicochemical and biological properties of the compounds [24,25]. Therefore, great attention has been paid to developing efficient synthesis methods for constructing fluoroalkyl imidazo[1,2-a]pyridine derivatives [26,27,28]. Langlois’ reagent (sodium triflinate, CF3SO2Na), a classic trifluoromethylation reagent, is commonly used to incorporate the CF3 group into imidazo[1,2-a]pyridine frameworks [29]. In 2016, Rueping’s study group first reported the trifluoromethylation of imidazo[1,2-a]pyridines under near-UV irradiation (λ = 350 nm) by using 4,4′-dimethoxybenzophenone as a photoredox catalyst (Scheme 1) [30]. However, this article only reported an example of the C3-trifluoromethylation of imidazo[1,2-a]pyridine, providing 2-phenyl-3-trifluoromethylimidazo[1,2-a]pyridine 3a in a 42% yield.
In 2019, Zhang’s study group [31] reported a transition-metal-free, visible light-induced method for the trifluoromethylation of imidazo[1,2-a]pyridines, by using anthraquinone-2-carboxylic acid (AQN-2-CO2H) as a photoorganocatalyst in the presence of K2CO3 and trifluoroacetic acid (Scheme 2a). Under the optimized reaction conditions, the authors subsequently investigated the substrates’ scope (Scheme 2b). A variety of substituents, such as methyl, methoxyl, phenyl, fluoro, chloro, bromo, trifluoromethyl, and methylsulfonyl groups on the C2-phenyl ring of 2-phenyl imidazo[1,2-a]pyridines were well tolerated under the standard conditions, giving the desired products 3a–3m in 49–81% yields. Moreover, 2-(3,4-Difluorophenyl)imidazo[1,2-a]pyridine and 2-(naphthalen-1-yl)- imidazo[1,2-a]pyridine were also suitable for this conversion, to afford the corresponding products 3n and 3o, respectively. Substrates with the methyl or halogen group on the pyridine ring could proceed smoothly under the standard conditions and provided the desired products 3p–3r. Both 2-thienylimidazo[1,2-a]pyridine and 2-methylimidazo- [1,2-a]pyridine could also provide the corresponding products 3s and 3t in 71% and 50% yields, respectively. In addition, 2-Alkenyl chain imidazo[1,2-a]pyridine and imidazo[1,2-a]pyridine could be converted into the desired products under the standard conditions, but the yields were relatively low (3u, 29% and 3v, 11%).
The radical pathway of the reaction was established by radical scavenging experiments. The plausible mechanism of this reaction has been described in Scheme 2c. Firstly, CF3 radical was generated through photoredox-catalyzed cyclization reactions. Subsequently, the CF3 radical reacted with imidazo[1,2-a]pyridine to form the radical intermediate 3A. Then, 3A was oxidized by the CF3 radical to obtain the carbocation intermediate 3B. Finally, 3B underwent deprotonation to form the final product 3.
In 2019, Cui’s study group also achieved visible light-promoted C-H trifluoromethylation of imidazopyridines by using mesityl acridinium as a photoredox catalyst at room temperature (Scheme 3) [32]. Unfortunately, the unsubstituted imidazo[1,2-a]pyridines on the C2 position could not proceed smoothly in this catalytic system.
Polyfluoroalkyl halides can also be used for the visible light-induced polyfluoroalkylation of imidazo[1,2-a]pyridines. For example, Xu’s research group developed a visible light-catalyzed C3 trifluoroethylation of imidazo[1,2-a]pyridines by using 1,1,1-trifluoro-2-iodoethane 4a as a trifluoroethyl reagent (Scheme 4a) [33]. The photocatalyst fac-Ir(ppy)3 was essential for this conversion. Various substituted 2-arylimidazopyridines could obtain the corresponding products with moderate to excellent yields (Scheme 4b), indicating that the protocol has good substrate applicability. It is noteworthy that 2-methylimidazo[1,2-a]pyridine was also suitable for this conversion, yielding the desired product 5n in a 77% yield.
A plausible mechanism was presented in Scheme 4c. Initially, fac-Ir(ppy)3 was transformed into an excited state [fac-Ir(III)(ppy)3*] under light irradiation, and then oxidized by CF3CH2I to generate a [fac-Ir(IV)(ppy)3]+ complex and a •CH2CF3 radical species 4A. Subsequently, 4A reacted with imidazo[1,2-a]pyridine 1a to form the radical intermediate 4B, and was oxidized by [fac-Ir(IV)(ppy)3]+ to produce the carbocation intermediate 4C. Finally, 4C underwent deprotonation to form the final product 5a.
A similar synthetic strategy can also achieve the difluoroacetylation and (phenylsulfonyl)difluoromethylation of imidazo[1,2-a]pyridines, respectively [34,35]. For example, Fu’s research group developed a visible light-mediated protocol for direct difluoroacetylation of imidazo[1,2-a]pyridines with BrCF2CO2Et by using fac-Ir(ppy)3 as a photocatalyst in the presence of K2CO3, yielding the corresponding products in 64–94% yields (Scheme 5a) [34]. PhSO2CF2I also achieved the same transformation, resulting in the (phenylsulfonyl)difluoromethylation of imidazo[1,2-a]pyridines in 62–91% yields (Scheme 5b) [35].
In addition to metal photocatalysts, many organic photocatalysts also exhibit good catalytic activity in visible light-induced reactions, and are widely used in the field of organic synthesis. In 2019, Hajra’s research group reported the organophotoredox-catalyzed difluoromethylenephosphonation of imidazo[1,2-a]pyridines under visible light irradiation by using rose bengal (RB) as a photoredox catalyst (Scheme 6) [36]. The desired coupling product was not formed in the absence of Bis(pinacolato)diboron (B2pin2), indicating that its presence was essential in this reaction. Other heterocycles, such as imidazo[2,1-b]thiazole, benzo[d]imidazo-[2,1-b]thiazole, and indole, were also suitable for this conversion. Mechanism experiments showed that the reaction possibly proceeds through a single electron transfer (SET) process.
Photoinduced radical reactions driven by electron donor–acceptor (EDA) complexes are also widely used in the C-H functionalization of imidazo[1,2-a]pyridines. In 2021, Zhu’s group reported a visible light-induced perfluoroalkylation of the imidazo[1,2-a]pyridines with perfluoroalkyl iodides via photoactive EDA complexes at room temperature (Scheme 7a) [37]. This method showcases a broad substrate scope and wide functional group tolerance. A series of 3-perfluoroalkyl substituted imidazo[1,2-a]pyridines could be obtained with moderate to excellent yields (Scheme 7b). It should be noted that 5-methyl-2-phenylimidazo[1,2-a]pyridine could not provide the desired product 13w under the conditions. A possible visible light-induced mechanism through photoactive EDA complexes was proposed in Scheme 7c [38,39]. In the same year, a similar strategy of perfluoroalkylation via photoactive EDA complexes and DUB as a base was reported by Wu’s group [40].
In 2019, Xu et al. reported photochemical direct C-H difluoroalkylation of imidazo[1,2-a]pyridines with bromodifluoromethylaryl ketones via visible light irradiation (Scheme 8) [41]. Tetramethylethylenediamine (TMEDA), a mild organic base, played a key role in this reaction by removing the proton of a radical intermediate to obtain a radical anion. Compared with the 33 W compact fluorescent light (CFL), the reaction efficiency was very low under the 5 W blue light-emitting diode (LED), with a wavelength of only ~460 nm.

