Access to Bis(imidazopyridine)sulfanes by Employing Cysteine as Greener Sulfur Source

An efficient and environmentally friendly protocol has been developed for synthesizing bis(imidazopyridine)sulfanes, employing cysteine as sulfur source. This method demonstrates broad substrate scope, accommodating diverse electron-donating and electron-withdrawing functional groups on the imidazopyridine core. Furthermore, the protocol is successfully extended to other heterocycles: indole, azaindole, and benzo[ d ]imidazo[2,1 -b ]thiazole. Notably, this unique mechanistic approach obviates the need for transition metal catalysts, and additives, offering a significant advantage over existing synthetic methodologies Cite this paper: Chin. J. Chem. 2024, 42, XXX—XXX. DOI: 10.1002/cjoc.202400XXX Access to Bis(imidazopyridine)sulfanes by Employing Cysteine as Greener Sulfur Source Pallavi Saha, a Mohit Kumar, a Deepak K. Sharma *a a Department of Pharmaceutical Engg. & Tech., IIT-Banaras Hindu University, Varanasi, UP, 221005. | Bis(imidazopyridine)sulfanes | Cysteine | Environmentally friendly protocol | Imidazopyridine | Indole | Azaindole | Benzo[ d ]imidazo[2,1 -b ]thiazole | Transition metal free | | Comprehensive Summary | | An efficient and environmentally friendly protocol has been developed for synthesizing bis(imidazopyridine)sulfanes, employing cysteine as sulfur source. This method demonstrates broad substrate scope, accommodating diverse electron-donating and electron-withdrawing functional groups on the imidazopyridine core. Furthermore, the protocol is successfully extended to other heterocycles: indole, azaindole, and benzo[ d ]imidazo[2,1 -b ]thiazole. Notably, this unique mechanistic approach obviates the need for transition metal catalysts, and additives, offering a significant advantage over existing synthetic methodologies |

and Originality Content The advancement of medicinal chemistry has been significantly propelled by the development of sulfur-substituted therapeutics. The inclusion of sulfur-containing moieties in the design of biologically active compounds can profoundly impact their pharmacokinetic and pharmacodynamic profiles. [1, 2] For instance, a report indicates that the U.S. Food and Drug Administration had approved a total of 249 small-molecule drugs containing sulfur by 2016. Within these therapeutic agents, sulfur is predominantly integrated through functionalities such as thioethers, thiocarbonyls, sulfones, sulfoxides, sulfates, and sulfur-containing heterocycles. [3] Recognizing the critical role of heterocyclic moieties in therapeutic agents due to their inherent medicinal properties, considerable global research efforts are focused on the design and synthesis of bisheterocyclic compounds, linked by a -C-S-C- linker . This strategic approach seeks to synergistically combine the therapeutic potential of heterocycles with the pharmacokinetic and biochemical advantages bestowed by sulfur incorporation, ultimately enhancing the overall efficacy and specificity of the resulting molecular entities. [4] Figure 1 illustrates several biologically significant heterocycles featuring a thioether linkage. [5a-g] Figure 1. Biologically active sulfur-containing bisheterocycles. Imidazopyridines are well-established as privileged drug scaffolds, frequently found in natural products and demonstrating significant therapeutic efficacy. [6, 7] Among their various derivatives, bis(imidazoheterocycle) compounds connected by thioether bridges are particularly noteworthy for their diverse biological activities, including antitumor and anthelmintic properties (Figure 1). While the importance of bis(imidazopyridine)sulfanes is lucid, their synthesis has typically relied on methods that present certain limitations. Current literature reports several approaches for their synthesis, employing various sulfur sources. For example, Tian et al. (2019) developed a copper iodide-mediated protocol utilizing aryl thiocyanate as the sulfur source (Scheme 1a). [8a] Ting-Ma et al. introduced a method employing deoxofluor to construct the thioether linkage (Scheme 1b). [8b] Reddy et al. reported a synthesis using elemental sulfur as the sulfur source (Scheme 1c). [8c] More recently, Saha et al. described a visible light-mediated approach that also uses thiocyanate salts (Scheme 1d). [8d] A critical observation across all these existing methodologies is their consistent reliance on either transition metal catalysts, elemental sulfur, or sulfur salts. These requirements often lead to challenges such as harsh reaction conditions, the generation of hazardous waste, side products, and limited functional group tolerance, highlighting the need for more sustainable and efficient synthetic routes. In this work, we aimed to develop a more environmentally friendly method for synthesizing bis(imidazopyridine)sulfanes by exploring the