Visible-light-induced regioselective sulfenylation of imidazopyridines with thiols under transition metal-free conditions

Abstract

A metal-free visible-light-promoted regioselective C-3 sulfenylation of imidazo[1,2-a]pyridines and indoles using thiols has been developed via C(sp²)–H functionalization. This method provides direct access to a wide range of structurally diverse 3-sulfenylimidazopyridines of biological interest. The operational simplicity, eco-energy source, high atom efficiency, and the use of green solvents under ambient conditions are some of the attractive features of this methodology.

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Introduction

The introduction of a sulfur moiety into organic molecules is one of the fundamental processes in organic synthesis. Sulfur-containing compounds are of much importance as they are present in many biologically and medicinally active molecules.¹ Among bioactive azaheterocycles, pyridine derivatives play a significant role in medicinal chemistry.²

Over the past few decades, considerable attention has been paid to imidazo[1,2-a]pyridines as these are core units of many natural products³ and have substantial applications in pharmaceutical and biological activities such as antiviral, antibacterial, fungicidal, cytotoxic, and anti-inflammatory activities,⁴ including GABA inhibitors.⁵ Imidazo[1,2-a]-pyridine derivatives are also used as commercially available drugs including alpidem, olprinone, minodronic acid (to treat anxiety, heart failure, and osteoporosis), zolimidine (peptic ulcer), zolpidem, necopidem, saripidem, and optically active GSK812397 (HIV infection) (Fig. 1).⁶ In addition, these are used as charge transporters in materials science.⁷ Moreover, due to their antiproliferative activity against melanoma cells, functionalized imidazo[1,2-a]pyridines have also been recognized in medicinal chemistry, used in tubulin polymerization, and as protein kinase inhibitors.⁸

Fig. 1 Examples of imidazo[1,2-a]pyridine-based therapeutics.

A combination of the structural unity between imidazo[1,2-a]pyridine and an organosulfur group was successfully used in the design of a new class of inhibitors of human rhinovirus (A),⁹ with similar effects to enviroxime, and the new anthelminthic B (Fig. 2).¹⁰

Fig. 2 3-Thioimidazo[1,2-a]pyridines A and B with similar effects to enviroxime.

Thus sulfenylation at C3 of imidazo[1,2-a]pyridines is of much significance and has been accomplished using copper,¹¹ iodine derivatives,¹² hypervalent iodine reagents,¹³ strong acidic conditions,¹⁴ ionic liquids,¹⁵ or silica-supported CeCl₃·7H₂O/NaI,¹⁶ NCS,¹⁷ NBS,¹⁸ and ArSCl derivatives.¹⁹ Recently, Adimurthy et al. described a catalyst-free 3-sulfenylation of imidazopyridine with sulfonothioate as the sulfur source at high temperature.²⁰ These methods involve thiolating agents such as thiols, disulfides, arylsulfonyl chlorides, sulfonyl hydrazides, sodium sulfinates, sulfur powder and sulfonothioate (Scheme 1). Though these strategies were successfully employed for the sulfenylation of imidazo[1,2-a]pyridines, most of them have various disadvantages. Some of the sulfenylating reagents such as sulfonyl hydrazides are not commercially available and are also expensive. In addition, the use of metals and bases limited the further application of these protocols in industry and pharmacy.^11a,21 The separation of the metal catalyst from products is of particular importance for the synthesis of pharmaceuticals because of their residual toxicity in the target compound which is a central issue to consider. Moreover, transition-metal-catalyzed reactions also generate environmentally problematic hazardous waste, and hence, should be avoided wherever possible. So, it is highly desirable to develop environmentally benign chemical processes to access pharmacologically potent thiolated imidazoheterocyclic compounds without the use of any metal catalysts. The direct sulfenylation of aromatic C–H bonds, which does not require the presence of reactive functional groups (such as metal moieties or halogens), is an attractive strategy in organic synthesis.²²

Scheme 1 Different routes for the synthesis of 3-sulfenylimidazopyridine derivatives.