2.2. C3-Cyanomethylation of Imidazo[1,2-a]pyridines

In the field of organic synthesis and drug synthesis, cyano groups can be converted into various useful functional groups [42]. Therefore, the cyanation of heterocyclic compounds has received widespread attention from organic chemists [43]. In 2017, Sun’s group explored a visible light-promoted protocol for C3 cyanomethylation of imidazo[1,2-a]pyridines with bromoacetonitrile by using fac-Ir(ppy)3 as a photocatalyst (Scheme 9a) [44]. The substrates bearing electron-donating groups (such as methyl, methoxyl, tertbutyl, and phenyl) and electron withdrawing groups (such as Cl-, Br-, and CF3) on the para- or meta- position of the benzene ring could produce the desired products 17a–17i in 65–96% yields (Scheme 9b). However, the reaction efficiency of ortho-Cl substrates was significantly reduced (17j, 12% and 17k, 30%). Furthermore, 2-Phenyl imidazo[1,2-a]pyridines with the Me-, Cl-, and Br- groups on the pyridine ring proceeded smoothly and provided the corresponding products 17l–17p in moderate to excellent yields (52–93%). Imidazo[1,2-a]pyridines with an aryl group (such as thiophen-2-yl, naphthalen-1-yl, or naphthalen-2-yl) or an alkyl group (such as methyl or isobutyl) on the C2 position also worked well and afforded the corresponding products 17t–17x in satisfactory yields. A possible mechanism for visible light-induced cyanomethylation by using fac-Ir(ppy)3 as a photocatalyst was proposed in Scheme 9c.
The effective synthesis of Zolpidem and Alpidem can be achieved via the derivatization of C3 cyanomethylation products [45]. Firstly, the 3-cyanomethylation products 17r and 17s undergo hydrolysis and an esterification reaction to obtain the corresponding esters 18a and 18b, followed by aminating to obtain Zolpidem (19a) and Alpidem (19b), respectively (Scheme 10).