use of sulfur-containing amino acid cysteine. The application of cysteine, especially for forming thioether linkages, has been relatively underexplored in organic synthesis. While Yan Xiao et al. reported that aryl iodides and heteroarenes could react with L-cysteine in the presence of cuprous iodide (CuI) as a metal catalyst and additive to form diaryl sulfides and heteroarene thiols,8e our approach takes a significant step forward. Building on our extensive research in functionalizing imidazo-heterocycles, [8d, 9a-c] we have developed a novel protocol for synthesizing bis(imidazopyridine)sulfanes that exclusively utilize cysteine as the sulfur source, crucially eliminating the need for any additives or metal catalysts (Scheme 1e). This advancement offers a more sustainable and efficient route for preparing these important compounds. Scheme 1: Strategies for synthesis of bis(imidazopyridine)sulfanes. Results and Discussion Our investigation began by selecting 2-phenylimidazo[1,2- a ]pyridine (1a) as our model substrate and 1 equivalent of cysteine as the sulfur source. We were pleased to find that using a DMSO:water solvent system at 80 °C, yielded the desired product 3a in 40% yield (Table 1, entry 1). Systemati optimization of the reaction conditions were pursued then. On gradually increasing the temperature to 110 °C significantly improved the yield of 3a to 80% (Table 1, entries 2 and 3). Further enhancement was achieved by increasing the amount of cysteine from 1 to 1.5 equivalents, which improved the yield to 90% (Table 1, entries 4 and 5). Next, on increasing the temperature beyond 110 °C did not lead to a substantial improvement in 3a yield (Table 1, entry 6), and a negligible 1% increase was observed when cysteine equiv. was raised to 2 equivalents (Table 1, entry 7). A solvent screening was conducted thereafter. Our findings indicated that DMSO:H 2 O was the optimal solvent system. Deviating from this by using DMSO (Table 1, entry 8), H 2 O (Table 1, entry 9), or 1,4-dioxane (Table 1, entry 10) consistently resulted in a detrimental impact on the reaction’s efficiency. Lastly, the impact of using alternative amino acid as potential sulfur sources was examined. Use of methionine proved unsuitable for this protocol, yielding only negligible amounts of 3a (Table 1, entry 11). Table 1 Optimization of reaction conditions for synthesis of bis(imidazopyridine)sulfane ( 3a ) a | 1 | Cysteine (1 equiv.) | DMSO:H 2 O (1:1) | 80 | 40 | | 2 | Cysteine (1 equiv.) | DMSO:H 2 O (1:1) | 95 | 65 | | 3 | Cysteine (1 equiv.) | DMSO:H 2 O (1:1) | 110 | 80 | | 4 | Cysteine (1.2 equiv.) | DMSO:H 2 O (1:1) | 110 | 84 | | 5 | Cysteine (1.2 equiv.) | DMSO:H 2 O (1:1) | 110 | 90 | | 6 | Cysteine (1.5 equiv.) | DMSO:H 2 O (1:1) | 110 | 90 | | 7 | Cysteine (2 equiv.) | DMSO:H 2 O (1:1) | 110 | 91 | | 8 | Cysteine (1.5 equiv.) | DMSO | 110 | traces | | 9 | Cysteine (1.5 equiv.) | H 2 O | 110 | 72 | | 10 | Cysteine (1.5 equiv.) | 1,4 dioxane | 110 | traces | | 11 | Methionine | DMSO:H 2 O (1:1) | 110 | - | a Reaction conditions: 1a (0.5 mmol), solvent (2 ml), 24 h. b Isolated yield. After successfully optimizing the reaction conditions (Table 1, entry 5), we proceeded to thoroughly investigate the substrate scope of our new method. To our delight, we witnessed that the nature of the substitution on the 2-aryl imidazopyridines had limited effect on the yield of the desired products, with compounds 3b-3j obtained in 75-93% yield (Scheme 2). Our observations revealed that imidazopyridines bearing electron-donating groups (EDGs), whether on the core scaffold or the side motif, exhibited excellent reactivity. The desired products with methyl substitutions ( 3b and 3f ) and methoxy substitutions ( 3c and 3h ) were isolated in 88-93% yields. The derivatives substituted with electron-withdrawing groups (EWGs) ( 3d, 3e and 3i ), the yields remained in range from 78-84%. Pleasingly, our proposed methodology demonstrated good tolerance for a naphthyl substitution at the C-2 position of imidazopyridine, resulting the corresponding product 3i in 77% yield. Furthermore, when a thiophene heterocycle was present at the C-2 position, the desired product 3j was obtained in 75% yield. These results highlight the broad applicability and versatility of our method across a range of substituted imidazopyridines. Scheme 2. Substrate scope with various substituted of imidazo[1,2- a ]pyridines. a a Reaction conditions: imidazo[1,2- a ]pyridines (0.5 mmol), Cysteine (1.5 equiv.), DMSO: H 2 O (1:1, 2 mL), 80 ℃, 24-48h. b Isolated yield. Next, we evaluated the reproducibility of our methodology using 3-phenyl imidazo[1,5- a ] derivatives. Regardless of the substitution pattern, the corresponding bis(imidazopyridine)sulfanes were obtained in very good yields. The anticipated product with no substitution, 5a, was isolated in 85% yield. The desired products