Thiols and disulfides are the most frequently employed thiolating reagents for the sulfenylation of imidazopyridines and indoles.²³ Due to their ready availability, low cost as well as general functional group tolerance thiols and disulfides have been utilized as attractive starting materials for sulfenylation reactions. The main problem of using disulfide as the thiolating reagent is that disulfides need to be synthesized from thiols via the oxidative coupling reaction, which includes an additional operation step and low atom economics.²⁴ Thus we consider thiol as our choice of reagent.

In the past decade, a novel application of visible-light-mediated photoredox catalysis in synthetic organic chemistry has been extensively exploited because of its environmental compatibility, inexpensiveness, excellent functional group tolerance, high reactivity and versatility in promoting a large number of synthetically important reactions.²⁵ Organic dyes have been used as an effective alternative to transition metal photoredox catalysts.²⁶ Recently, Hajra et al. developed the thiocyanation of imidazoheterocycles through visible light photoredox catalysis.²⁷ Very recently, Guo reported a visible-light promoted protocol for the synthesis of 3-arylthioindoles with thiols as the sulfenylating agent and rose bengal as the photocatalyst.²⁸ However, there is no report on visible-light induced sulfenylation of imidazo[1,2-a]pyridine derivatives with thiols.²⁹ As a part of our continued studies focusing on the synthesis of sulfur-containing compounds,³⁰ herein, we wish to report a novel and mild protocol for the sulfenylation of imidazo[1,2-a]pyridines using thiol in the presence of rose bengal as the photosensitizer under ambient conditions through a visible-light irradiated process (Scheme 1).

Results and discussion

We began our investigation with 2-phenylimidazo[1,2-a]pyridine (1a) and thiophenol (2a) as the model substrate to find suitable reaction conditions, as summarized in Table 1. Initially, the reaction was carried out by employing 0.5 equiv. of 1a with 0.5 equiv. of thiophenol (2a) and eosin Y (5 mol%) as the photocatalyst in CH₃CN in open air in the presence of blue LED light. The desired product 3a was obtained in 35% yield after 6 h (Table 1, entry 1). The structure of 3a was characterized by ¹H and ¹³C NMR spectroscopy. A higher yield of 3a was obtained when 1.1 equiv. of 2a was employed (Table 1, entry 2). When the reaction was performed under nitrogen the desired sulfenylation product 3a was obtained in 30% yield (Table 1, entry 3). Other photocatalysts such as rose bengal, rhodamine B, Ru(bpy)₃Cl₂·6H₂O and acridine red were also examined (Table 1, entries 4–7). Among these, rose bengal demonstrated the best catalytic activity to give the desired product 3a in 75% yield (Table 1, entry 4). Among a variety of solvents such as DMF, DMSO, THF, methanol, DCE, toluene, 1,4-dioxane and acetone investigated (Table 1, entries 8–15) the best result (90%) was obtained in DMSO (Table 1, entry 9). An improved yield of 3a was obtained when 4 Å molecular sieves were added (Table 1, entry 16). When green LED (4 W) was used instead of blue LED, a decrease in yield (65%) was observed even after 12 h (Table 1, entry 17). Moreover, no product formation was observed in the absence of visible-light irradiation, and only a trace amount of 3a was obtained without any photocatalysts (Table 1, entries 18 and 19). The loading of the photocatalyst was also optimized and the results are summarized in Table 1 (entry 20).