2.3. C3-Aminoalkylation of Imidazo[1,2-a]pyridines

The direct α-amino C-H functionalization by photocatalysis can achieve the preparation of important pharmaceutical skeletons via the formation of C-C and C-heteroatom bonds, which is of great significance in organic synthesis. In 2018, Hajra’s group reported an aerobic visible light-promoted oxidative cross-dehydrogenative coupling between imidazo[1,2a]pyridines and N-phenyltetrahydroisoquinoline by using rose bengal (RB) as a photocatalyst (Scheme 11a) [46].
Imidazo[1,2-a]pyridine substrates with a series of substituents, such as methyl, methoxyl, fluoro, chloro, bromo, cyan, trifluoromethyl, hydroxyl, methylsulfonyl, cyclopropyl, thiophen-2-yl, and naphthalen-2-yl on the benzene or pyridine ring were well tolerated, and gave the corresponding products 21a–21t in 78–91% yields (Scheme 11b). Various substituted N-aryl tetrahydroisoquinolines bearing methyl, bromo, and trifluoromethyl groups could also provide the desired products 21u–21z in 78–86% yields. A plausible mechanistic pathway was shown in Scheme 11c. Based on the redox potential values of excited state RB* and N-phenyltetrahydroisoquinoline, it is believed that this reaction may be involved in energy transfer processes rather than photoredox catalysis.
Excitingly, N, N-dimethylaniline substrates were also suitable for the protocol, giving the coupling aminomethylation products 23a–23c in 82–86% yields (Scheme 12). However, only three examples were reported in this paper.
Visible light-induced decarboxylation coupling is another strategy for achieving the aminomethylation of imidazo[1,2-a]pyridines [47]. In 2019, Le et al. reported visible light-promoted decarboxylative aminomethylation of imidazo[1,2-a]pyridines with N-aryl glycines in the air in the absence of expensive photosensitizers and extra additives at room temperature (Scheme 13a) [48]. As shown in Scheme 13b, an array of N-aryl glycines with electron-donating groups (such as methyl and methoxyl) or halogen (such as chloro, bromo, and iodine) on the aryl rings could react smoothly with 2-phenylimidazo[1,2-a]pyridines, providing the corresponding products 25a–25g in 50–82% yields. Various imidazo[1,2-a]pyridines containing aryl and methyl substituents on the C2 position could also react with N-4-methylphenylglycine, providing the corresponding products 25h–25x in 40–95% yields. Overall, this protocol exhibited good functional group tolerance and substrate applicability.
A plausible mechanism for this decarboxylative coupling process was also proposed, as shown in Scheme 13c. Firstly, singlet oxygen generated from O2 under blue LED illumination reacted with N-aryl glycine 24 to form amino cation radical 24A and superoxide radical anion (O2−•). Subsequently, superoxide radical anion (O2 −•) induced 24A to undergo a proton transfer and decarboxylation process, transforming it into α-amino radical 24B. Then, α-amino radical 24B was further oxidized by HO2• to obtain imine intermediate 24C. Finally, 24C underwent electrophilic addition with 2-phenylimidazo[1,2-a]pyridine 1a to obtain the final product 25.
In 2020, Yu et al. developed a decarboxylative coupling method for the direct aminomethylation of imidazo[1,2-a]pyridines with N-aryl glycines by using 5 mol% CsPbBr3 as a photocatalyst under the irradiation of white LEDs (Scheme 14) [49]. Both imidazo[1,2-a]pyridines and N-aryl glycine substrates with different substituents were investigated, providing the corresponding product 25 in 44–94% yields. The recycling experiments showed that the photocatalyst CsPbBr3 has a high catalyst economy, can be reused at least five times, and has no significant decrease in activity. Meanwhile, a gram-scale experiment (5 mmol) was also successfully carried out, obtaining the desired N-((2-phenylimidazo[1,2-a]pyridin-3-yl)methyl)aniline 25a (1.32 g) in an 87% yield.
A dehydrogenation coupling reaction can also be achieved between imidazo[1,2-a]pyridines and N-arylglycine esters (26) under blue light irradiation by using copper (II) salt as the sole catalyst, resulting in α-heteroaryl-substituted α-amino acid ester derivatives (27) with good to excellent yields (Scheme 15a) [50]. It should be pointed out that N-arylglycine ester substrates with strong electron-withdrawing groups (cyano, nitro, and triflouromethyl) on the N-benzene rings could not transform to the desired products under the optimized conditions. The authors proposed a possible mechanism for this conversion, as shown in Scheme 15b. Initially, Copper (II) induced the conversion of N-aryl glycine ester 26a into intermediate 26A ([Cu(I)-NH+•]) via a single electron transfer (SET) [51]. Concurrently, singlet oxygen produced via the reaction of 26A and molecular oxygen under visible light irradiation further oxidized the NH+• of 26 into imine intermediate 26B. Finally, the electrophilic addition process occurred between 26B and 1a, resulting in the desired product 27a.

2.4. C3-Formylation of Imidazo[1,2-a]pyridines

The formyl group is an important functional group in the derivatization of organic and pharmaceutical molecules, so the formylation of imidazopyridine has always been a research hotspot for chemists [52,53]. In 2018, Hajra’s group reported a visible light-induced rose bengal-catalyzed C3 formylation of imidazo[1,2a]pyridines with tetramethylethylenediamine (TMEDA) (Scheme 16a) [54]. As shown in Scheme 16b, this method exhibited good substrate applicability and provided the corresponding products in 81–95% yields. In addition, 2-phenyl imidazo[1,2a]pyridines (1a) and TMEDA were subjected to a gram-scale reaction (6 mmol), resulting in 29a in a 79% yield (1.053 g). The control experiments indicated that the reaction may undergo the superoxide radical pathway. A plausible mechanism for the photocatalytic formylation process was proposed in Scheme 16c.

2.5. C3-Alkoxycarbonylation of Imidazo[1,2-a]pyridines

Alkoxycarbonylation of imidazo[1,2-a]pyridine is a meaningful synthetic conversion, as the introduced ester groups [55] can be further converted into various functional groups, such as amide, hydroxymethyl, and carbonyl, etc. In 2022, Kshirsagar’s group developed a rose bengal-catalyzed C3-alkoxycarbonylation of imidazo[1,2-a]pyridines with carbazates by using (NH4)2S2O8 as a oxidant under blue LED illumination at room temperature (Scheme 17a) [56]. As shown in Scheme 17b, the substrate suitability study showed that this transformation has good functional group tolerance. Both methyl carbazate 30a and ethyl carbazate 30b were suitable for this reaction and provided the corresponding products 31a and 31b in 71% and 61% yields, respectively. However, tertbutyl carbazate could not smoothly convert to the desired product 31c under the optimal conditions. The 2-arylimidazopyridines substrates with different substituents (such as methyl, methoxy, halogen, and nitro) were also compatible with this system, and afforded the corresponding products 31d-31ae in moderate yields.
Based on the results of control experiments, a plausible mechanism was proposed by the authors (Scheme 18). Firstly, the excited state species of rose bengal formed by visible-light irradiation reduced the persulfate to the sulfate radical anion via a single electron transfer. Subsequently, nitrogen radical intermediate 30A was generated via hydrogen atom transfer between methyl carbamate and sulfate anions, which further underwent sequential dehydrogenation and released nitrogen molecules to form alkoxycarbonyl radical intermediate 30B. Then, radical intermediate 30C was formed through a free radical addition of 30B and 1a, which was further oxidized and deprotonated to obtain the final product 31a.

2.6. C3-Arylation of Imidazo[1,2-a]pyridines

Furthermore, 2,3-Diarylimidazo[1,2-a]pyridine derivatives have significant biological activity potential [57,58], so their synthesis methods have been highly studied. In traditional methods for the synthesis of 2,3-diarylimidazo[1,2-a]pyridine derivatives, the transition-metal-catalyzed C-H bond arylation of 2-arylimidazo[1,2-a]pyridines is the most common synthetic strategy, which inevitably leads to serious heavy metal pollution problems [59]. In 2022, Mahdavi’s group disclosed a visible light-mediated method for the arylation of imidazo[1,2 a]pyridines using diazonium salts 32 by using chlorophyll as a photocatalyst (Scheme 19a) [60]. Various imidazo[1,2-a]pyridines and diazonium salts bearing electron-withdrawing and electron-donating groups were well tolerated to construct 2,3-Diarylimidazo[1,2-a]pyridine compounds 33 in moderate to good yields (Scheme 19b).