containing an electron-donating group (EDG), 5b, and an electron-withdrawing group (EWG), 5c, were obtained in 88% and 82% yield, respectively (Scheme 3). The versatility of our established reaction protocol was then further assessed on other significant heterocyclic scaffolds. We successfully applied our optimized method to indole, azaindole, and benzo[ d ]imidazo[2,1- b ]thiazole derivatives. These yielded the desired products ( 7a - 7c ) in excellent yields, ranging from 90-92% (Scheme 4). These results strongly underscore the broad applicability, and robustness of our additive- and metal-free approach, extending its utility beyond imidazopyridines to other important heterocyclic systems. Scheme 3. Substrate scope with various substituted imidazo[1,5 a ]pyridines. a a Reaction conditions: imidazo[1,5- a ]pyridines (0.5 mmol), Cysteine (1.5 equiv.), DMSO: H 2 0 (1:1, 2 mL), 80 ℃, 24-30h. b isolated yield. Scheme 4. Substrate scope with diverse heterocycles. a a Reaction conditions: 6a - 6c (0.5 mmol), Cysteine (1.5 equiv.), DMSO: H 2 O (1:1, 2 mL), 80 ℃, 24. b Isolated yield. Next, we explored the reproducibility of our method when two different heterocycles were reacted together. When 2-phenylimidazo[1,2- a ]pyridine (1a) and indole (6a) were subjected to the optimized reaction conditions, we remarkably obtained the dual C-H sulfenylated product 8a in 65% yield, alongside 25% of 3a and a negligible yield of 7a . A similar outcome was observed when 1a was reacted with azaindole (6b), yielding the corresponding dual C-H sulfenylated product 8b in 68% yield, with 3a (15%) and a negligible amount of 7b . Finally, reacting 1a with 7-fluoro-2-phenylimidazo[1,2- a ]pyridine furnished 8c in 40% yield, along with 23% of 3d and 20% of 3a . We infer from these reactions that the yield of the final product depends on the reactivity of the individual starting materials with respect to each other (Scheme 5). Our proposed methodology further proved reproducible on a gram-scale, a crucial factor for its practical application (Scheme S1). This successful scale-up validates the robustness and utility of our approach. Scheme 5. Substrate scope with two different heterocycles. a a Reaction conditions: 1a (0.25 mmol), 6a, 6b, or 1d (0.25 mmol), Cysteine (1.5 equiv.), DMSO: H 2 O (1:1, 2 mL), 80 ℃, 24-30 h. b Isolated yield. To get an insight into the reaction mechanism of the proposed methodology, and the impact of the presence of amino and carboxyl group of cysteine on reaction, we conducted a few control experiments. First, when the reaction of 1a was carried out under optimized conditions but in the absence of D-cysteine , 3a was not detected (Scheme S2a). This conclusively confirms that D-cysteine serves as the essential sulfur source in our method. Next, we investigated the involvement of radical species. We performed the reaction with 1a in the presence of 2.5 equivalents of the radical scavenger TEMPO under optimized conditions. The reaction proceeded smoothly, and the yield of the desired product 3a remained unaffected by TEMPO’s presence ( Scheme S2b). This suggests that a radical pathway is unlikely to be the primary mechanism. On performing reaction with thioglycolic acid ( 9 ), with 1a, in the standard reaction condition afforded the product 2-((2-phenylimidazo[1,2- a ]pyridin-3-yl)thio)acetic acid ( 10 ) in 65% yield, along with 3a in 20% yield after 12 hours (Scheme S2c). On further continuing the reaction up to 24 h, 3a was obtained with 75% yield with a 20% yield of 10 . Such observation indicates gradual conversion of 10 to our desired final product 3a . On reaction of 1a with 2-aminoethanethiol (11) the desired product 3a was obtained in 54%, along with 12, with 25% yield. Such observations marked the superiority of cysteine, possessing both amino and carboxyl functionality as sulfur source; over either of 9, and 11 (Scheme S2d). From the observations of the control experiments and literature report, [8e] we have established a plausible reaction mechanism of our proposed method. At first, following prolonged heating of 1a and 2, in DMSO/water solvent system, lead to the formation of disulfide intermediate A. Subsequently, electrophilic addition of A at the nucleophilic C-3 position of 1a, leads to the formation of thioether intermediate B . B is further converted to the diimidazopyridinyl disulfide intermediate C, following an intramolecular nucleophilic attack by the terminal amine and carboxylate group. Formation of B and C as reaction intermediate were identified from the HRMS spectra of the reaction mixture, after 12 hours of commencement of the reaction. Next, in presence of another molecule of nucleophile 1a, cleave of the disulfide linker takes place, with a concomitant formation of C(Sp 2 )-S bond; thereby harboring the anticipated final compound 3a (Scheme 6). Scheme 6. Plausible reaction mechanism.