Table 1 Optimization of the reaction parameters^a

Entry	1a/2a (equiv.)	Photocatalyst (mol %)	Light source	Solvent	Yield^b (%)
a Reaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), photocatalyst (5.0 mol%), solvent (2.0 mL), 4 Å molecular sieves (80 mg), r.t., air, blue LED (450 nm) for 6 h. b Isolated yield. c Nitrogen atmosphere. d Rose bengal (1.0 mol%) was used. e Rose bengal (3.0 mol%) was added. f Rose bengal (10 mol%) was used.
1	0.5/0.5	Eosin Y	Blue LED	CH3CN	35
2	0.5/0.6	Eosin Y	Blue LED	CH3CN	55
3	0.5/0.6	Eosin Y	Blue LED	CH3CN	30^c
4	0.5/0.6	Rose bengal	Blue LED	CH3CN	75
5	0.5/0.6	Rhodamine B	Blue LED	CH3CN	45
6	0.5/0.6	Ru(bpy)3Cl2·6H2O	Blue LED	CH3CN	20
7	0.5/0.6	Acridine red	Blue LED	CH3CN	55
8	0.5/0.6	Rose bengal	Blue LED	DMF	60
9	0.5/0.6	Rose bengal	Blue LED	DMSO	90
10	0.5/0.6	Rose bengal	Blue LED	THF	35
11	0.5/0.6	Rose bengal	Blue LED	Methanol	18
12	0.5/0.6	Rose bengal	Blue LED	Chloroform	15
13	0.5/0.6	Rose bengal	Blue LED	DCE	45
14	0.5/0.6	Rose bengal	Blue LED	Toluene	25
15	0.5/0.6	Rose bengal	Blue LED	1,4-Dioxane	20
16	0.5/0.6	Rose bengal	Blue LED	DMSO	95
17	0.5/0.6	Rose bengal	Green LED	DMSO	65
18	0.5/0.6	Rose bengal	—	DMSO	—
19	0.5/0.6	—	Blue LED	DMSO	Trace
20	0.5/0.6	Rose bengal	Blue LED	DMSO	35^d, 65^e, 85^f

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A reaction between unsubstituted imidazo[1,2-a]pyridine 4 with thiophenol under optimized reaction conditions (Table 1, entry 16) produced C-3 sulfenylated product 4a in 85% yield (Scheme 2). When C3 substituted 3-methylimidazo[1,2-a]pyridine 5 was subjected to the reaction under the same conditions no C2 sulfenylated product 5a was observed. These experiments (Scheme 2) indicate that when the C-3 position of imidazo[1,2-a]pyridine is blocked by any group, no reaction takes place to yield the thiolated product. The reported method proceeds with only C-3 unsubstituted imidazopyridine core units to yield 3-sulfenylated imidazo[1,2-a]pyridines in a selective manner.

Scheme 2 Selective sulfenylation.

To explore the generality of this direct sulfenylation reaction, different substituted imidazo[1,2-a]pyridines 1 were reacted with various thiols 2 under optimized conditions (Table 1, entry 16). The results are listed in Table 2. A variety of imidazo[1,2-a]pyridines underwent the direct sulfenylation with thiols (2) smoothly to afford the desired products 3 in good to excellent yields. The imidazopyridine moiety bearing electron-donating groups (–Me, –Et and –OMe) of the phenyl ring at the C-2 position provided the C-3 sulfenylated products in excellent yields (3b–3d). Similarly, the presence of electron-withdrawing groups (–Cl, –Br, and –CN) of the phenyl ring produced the corresponding products with good yields (3e–3g). As evident from the results 3a–3g, the electronic effects associated with electron-donating/-withdrawing substituents on the phenyl ring at the C-2 position of imidazopyridines do not affect the efficiency of the reaction. The reaction between 2 with various substituted imidazo[1,2-a]pyridines (substituents on the pyridine ring of imidazopyridine, such as –Br, –Me, and –OMe) under the optimized conditions provided the corresponding C3-sulfenylated products in moderate to excellent yields (3h–3k). Interestingly, important heterocycles such as 3-(phenylthio)-2-(thiophene-2-yl)imidazo[1,2-a]-pyridine gave the product in excellent yields under the present conditions (3l). Alkyl substituents imidazopyridine such as 2-Me and 2-tert-butyl also worked well in this reaction to give the expected products without any difficulties (3m–3o). Interestingly, when indole was used as a substrate instead of imidazopyridine, under the standard conditions C3-sulfenylated indole was obtained in high yields (3q–3t).

Table 2 Scope of imidazo[1,2-a]pyridines 1 and thiols 2 ^a

a Reaction conditions: 1 (0.5 mmol), 2 (0.6 mmol), rose bengal (5.0 mol%), DMSO (2.0 mL), 4 Å molecular sieves (80 mg), r.t., air, blue LED (450 nm) for 6–7 h. Isolated yield.