2.7. C3-Carbosilylation of Imidazo[1,2-a]pyridines

The synthesis methodology of organosilicon molecules has attracted widespread attention from chemists due to these molecules’ significant applications in the fields of materials, polymer science, and agricultural chemistry. In 2021, Hajra’s group established a visible light-induced protocol for FeCl2 and eosin Y-catalyzed three-component carbosilylation of imidazo[1,2-a]pyridines with alkenes and silanes at room temperature (Scheme 20a) [61]. The substrates were studied systematically and obtained the corresponding products in satisfactory yields (Scheme 20b). This transformation exhibited good functional group tolerance and substrate adaptability.
A possible mechanism proposed by the authors was shown in Scheme 20c. Initially, radical anion EY•− and Fe(III) were generated through a single electron transfer (SET) occurring from Fe(II) to the excited state of EY (EY*), which then transferred SET to DTBP to generate tBuO• and regenerate the photocatalyst. (TMS)3Si• produced via the reaction of tBuO• and silane (35) reacted with olefin (34) to form radical intermediate 35B. Subsequently, 35B underwent SET with iron(III) to obtain the regenerated Fe(II) catalyst and cationic intermediate 35C. Then, the electrophilic addition process occurred between 35C and imidazo[1,2-a]pyridine 1a to obtain 35D. Finally, 35D underwent deprotonation to obtain the desired product 36.

2.8. C5-Alkylation of Imidazo[1,2-a]pyridines

At present, the C3 functionalization of imidazo[1,2-a]pyridine has been widely explored, but the visible light-induced C5 functionalization of imidazo[1,2-a]pyridines has still rarely been investigated [62]. In 2020, Jin’s group developed a visible light-induced method for the C5 alkylation of imidazo[1,2-a]pyridines with alkyl N-hydroxyphthalimides 37, by using eosin Y as a photocatalyst at room temperature (Scheme 21) [63].
As shown in Scheme 21b, various imidazo[1,2-a]pyridine substrates containing different electron-donating groups (such as methyl and methoxy) and electron-withdrawing groups (such as fluorine, chlorine, bromine, trifluoromethyl, and cyano) on either C2-benzene or pyridine rings were all compatible with this reaction system, generating the corresponding products 38a–38s in moderate to good yields. N-hydroxyphthalimides substrates 37 with different substituents (such as primary, secondary, and tertiary alkyl esters) were also suitable for this conversion, obtaining the corresponding products 38t–38ac in 38–75% yields. In addition, gram-scale experiments have also been successfully completed under the standard conditions.

3. C-N Bond Formation

3.1. C3-Sulfonamidation of Imidazo[1,2-a]pyridines

Sulfonamide moieties are one of the most important pharmacophores in many clinical drugs, such as rosuvastatin and benazepril [64]. Therefore, the introduction of sulfonamide moieties on the N-heterocycle backbones is a hot topic of research for chemists [65]. In 2018, Sun’s group developed a visible light-promoted protocol for C3 sulfonamidation of imidazo[1,2-a]pyridines with sulfamides 39 by using NaClO solution as the oxidant and Ir(ppy)2(dtbbpy)PF6 as a photosensitizer under mild conditions (Scheme 22) [66].
Various imidazo[1,2-a]pyridine substrates with different electron-donating (such as Me, OMe, and tBu) and electron-withdrawing groups (such as Cl, Br, CN, andCF3, etc.) on either C2-aryl or pyridine rings were tolerated under the optimal reaction conditions and generated the corresponding products 40a–40q in 67–92% yields (Scheme 22b). N-methyl-arylsulfonamide substrates containing different substituents could proceed smoothly and generated the desired products 40aa–40ai in 36–93% yields. Furthermore, both N-ethyl-p-toluenesulfonamide and N-butyl-p-toluenesulfonamide were also suitable for this method, yielding the corresponding products 40aj and 40ak in 88% and 73% yields, respectively. Unfortunately, N-phenyl-benzenesulfonamide and benzenesulfonamide substrates failed to give the corresponding products 40al and 40am.

3.2. C3-Amination of Imidazo[1,2-a]pyridines

Heteroarylamines are prominent in many natural products, drugs, and functional materials [67]. Therefore, the direct C-H amination of heteroarenes has received widespread attention from synthetic and pharmaceutical chemists. In 2018, Lei’s group reported a photo-induced external oxidant-free protocol for C3-amination of imidazo[1,2-a]pyridines with azoles, by employing acridinium as a photosensitizer and cobaloxime as the catalyst at room temperature (Scheme 23) [68]. A series of 2-arylimidazo[1,2-a]pyridines substituted with various electron-donating and electron-withdrawing groups were well tolerated to obtain the corresponding products 42a–42n in good to excellent yields (69–99%). Various types of azoles, including pyrazoles, imidazoles, 1,2,3-triazole, and 1,2,4-triazole, could also achieve this conversion to provide the corresponding products 42aa–42ag in 43–99% yields. In addition, Adimurthy’s team also successfully achieved a similar amination reaction of imidazo[1,2-a]pyridines with heteroamines (such as benzotriazoles, benzoimidazoles, triazoles, pyrazoles, imidazoles, and indazoles) by employing Acr+-MesClO4- as a photocatalyst and K2S2O8 as an oxidant [69].

4. C-P Bond Formation

Phosphorus moieties are widely present in natural products, pharmaceutical molecules, and agricultural chemicals, possessing good physicochemical properties and certain bioactive activities [70]. The synthesis method of imidazo[1,2-a]pyridines containing phosphorus has also attracted the attention of chemists [71]. In 2021, Yu’s group reported a visible light-induced reaction for the phosphorylation of imidazo[1,2-a]pyridines with phosphine oxides 43 by using rhodamine B as a photocatalyst at room temperature (Scheme 24) [72]. A wide range of imidazo[1,2-a]pyridines and phosphine oxide substrates were systematically explored and provided the corresponding products 44a–44w in 43–93% yields. The great advantages of this method are mild reaction conditions, wide substrate applicability, and good functional group tolerance.