In summary, we have developed an additive-free protocol for the synthesis of bis(imidazopyridine)sulfanes, utilizing cysteine as a sulfur source. The reaction proceeded smoothly, yielding good to excellent results for all synthesized derivatives. This methodology demonstrates a broad substrate scope, effectively accommodating various heterocyclic scaffolds, including imidazo[1,2- a ]pyridines, imidazo[1,5- a ]pyridines, indole, azaindole, and benzo[ d ]imidazo[2,1- b ]thiazole moieties. Furthermore, it exhibits substantial functional group tolerance to both electron-donating and electron-withdrawing substitutions. Preliminary mechanistic studies suggest that an electrophilic substitution reaction is the likely mechanism underlying this established protocol. Experimental General procedure for the synthesis of bis(imidazopyridine)sulfanes (3a-3j, 5a-5c): An oven-dried round bottom flask was charged with the corresponding 2-arylimidazo[1,2- a ]pyridines (1a-j, 0.5 mmol), or 3-aryl imidazo[1,5- a ]pyridines (4a-c, 0.5 mmol), cysteine (1.5 equiv.) in 2 mL of DMSO:H 2 O in 1:1 ratio. The reaction mixture was stirred at 80 ℃ for 24-48h. On completion of the reaction, next reaction mixture was extracted with ethyl acetate (15 mL × 3). The combined organic layers were washed with brine, dried over Na 2 SO 4, and concentrated in vacuum. The residue was purified by column chromatography on silica gel using a solvent system of ethyl acetate/ n -hexane to afford the desired product. General procedure for the synthesis of 7a-7c: An oven-dried round bottom flask was charged with the corresponding heterocycles (6a-c, 0.5 mmol), cysteine (1.5 equiv.) in 2 mL of DMSO:H 2 O in 1:1 ratio. The reaction mixture was stirred at 80 ℃ for 24. On completion of the reaction, next reaction mixture was extracted with ethyl acetate (15 mL × 3). The combined organic layers were washed with brine, dried over Na 2 SO 4, and concentrated in vacuum. The residue was purified by column chromatography on silica gel using a solvent system of ethyl acetate/ n -hexane to afford the desired product. General procedure for the synthesis of 8a-8c: An oven-dried round bottom flask was charged with 2-phenylimidazo[1,2- a ]pyridines (1a, 0.25 mmol), and the corresponding heterocycle (6a-b, 1d, 0.25 mmol), cysteine (1.5 equiv.) in 2 mL of DMSO:H 2 O in 1:1 ratio. The reaction mixture was stirred at 80 ℃ for 24-48h. On completion of the reaction, next reaction mixture was extracted with ethyl acetate (15 mL × 3). The combined organic layers were washed with brine, dried over Na 2 SO 4, and concentrated in vacuum. The residue was purified by column chromatography on silica gel using a solvent system of ethyl acetate/ n -hexane to afford the desired product. Supporting Information The supporting information for this article is available on the WWW under https://doi.org/10.1002/cjoc.202400xxx.

Acknowledgements are extended to the individuals, laboratories, or organizations that have provided financial support and assistance for this work. We are thankful to the ICMR, New Delhi, and SERB-CRG, Gov. of India, for awarding research grant to D.K. (IIRP2023-0227, and CRG/2023/003559). P.S. is thankful to IIT(BHU), and the Ministry of Education, Gov. of India, for providing the teaching assistantship.

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Authors Metrics & Citations Metrics Article Usage 150views 171downloads Citations Download citation Pallavi Saha, Mohit Kumar, Deepak K. Sharma. Access to Bis(imidazopyridine)sulfanes by Employing Cysteine as Greener Sulfur Source. Authorea. 14 July 2025. DOI: https://doi.org/10.22541/au.175247487.73060988/v1 DOI: https://doi.org/10.22541/au.175247487.73060988/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu.