English

• Calcium carbide (CaC₂) is a crucial product in the coal chemical industry, primarily serving as a fundamental raw material to produce acetylene, a significant C2 synthon in organic synthesis [1,2]. However, the flammable and explosive nature of acetylene gas presents substantial challenges for its storage, transportation, and transformation. In contrast, CaC₂ stands for a green, inexpensive, and safe alternative to acetylene gas, capable of being directly used as an alkyne source to synthesize various organic compounds [3–9]. Despite its advantages, the direct uptake of CaC₂ in organic synthesis is restricted by its high lattice energy and low solubility in organic solvents. Currently, two methods have been identified for the in situ release of acetylene from CaC₂: Mechanical disruption via ball milling (Scheme 1A) [10–12] and depolymerization with water. CaC₂ depolymerized with water is particularly attractive in organic chemistry since it can accommodate a series of downstream transformations, such as addition [13–16], coupling [17–21], and cyclization reactions [22–27] (Scheme 1B). Despite the advancements, the current methods for the utilization of CaC₂ remain relatively limited. Hydrogen sulfide (H₂S), a toxic and harmful gas by-product of the coal chemical industry [28], has not been efficiently utilized or treated in industrial processes (Scheme 1C). Typically, H₂S is converted into sulfur through the Claus process, leading to minimal economic benefits [29,30]. Therefore, a novel strategy to utilize H₂S as a valuable sulfur resource for synthesizing sulfur-containing fine chemicals remains an important yet unmet need.

  Scheme 1

  

  Scheme 1.  The previous and our strategy of CaC₂ depolymerization.

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  Thioamides find widespread applications in the fields of agriculture and pharmaceutical industry [31–34]. Moreover, thioamides can serve as key intermediates to construct sulfur-containing heterocyclic compounds of great value, such as thiophene [35,36], thiazole [37,38], and benzothiazine [39]. Therefore, the development of the method for synthesizing and applying thioamide derivatives is of great significance. Currently, the Willgerodt-Kindler (WK) reaction from aldehydes, amines, and S₈ are commonly utilized to synthesize thioamide derivatives (Scheme 2A₁) [40–42]. In addition, an elegant three-component cascade involving amines, S₈, and alkynes has been introduced to expand the horizon of thioamides synthesis (Scheme 2A₂) [43]. We envisioned that H₂S, as an acidic gas, can react with the basic CaC₂ to form acetylene and HS⁻ ions, and the nucleophilic HS⁻ ions are particularly useful for constructing sulfur-containing fine chemicals.

  Scheme 2

  

  Scheme 2.  The synthesis methods of thioamides and some strategies for preparing organic fine chemicals using CaC₂ depolymerized by H₂S.

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  Following the strategy to in situ generate acetylene and HS⁻ enabled by depolymerization of CaC₂ with H₂S, it is speculated that the nucleophile HS⁻ attacks acetylene to form an ethenethiol intermediate Ⅰ (Scheme 2B), which would tautomerize to the more stable ethanethial Ⅱ. By introducing amines, ethanethial Ⅱ undergoes an addition reaction with amines to form the intermediate Ⅲ, and then is oxidized to yield thioamides. Furthermore, the sulfur atom of thioamides exhibits nucleophilicity, while the carbon atom shows electrophilicity, allowing the further versatile addition cascades (Scheme 2C). As a proof of concept, three types of novel tandem reactions involving CaC₂, H₂S, and various substrates were developed to construct structurally diverse heterocyclic compounds (Schemes 2D-F).

  To initialize the study, phenylmethanamine (S1) was chosen as a model substrate to prepare N-benzylethanethioamide (1) directly with CaC₂ and H₂S (Tables S1-S3 in Supporting information). First, the molar ratios of CaC₂ and H₂S were screened, and it was found that optimal reactivity was achieved when a feed ratio of CaC₂ to H₂S is 1:2. Subsequently, the effect of the temperature, time and solvents was investigated. The optimal reaction condition is S1 as the limiting substrate, 3 equiv. of CaC₂, and 6 equiv. of H₂S in NMP (2 mL) at 130 ℃ for 18 h, and compound 1 can be produced in 99% yield (Table S3, entry 2).