5. C-O Bond Formation

Due to the widespread presence of C-O bonds in natural products and bioactive molecules, the development of efficient methods for constructing C-O bonds has received widespread attention from synthetic chemists [73]. In 2017, Hajra’s group achieved the visible light-induced C-3 alkoxylation of imidazopyridines with alcohols by using rose bengal as a photoredox catalyst under mild conditions (Scheme 25) [74]. The 2-arylimidazo[1,2-a]pyridines with various substituents (such as Me, OMe, F, Cl, CF3, NO2, and OH) on the phenyl ring could proceed smoothly and afforded the expected methoxylated products 46a–46h in moderate to good yields. Both thiophen-2-yl and naphtha-2-yl-substituted imidazo[1,2-a]pyridines were well tolerated and gave the desired products 46i and 46j in 83% and 86% yields, respectively. However, no corresponding methoxylated products were detected under the standard conditions with C2-position-alkyl-substituted or -unsubstituted imidazo[1,2-a]pyridine substrates. Various primary and secondary alcohol substrates with alkyl, vinyl, alkynyl, hydroxyl, halogen, phenyl, and thiophene groups reacted well with 8-methyl-2-phenylimidazo[1,2-a]pyridine, providing the alkoxylation products 46aa–46an in 75–90% yields.

6. C-S Bond Formation

6.1. C3-Thiocyanation of Imidazo[1,2-a]pyridines

Heterocyclic compounds containing thiocyanates are widely found in many natural products and have a wide range of biological activities [75]. Visible light mediated-thiocyanation is one of the effective methods for synthesizing organic heterocyclic thiocyanates [76].
Hajra’s group developed a visible light-mediated protocol for the C3 thiocyanation of imidazo[1,2-a]pyridines with NH4SCN by using eosin Y as a photoredox catalyst in acetonitrile solution in an ambient air atmosphere (Scheme 26) [77]. A plausible mechanism of this reaction was investigated through the controlled experiments. Initially, thiocyanate radical was obtained from the oxidation of thiocyanate anion by SET mechanism via a reductive quenching cycle, which interacted with 1 to obtain the radical intermediate 48A. Then, 48A was oxidized to the cationic intermediate 48B. Finally, 48B underwent deprotonation to obtain the desired product 48 (Scheme 26a). For the substrate expansion experiment, 2-Aryl imidazo[1,2-a]pyridines with various electron-donating and electron-withdrawing groups on the phenyl ring were compatible with the reaction system to provide the corresponding products 48a–48i in 67–93% yields. Trifluoromethyl and isobutylethyl-substituted or -unsubstituted substrates on the C2 position of imidazo[1,2-a]pyridines could also proceed smoothly under the optimal reaction conditions and obtained the corresponding products 48l–48n in 54–69% yields. The gram-scale experiment of this conversion (6 mmol scale) was also performed and successfully afforded the target product in an 89% yield (1.34 g). In 2023, a similar photoredox-catalyzed methodology for the synthesis of 2-aryl-3-thiocyanatoimidazo-[1,2-a]pyridines by using naphthalimide as a photoredox catalyst was reported by Sharma’s group [78].

6.2. C3-Sulfonylation of Imidazo[1,2-a]pyridines

Heterocyclic compounds containing sulfone groups have a wide range of applications in the fields of agrochemicals, materials science, and medicinal chemistry. Therefore, the synthesis methodology for visible light-induced sulfonylation of imidazo[1,2-a]pyridines has been studied [79,80].
In 2020, Piguel’s group reported a visible light-induced three-component reaction for C3-sulfonylation of imidazo[1,2-a]pyridines 1 with diaryliodonium salts 49 and DABCO-bis(sulfur dioxide) 50 by using an organic photoredox catalyst at room temperature (Scheme 27) [81]. This method had good substrate compatibility and provided the corresponding products in moderate yields. However, C2 position-unsubstituted imidazo[1,2-a]yridine and 2-(phenylsulfonyl)imidazo[1,2-a]pyridine substrates are not suitable for this reaction system and failed to yield the sulfonylated products 50ac and 50ad, respectively.

6.3. C3-Sulfenylation of Imidazo[1,2-a]pyridines

Considering that aryl sulfide moieties are widely present in bioactive molecules and natural products, visible light-induced direct sulfenylation of C-H bonds with sulfinic acids has been developed [82,83]. In 2017, Wang et al. developed a visible light-initiated method for the sulfenylation of imidazo[1,2-a]pyridines with sulfinic acids by using Eosin B as a photosensitizer and hydroperoxide (TBHP) as an oxidant in DCE at room temperature (Scheme 28a) [84]. For the substrate screening, a series of 2-arylimidazo[1,2-a]pyridines and sulfinic acid substrates bearing electron-donating groups as well as halogen substituents were well tolerated in this transformation, resulting in the corresponding imidazo[1,2-a]pyridine derivatives in moderate yields (Scheme 28b).
A radical pathway mechanism was proposed based on the results of control experiments (Scheme 29). Initially, the excited Eosin B* generated from Eosin B via visible light irradiation underwent a single electron transfer (SET) process to TBHP, generating tertbutoxyl radical and a hydroxyl anion. Then, the arylsulfinic acid reacted with tertbutoxyl radical to obtain the sulfonyl radical 52A. Subsequently, the sulfinyl radical 52C was formed via the reaction of 52A with arylsulfinate 52B, which was reduced by tBuOH or H2O to generate the thienyl radical intermediate 52D. Then, 52D underwent addition reaction with imidazo[1,2-a]pyridine 1 and further transformed into the carbocation intermediate 52F through SET with Eosin B•+. Finally, 52F underwent dehydrogenation under the action of the sulfonic acid anion and resulted in the desired product 53.
Soon afterwards, Barman’s group established another visible light-promoted procedure for the C3-sulfenylation of imidazo[1,2-a]pyridines with thiols by using rose bengal as a photoredox catalyst in ambient conditions (Scheme 30) [85]. This protocol could conveniently and efficiently provide a series of C3-sulfenylimidazo[1,2-a]pyridines in good to excellent yields, characterized by operational simplicity, broad substrate scope, and high atom efficiency.