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Subsequently, the scope of thiols (2) was also investigated with 1 under the optimized reaction conditions. Thiols containing an electron-rich group (–OMe, –Me) on the para-positions of the benzene rings reacted with 1 to give the corresponding products (3u, 3v and 3ab) in 85–88% yields. Meanwhile, the reactions of 1 with thiols attached to relatively electron-deficient groups such as –Cl and –Br on the para-positions of the phenyl ring afforded the corresponding products (3w and 3x) in excellent yields. The reactions of naphthyl thiol with 1a also gave the desired product 3y in good yield. Besides aryl thiols this protocol was also successfully applied to alkyl thiols such as methanethiol and butanethiol which gave good to excellent conversions to the desired products (3z, and 3aa).

The gram-scale reaction was also performed under the optimized reaction conditions as shown in Scheme 3, which demonstrates the practical applicability of the new protocol. The reaction between 2-phenylimidazo[1,2-a]pyridine (1a) and thiophenol (2a) in the presence of rose bengal (5 mol%), 4 Å molecular sieves in DMSO irradiated by blue LED (450 nm) for 9 h under an open atmosphere afforded the desired product 3a in 87% yield.

Scheme 3 Gram-scale reaction.

To understand this transformation, some selective and control experiments were conducted and the results are listed in Scheme 4. The sulfenylation reaction did not proceed when 2.5 equiv. of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), a radical scavenger, was present in the reaction system (Scheme 4, entry 1). This indicates that a radical pathway might be involved in this transformation. We also carried out the reaction in the dark (Scheme 4, entry 2) and found that the reaction did not proceed at all, which also suggests the radical mechanism. Moreover, the reaction was performed in the presence of DABCO with diphenyl disulfide under the standard conditions (Scheme 4, entry 3). The reaction delivered product 3a with equal ease, which indicates that singlet oxygen is not involved in the reaction.^26m In addition, when 2-phenylimidazo[1,2-a]pyridine (1a) reacted with 1,2-diphenyldisulfane under standard conditions, none of the product 3a was detected, which showed that the disulfide might not be involved in this transformation.

Scheme 4 Control experiments.

On the basis of previous reports^{25p,26i,o,p,q,27,28} and the control experiments, we proposed a plausible reaction pathway for the photocatalytic sulfenylation of imidazo[1,2-a]pyridine as shown in Scheme 5. Initially, rose bengal (RB) was excited under the presence of blue LED light to produce RB*. Then, a single electron transfer (SET) from thiol 2a to RB* afforded the radical cation A and RB˙⁻. RB˙⁻ oxidized by air generated the ground state rose bengal and O₂˙⁻. The deprotonation of radical cation A by O₂˙⁻ leads to the formation of stabilized thiyl radical B. The resulting thiyl radical (B) reacts with 1a to produce the radical intermediate C. Finally, C is oxidized to intermediate D along with the generation of HOO⁻. The deprotonation (aromatization process) of D afforded the desired product 3a along with the release of H₂O₂. To complete the photoredox cycle aerobic oxygen probably plays a crucial role to oxidize RB radical anions to the ground state.

Scheme 5 Plausible reaction mechanism.

Conclusions

In summary, we have developed an efficient and facile strategy for the regioselective C-3 sulfenylation of imidazo[1,2-a]pyridine derivatives and indoles with thiols at room temperature through a visible-light-promoted process in the presence of rose bengal. A series of biologically important C-3 thiolated imidazo[1,2-a]pyridines was obtained conveniently and efficiently in good to excellent yields using readily available materials. The reported method is endowed with several important features including operational simplicity, eco-energy source, high atom efficiency, and green solvent under ambient conditions. Most importantly, the present methodology is scalable. Notably, this methodology in contrast to the previous reports is free from using transition metal catalysts and strong oxidants. To the best of our knowledge, there is no report on the C-3 sulfenylation of imidazopyridines using thiol under a photo-catalysed process. Because of the importance of thiolated imidazopyridines for biological and pharmaceutical sciences this procedure will find wider applications in industry as well as in academia.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are thankful to Prof. Brindaban C. Ranu, Indian Association for the Cultivation of Science, Kolkata, for valuable discussion and suggestions. MHRD, Govt. of India is acknowledged for the doctorate fellowship (MHRD GATE fellowship) received by R. R. The authors also acknowledge CBMR, Lucknow and SAIC, Tezpur University for spectral analysis.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: General information, experimental procedures, characterization data and copies of ¹H NMR and ¹³C NMR spectra of synthesized compounds. See DOI: 10.1039/c7gc02906c

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