  Under the optimized reaction conditions, the substrate scope of amines was evaluated with CaC₂ and H₂S (Scheme 3). The reaction proceeded smoothly for aliphatic primary amines containing both electron-donating and electron-withdrawing groups, and the corresponding thioamides (1–17) were efficiently produced in good to excellent yields. However, aromatic amines, such as aniline, exhibited lower reactivity due to their low nucleophilicity, resulting in a diminished yield of thioamide (18). When heterocyclic methanamines were used, such as pyridinyl, furanyl, thiophenyl or tetrahydrofuranyl, the corresponding products (19–22) were obtained in good yields. Additionally, 1-(naphthalen-1-yl)ethan-1-amine react with CaC₂ and H₂S to produce the corresponding thioamide (23) in excellent yields. Secondary amines were also viable in the preparation of the target thioamides (24–28) in good to excellent yields. Similarly, diamines were effectively transformed into dithioamides (29, 30) in high yield. Heterocyclic amines, such as morpholine or tetrahydropyrrole, were also compatible under the optimal conditions, producing the corresponding thioamides (31, 32) in good yields.

  Scheme 3

  

  Scheme 3.  Synthesis of various thioamides from amine with CaC₂ and H₂S. Reaction conditions: Amine (1.0 mmol), CaC₂ (3.0 mmol), H₂S (6.0 mmol), NMP (2.0 mL) at 130 ℃ for 18 h. Isolated yield. ^a S29: 1,3-phenylenedimethanamine (0.5 mmol); ^b S30: piperidin-4-amine (0.5 mmol).

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  Thioamides, which feature both nucleophilic sulfur and electrophilic carbon sites, serve as crucial intermediates in organic synthesis. To explore their reactivities, a series of tandem reactions were designed to react with electrophiles or nucleophiles. Leveraging the nucleophilicity of the sulfur atom, electrophilic groups such as alkynes can be introduced to synthesize 2-methyltetrahydrothiazole derivatives (Scheme 2D). Therefore, a three-component reaction of 2-methyl-4-phenylbut-3-yn-2-amine (S33), CaC₂, and H₂S was used as the model reaction to synthesize 5-benzylidene-2,4,4-trimethylthiazolidine (33). The optimal conditions for the reaction of S33 with CaC₂ and H₂S were established (Tables S4-S9 in Supporting information). Under these optimized conditions, the substrate scope was systematically investigated (Scheme 4). A wide range of primary propargyl amines were compatible with the protocol, producing the 2-methyltetrahydrothiazole derivatives in good to excellent yields (33–41). Unfortunately, steric hindrance posed by the N-Bu group in propargylamine (S42) impeded the formation of the corresponding product 42. Furthermore, when a terminal alkyne without an aromatic group (R¹) was used as substrate (S43), no target product was obtained (43). These experimental results suggest that the presence of an aromatic ring may enhance electron delocalization in alkynes, promoting electrophilic addition. In contrast, the aromatic ring replaced with a hydrogen atom would hinder the formation of the desired product. Moreover, when the methyl groups of R² and R³ were replaced with hydrogen atoms, the target product 44 could not be obtained either. These findings indicate that our method is significantly influenced by the Thrope−Ingold effect [44] and is more suitable for propargyl amines with bulky R² and R³ substituents.

  Scheme 4

  

  Scheme 4.  Substrate scope of propargylamine. Reaction conditions: propargylamine (0.2 mmol), CaC₂ (0.6 mmol), H₂S (0.9 mmol), TBD (0.2 mmol), DMSO (2.0 mL) at 110 ℃ for 4 h. Isolated yield.

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  Additionally, for some nucleophilic substrates with two nucleophilic moieties, like o-phenylenediamine, one of the amino groups can react with CaC₂ and H₂S to yield thioacetamide. Subsequently, the other amino group can undergo an intramolecular nucleophilic attack on the carbon atom of the thioacetamide, leading to the formation of 2-methylbenzoimidazole derivatives (Scheme 2E). The optimal reaction conditions for the synthesis of 2-methylbenzimidazole (45) were obtained, in which o-phenylenediamine (S45–1) was chosen as the model substrate to react with CaC₂, and H₂S (Tables S10-S13 in Supporting information). Under these optimized conditions, the reactions of CaC₂ and H₂S with various substrates were investigated (Scheme 5). It was observed that o-phenylenediamine with either electron-donating or electron-withdrawing groups produced the desired products in moderate to excellent yields (45–57). Only moderate yields of 2-methyl-1H-imidazo[4,5-b]pyridine (58) were obtained when 2,3-diaminopyridine was employed as a substrate. This lower yield may be attributed to the lower nucleophilicity of the amino group on the pyridine ring. When both naphthalene-1,8-diamine and naphthalene-2,3-diamine are used as substrates, the reaction proceeds unaffected, yielding the target product with excellent efficiency (59, 60). Notably, o-phenylenediamine contained N-methyl or N-phenyl could react with CaC₂ and H₂S to produce the target product in excellent yields (61, 62). However, the amino group containing protecting groups, such as N-acetyl (S45–2) or N-Boc (S45–3), only the 2-methylbenzimidazole (45) was generated, whereas the target product containing protecting groups was not obtained. It is speculated that the alkaline reaction conditions may remove these protecting groups. For aliphatic diamines, such as 1,2-diphenylethane-1,2-diamine (S63), the product imidazoline (63) can also be achieved in high yields. However, the N¹-phenylethane-1,2-diamine (S64) can not react with CaC₂ and H₂S to produce the target product imidazoline, and only the N-(2-(phenylamino)ethyl)ethane-thioamide (64) is produced. This result demonstrates that the weak nucleophilicity of the amino group in aniline prevents the intramolecular cyclization to form the imidazoline product. Additionally, 2-aminobenzenethiol derivatives react with CaC₂ and H₂S to produce the benzothiazole derivatives in excellent yields (65, 66). When 2-aminobenzenethiol is replaced with o-aminophenol, the 2-methylbenzoxazole derivatives are obtained only in medium yield due to the weaker nucleophilicity of the oxygen atom (67, 68). Similarly, when 2-(aminomethyl)aniline, 2-aminobenzamide, or 2-aminobenzenesulfonamide is used as the starting material, the target product is obtained only in moderate yields (69–71).

  Scheme 5

  

  Scheme 5.  Substrate scope of o-phenylenediamine derivatives. Reaction conditions: o-phenylenediamine derivatives (1.0 mmol), CaC₂ (3.0 mmol), H₂S (6.0 mmol), NMP (2.0 mL) at 140 ℃ for 24 h. Isolated yield.

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  Since many substituted aniline are industrially produced by reducing nitro compounds [45–48], it would be beneficial to directly employ nitro compounds to replace amines under the reduction of H₂S to form o-phenylenediamine, which then reacted with CaC₂ and H₂S to produce 2-methylbenzimidazole (Scheme 2F). This novel multi-step tandem reaction provides an improved method for synthesizing 2-methylbenzimidazole. Under the optimal conditions established for synthesizing 2-methylbenzimidazole from the reaction of o-phenylenediamine with CaC₂ and H₂S, various 2-nitroaniline derivatives were directly used to illustrate their efficiency (Table 1). Similar to o-phenylenediamine, 2-nitroaniline derivatives containing both electron-withdrawing and electron-donating groups successfully reacted with CaC₂ and H₂S, giving 2-methylbenzimidazole derivatives in moderate to excellent yields (Table 1, entries 1–13). Additionally, the N-methyl or N-phenyl substituted 2-nitroaniline could react with CaC₂ and H₂S to produce N-substituted 2-methylbenzimidazole in high yields (entries 14 and 15). Notably, N-(2-nitrophenyl)acetamide (S45–5) reacted with CaC₂ and H₂S generated 2-methylbenzimidazole instead of the expected product with protecting groups (Table 1, entry 16). Furthermore, when 3-nitropyridin-2-amine was used, the desired product (58) was obtained in a lower yield due to the reduced nucleophilicity of the amino group on the pyridine rings. When 2-nitrophenol was used as the substrate, the target product 2-methylbenzoxazole (67) was obtained only in a low yield due to the weaker nucleophilicity of the oxygen atom. These findings highlight the versatility of this synthetic approach, providing valuable insights for its industrial application.