7. C-Se Bond Formation

Selenium is an important trace element that has multiple effects on human health, such as antioxidant effects, immune enhancement, liver protection, and anticancer effects [86]. Considering the increasingly important role of organic selenium in the field of medicinal chemistry, research on the selenylation of imidazo[1,2-a]pyridines has also attracted the attention of chemists [87]. For example, Braga’s group established a rose bengal-catalyzed protocol for the selenylation of imidazo[1,2-a]pyridines with diorganoyl diselenides in the presence of blue LEDs (415 nm) at room temperature, providing the 2-phenyl-3-(phenylselanyl)imidazo[1,2-a]pyridine 57 in a 47% yield (Scheme 31, Path I) [88]. In addition, Bagdi’s group also successfully developed a similar method for the selenylation of imidazo[1,2-a]pyridines by using erythrosine B as a photocatalyst. (Scheme 31, Path II) [89]. Notably, the yield of this method was better than that of Braga’s method.

8. Conclusions

In summary, significant progress has been made in recent years in the direct C-H functionalization of imidazo[1,2-a]pyridines based on arylation, thiolation, formylation, oxidative cycloaromatization, and carbonylation strategies, providing an effective pathway for the synthesis of various substituted imidazo[1,2-a]pyridine derivatives. This review mainly summarized the significant developments in the C3 position functionalization of imidazo[1,2-a]pyridines under visible light-induced conditions, and outlined the mechanisms and substrate applicability of various methods. It is worth noting that the visible light-induced C-H functionalization of imidazo[1,2-a]pyridines remains to be a hot and cutting-edge topic in organic synthesis methodology. At the same time, greener and more environmentally friendly methods conforming to the principle of green chemistry are still desirable to be developed. Therefore, we believe that these methods can provide excellent supplements for the photochemical construction of heterocyclic compounds. This not only provides assistance for the C-H functionalization of other heterocyclic compounds, but also provides new research inspiration for the visible light-catalyzed C-H functionalization of imidazo[1,2-a]pyridines in the future.
Obviously, research on the C-H functionalization of imidazo[1,2-a]pyridines mainly focuses on its C3 position. To date, reports are rarely available on the functionalization of other positions of imidazo[1,2-a]pyridines. We believe that through the continuous efforts of synthetic chemists, the study of C-H functionalization of imidazo[1,2-a]pyridines in other positions promoted by visible light will lead to breakthroughs, and will contribute to the development of organic synthesis and drug synthesis.

Author Contributions

J.G.—visualization, writing—original draft. X.F.—visualization, writing—original draft. K.Y.—review and editing. Z.L.—funding acquisition, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Natural Science Foundation of Jiangxi Province (no. 20224BAB203010) and the Science and Technology Projects of Ganzhou (2023CYZ17866).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Conflicts of Interest

The authors declare no conflicts of interest.

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Molecules 30 00607 g001
Figure 1. Examples of marketed drugs containing imidazo[1,2-a]pyridine skeleton.
Figure 1. Examples of marketed drugs containing imidazo[1,2-a]pyridine skeleton.
Molecules 30 00607 g001
Molecules 30 00607 sch001
Scheme 1. Trifluoromethylation of 2-phenylimidazo[1,2-a]pyridine with sodium triflinate.
Scheme 1. Trifluoromethylation of 2-phenylimidazo[1,2-a]pyridine with sodium triflinate.
Molecules 30 00607 sch001
Molecules 30 00607 sch002
Scheme 2. Visible light-mediated trifluoromethylation of imidazo[1,2-a]pyridines with sodium triflinate. (a) Reaction equation. (b) Substrate scope of imidazo[1,2-a]pyridines. (c) The plausible reaction mechanism.
Scheme 2. Visible light-mediated trifluoromethylation of imidazo[1,2-a]pyridines with sodium triflinate. (a) Reaction equation. (b) Substrate scope of imidazo[1,2-a]pyridines. (c) The plausible reaction mechanism.
Molecules 30 00607 sch002
Molecules 30 00607 sch003
Scheme 3. Visible light-promoted trifluoromethylation of 2-arylimidazo[1,2-a]pyridines using mesityl acridinium as a photoredox catalyst.
Scheme 3. Visible light-promoted trifluoromethylation of 2-arylimidazo[1,2-a]pyridines using mesityl acridinium as a photoredox catalyst.
Molecules 30 00607 sch003
Molecules 30 00607 sch004
Scheme 4. Visible light-promoted trifluoroethylation of imidazo[1,2-a]pyridines with 1,1,1-trifluoro-2-iodoethane. (a) Reaction equation. (b) Substrate scope of imidazo[1,2-a]pyridines. (c) The plausible reaction mechanism.
Scheme 4. Visible light-promoted trifluoroethylation of imidazo[1,2-a]pyridines with 1,1,1-trifluoro-2-iodoethane. (a) Reaction equation. (b) Substrate scope of imidazo[1,2-a]pyridines. (c) The plausible reaction mechanism.
Molecules 30 00607 sch004
Molecules 30 00607 sch005
Scheme 5. (a) Visible light-induced photocatalytic difluoroacetylation of imidazo[1,2-a]pyridines. (b) Visible light-induced (phenylsulfonyl)difluoromethylation of imidazo[1,2-a]pyridines.
Scheme 5. (a) Visible light-induced photocatalytic difluoroacetylation of imidazo[1,2-a]pyridines. (b) Visible light-induced (phenylsulfonyl)difluoromethylation of imidazo[1,2-a]pyridines.
Molecules 30 00607 sch005
Molecules 30 00607 sch006
Scheme 6. Visible light-induced difluoromethylenephosphonation of imidazo[1,2-a]pyridines.
Scheme 6. Visible light-induced difluoromethylenephosphonation of imidazo[1,2-a]pyridines.
Molecules 30 00607 sch006
Molecules 30 00607 sch007
Scheme 7. Visible light-induced perfluoroalkylation of imidazo[1,2-a]pyridines with perfluoroalkyl iodides. (a) Reaction equation. (b) Substrate scope. (c) The plausible reaction mechanism.
Scheme 7. Visible light-induced perfluoroalkylation of imidazo[1,2-a]pyridines with perfluoroalkyl iodides. (a) Reaction equation. (b) Substrate scope. (c) The plausible reaction mechanism.
Molecules 30 00607 sch007
Molecules 30 00607 sch008
Scheme 8. Visible light-induced difluoroalkylation of imidazo[1,2-a]pyridines with bromodifluoroaryl ketones.
Scheme 8. Visible light-induced difluoroalkylation of imidazo[1,2-a]pyridines with bromodifluoroaryl ketones.
Molecules 30 00607 sch008
Molecules 30 00607 sch009
Scheme 9. Visible light-induced C3-cyanomethylation of imidazo[1,2-a]pyridines with bromoacetonitrile. (a) Reaction equation. (b) Substrate scope of imidazo[1,2-a]pyridines. (c) The plausible reaction mechanism.
Scheme 9. Visible light-induced C3-cyanomethylation of imidazo[1,2-a]pyridines with bromoacetonitrile. (a) Reaction equation. (b) Substrate scope of imidazo[1,2-a]pyridines. (c) The plausible reaction mechanism.
Molecules 30 00607 sch009
Molecules 30 00607 sch010
Scheme 10. Synthesis of Zolpidem and Alpidem.
Scheme 10. Synthesis of Zolpidem and Alpidem.
Molecules 30 00607 sch010
Molecules 30 00607 sch011
Scheme 11. Visible light-promoted cross-dehydrogenative coupling of imidazo[1,2-a]pyridines with tertiary amine. (a) Reaction equation. (b) Substrate scope. (c) The plausible reaction mechanism.
Scheme 11. Visible light-promoted cross-dehydrogenative coupling of imidazo[1,2-a]pyridines with tertiary amine. (a) Reaction equation. (b) Substrate scope. (c) The plausible reaction mechanism.
Molecules 30 00607 sch011
Molecules 30 00607 sch012
Scheme 12. Visible light-induced aminoalkylation of imidazo[1,2-a]pyridines with N, N-dimethylaniline.
Scheme 12. Visible light-induced aminoalkylation of imidazo[1,2-a]pyridines with N, N-dimethylaniline.
Molecules 30 00607 sch012
Molecules 30 00607 sch013
Scheme 13. Visible light-promoted decarboxylative aminoalkylation of imidazo[1,2-a]pyridines with N-aryl glycines. (a) Reaction equation. (b) Substrate scope. (c) The plausible reaction mechanism.
Scheme 13. Visible light-promoted decarboxylative aminoalkylation of imidazo[1,2-a]pyridines with N-aryl glycines. (a) Reaction equation. (b) Substrate scope. (c) The plausible reaction mechanism.
Molecules 30 00607 sch013
Molecules 30 00607 sch014
Scheme 14. Visible light-induced aminoalkylation of imidazo[1,2-a]pyridines with N-aryl glycines.
Scheme 14. Visible light-induced aminoalkylation of imidazo[1,2-a]pyridines with N-aryl glycines.
Molecules 30 00607 sch014
Molecules 30 00607 sch015
Scheme 15. Visible light-induced cross-dehydrogenative coupling reaction of imidazo[1,2-a]pyridines with N-arylglycine esters. (a) Reaction equation. (b) The plausible reaction mechanism.
Scheme 15. Visible light-induced cross-dehydrogenative coupling reaction of imidazo[1,2-a]pyridines with N-arylglycine esters. (a) Reaction equation. (b) The plausible reaction mechanism.
Molecules 30 00607 sch015
Molecules 30 00607 sch016
Scheme 16. Visible light-induced C3-formylation of imidazo[1,2-a]pyridines with tetramethylethylenediamine (TMEDA). (a) Reaction equation. (b) Substrate scope of imidazo[1,2-a]pyridines. (c) The plausible reaction mechanism.
Scheme 16. Visible light-induced C3-formylation of imidazo[1,2-a]pyridines with tetramethylethylenediamine (TMEDA). (a) Reaction equation. (b) Substrate scope of imidazo[1,2-a]pyridines. (c) The plausible reaction mechanism.
Molecules 30 00607 sch016
Molecules 30 00607 sch017
Scheme 17. Visible light-induced C3-alkoxycarbonylation of imidazo[1,2-a]pyridines with carbazates. (a) Reaction equation. (b) Substrate scope.
Scheme 17. Visible light-induced C3-alkoxycarbonylation of imidazo[1,2-a]pyridines with carbazates. (a) Reaction equation. (b) Substrate scope.
Molecules 30 00607 sch017
Molecules 30 00607 sch018
Scheme 18. Proposed mechanism for alkoxycarbonylation of imidazo[1,2-a]pyridines.
Scheme 18. Proposed mechanism for alkoxycarbonylation of imidazo[1,2-a]pyridines.
Molecules 30 00607 sch018
Molecules 30 00607 sch019
Scheme 19. C3-arylation of imidazo[1,2-a]pyridines with diazonium salts. (a) Reaction equation. (b) Substrate scope.
Scheme 19. C3-arylation of imidazo[1,2-a]pyridines with diazonium salts. (a) Reaction equation. (b) Substrate scope.
Molecules 30 00607 sch019
Molecules 30 00607 sch020
Scheme 20. C3-carbosilylation of imidazo[1,2-a]pyridines with alkenes and silanes. (a) Reaction equation. (b) Substrate scope. (c) The plausible reaction mechanism.
Scheme 20. C3-carbosilylation of imidazo[1,2-a]pyridines with alkenes and silanes. (a) Reaction equation. (b) Substrate scope. (c) The plausible reaction mechanism.
Molecules 30 00607 sch020
Molecules 30 00607 sch021
Scheme 21. Visible light-induced C5-alkylation of imidazo[1,2-a]pyridines with alkyl N-hydroxyphthalimides. (a) Reaction equation. (b) Substrate scope.
Scheme 21. Visible light-induced C5-alkylation of imidazo[1,2-a]pyridines with alkyl N-hydroxyphthalimides. (a) Reaction equation. (b) Substrate scope.
Molecules 30 00607 sch021
Molecules 30 00607 sch022
Scheme 22. Visible light-induced C3-sulfonamidation of imidazo[1,2-a]pyridines with sulfamides. (a) Reaction equation. (b) Substrate scope.
Scheme 22. Visible light-induced C3-sulfonamidation of imidazo[1,2-a]pyridines with sulfamides. (a) Reaction equation. (b) Substrate scope.
Molecules 30 00607 sch022
Molecules 30 00607 sch023
Scheme 23. Visible light-induced C3-amination of imidazo[1,2-a]pyridines with azoles. (a) Reaction equation. (b) Substrate scope.
Scheme 23. Visible light-induced C3-amination of imidazo[1,2-a]pyridines with azoles. (a) Reaction equation. (b) Substrate scope.
Molecules 30 00607 sch023
Molecules 30 00607 sch024
Scheme 24. Visible light-induced C3-phosphorylation of imidazo[1,2-a]pyridines with phosphine oxides. (a) Reaction equation. (b) Substrate scope.
Scheme 24. Visible light-induced C3-phosphorylation of imidazo[1,2-a]pyridines with phosphine oxides. (a) Reaction equation. (b) Substrate scope.
Molecules 30 00607 sch024
Molecules 30 00607 sch025
Scheme 25. Photoredox-catalyzed C3-alkoxylation of imidazo[1,2-a]pyridines with alcohols. (a) Reaction equation. (b) Substrate scope.
Scheme 25. Photoredox-catalyzed C3-alkoxylation of imidazo[1,2-a]pyridines with alcohols. (a) Reaction equation. (b) Substrate scope.
Molecules 30 00607 sch025
Molecules 30 00607 sch026
Scheme 26. Photoredox-catalyzed thiocyanation of imidazo[1,2-a]pyridines with NH4SCN. (a) Reaction equation. (b) Substrate scope.
Scheme 26. Photoredox-catalyzed thiocyanation of imidazo[1,2-a]pyridines with NH4SCN. (a) Reaction equation. (b) Substrate scope.
Molecules 30 00607 sch026
Molecules 30 00607 sch027
Scheme 27. Photoredox-catalyzed C3-sulfonylation of imidazo[1,2-a]pyridines with diaryliodonium salts and DABCO-bis(sulfur dioxide). (a) Reaction straegy. (b) Substrate scope.
Scheme 27. Photoredox-catalyzed C3-sulfonylation of imidazo[1,2-a]pyridines with diaryliodonium salts and DABCO-bis(sulfur dioxide). (a) Reaction straegy. (b) Substrate scope.
Molecules 30 00607 sch027
Molecules 30 00607 sch028
Scheme 28. Photoredox-catalyzed C3-sulfenylation of imidazo[1,2-a]pyridines with sulfinic acids. (a) Reaction equation. (b) Substrate scope.
Scheme 28. Photoredox-catalyzed C3-sulfenylation of imidazo[1,2-a]pyridines with sulfinic acids. (a) Reaction equation. (b) Substrate scope.
Molecules 30 00607 sch028
Molecules 30 00607 sch029
Scheme 29. A proposed mechanism for the C3-sulfenylation of imidazo[1,2-a]pyridines.
Scheme 29. A proposed mechanism for the C3-sulfenylation of imidazo[1,2-a]pyridines.
Molecules 30 00607 sch029
Molecules 30 00607 sch030
Scheme 30. Photoredox-catalyzed C3-sulfenylation of imidazo[1,2-a]pyridines with thiols.
Scheme 30. Photoredox-catalyzed C3-sulfenylation of imidazo[1,2-a]pyridines with thiols.
Molecules 30 00607 sch030
Molecules 30 00607 sch031
Scheme 31. Photoredox-catalyzed C3-selenylation of imidazo[1,2-a]pyridines with diorganoyl diselenides.
Scheme 31. Photoredox-catalyzed C3-selenylation of imidazo[1,2-a]pyridines with diorganoyl diselenides.
Molecules 30 00607 sch031
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Gao, J.; Fu, X.; Yang, K.; Liu, Z. Recent Advances in Visible Light-Induced C-H Functionalization of Imidazo[1,2-a]pyridines. Molecules 2025, 30, 607. https://doi.org/10.3390/molecules30030607

AMA Style

Gao J, Fu X, Yang K, Liu Z. Recent Advances in Visible Light-Induced C-H Functionalization of Imidazo[1,2-a]pyridines. Molecules. 2025; 30(3):607. https://doi.org/10.3390/molecules30030607

Chicago/Turabian Style

Gao, Juanjuan, Xinlei Fu, Kai Yang, and Zhaowen Liu. 2025. "Recent Advances in Visible Light-Induced C-H Functionalization of Imidazo[1,2-a]pyridines" Molecules 30, no. 3: 607. https://doi.org/10.3390/molecules30030607

APA Style

Gao, J., Fu, X., Yang, K., & Liu, Z. (2025). Recent Advances in Visible Light-Induced C-H Functionalization of Imidazo[1,2-a]pyridines. Molecules, 30(3), 607. https://doi.org/10.3390/molecules30030607

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