  Table 1

  

  Table 1.  Substrate scope of 2-nitroaniline derivatives.^a

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  Entry	2-Nitroaniline derivatives			Product	Yield (%)
  	R1	R2
  1	H	H	S45–4	45	95
  2	4-CH3	H	S46–2	46	98
  3	5-CH3	H	S46–3	46	80
  4	3-CH3	H	S47–2	47	94
  5	2-CH3	H	S47–3	47	97
  6	4-CH3, 5-CH3	H	S49–2	49	87
  7	5–OCH3	H	S50–2	50	59
  8	5-Cl	H	S52–2	52	42
  9	4-Cl	H	S52–3	52	94
  10	3-Cl	H	S53–2	53	67
  11	2-Cl	H	S53–3	53	60
  12	4-CF3	H	S55–2	55	92
  13	4-COPh	H	S57–2	57	85
  14	H	CH3	S61–2	61	97
  15	H	Ph	S62–2	62	96
  16	H	Boc	S45–5	45	97
  17			S58–2	58	33
  18			S67–2	67	20
  a Reaction conditions: 2-nitroaniline derivatives (1.0 mmol), CaC 2 (3.0 mmol), H 2S (6.0 mmol), NMP (2.0 mL) at 140 ℃ for 24 h. Isolated yield.

  To demonstrate the scalability of this synthetic method, the gram-scale synthesis of 45 was performed (Scheme 6A). When 10 mmol of o-phenylenediamine (S45–1) was employed to react with CaC₂ and H₂S under optimized conditions, the desired product 45 was obtained in 96% yield (1.273 g), highlighting the potential utility of our protocol. To further illustrate the practical applicability of this method, an antifungal drug Chlormidazole [49] was successfully synthesized in high yield by the reaction of N¹-(4-chlorobenzyl)benzene-1,2-diamine (S72–1) or N-(4-chlorobenzyl)-2-nitroaniline (S72–2) with CaC₂ and H₂S under optimized reaction conditions (Schemes 6B and C). Additionally, 2-methylbenzimidazole can be exploited as a key intermediate for synthesizing value-added scaffolds. For example, benzo[4,5]imidazo[1,2-a]quinoline (73) [50], 2-methyl-1-(phenylsulfonyl)-1H-benzo[d]imidazole (74) [51] and (Z)-2-methyl-1-(1-phenyl-2-(phenylsulfonyl)vinyl)-1H-benzo[d]imidazole (75) [52] were synthesized through nitrogen atom functionalization (Schemes 6D-F). These results further demonstrate the practicality and versatility of this synthetic method, making it a promising approach for industrial applications.

  Scheme 6

  

  Scheme 6.  Synthetic applications.

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  To elucidate the reaction mechanism, several control experiments were conducted under standard conditions. Initially, it was observed that a small amount of o-phenylenediamine could be obtained via the reduction of 2-nitroaniline in the presence of H₂S (Scheme 7A). In the presence of Ca(OH)₂, the reduction of 2-nitroaniline was completely reduced to o-phenylenediamine in 90% yield (Scheme 7B). These results collectively demonstrated that H₂S could efficiently reduce 2-nitroaniline into o-phenylenediamine under basic conditions. Next, the introduction of the radical scavenger 2,2,6,6-tetramethylpiperidoxyl (Tempo) did not inhibit the formation of compound 1 in the model reaction of S1 with CaC₂ and H₂S, precluding the radical pathway as the major component (Scheme 7C). In the subsequent control experiment, when acetylene and NaHS were directly used instead of CaC₂ and H₂S to react with S1, the target thioamides was not produced (Scheme 7D). This result is probably attributed to the lack of H⁺ in the reaction system, which impedes the addition reaction of acetylene with HS⁻. Fortunately, when S1 reacted with acetylene in a mixed system of NaHS and H₂S, the target product was successfully obtained in 87% yield (Scheme 7E). This result indicates that the initial step involves the reaction of CaC₂ and H₂S to form acetylene and HS⁻, followed by the nucleophilic attack of HS⁻ to acetylene to form ethenethiol, and then isomerized to form ethanethial, a process requiring the presence of H⁺. Notably, the formation of thioamides from the reaction of amine and ethanethial typically requires an oxidizing reagent. However, no external oxidant was added during the experimental process. Thus, air is speculated to probably acts as the oxidizing reagent. However, the target product was still obtained with a 93% yield even under the atmosphere of N₂ (Scheme 7F), indicating that oxygen was not the primary oxidizing reagent. To identify the source of the oxidizing reagent, the reaction solution of S45–1 with CaC₂ and H₂S was analyzed by ¹H NMR, revealing characteristic peaks of 1,2-diethyldisulfane at 1.32 ppm and 2.71 ppm (Fig. 1). It was found that 1,2-diethyldisulfane originates from the oxidization of ethanethiol during post-processing, which is derived from the reduction of ethanethial. Therefore, ethanethial acts as the oxidizing reagent during the formation of thioamide.

  Scheme 7

  

  Scheme 7.  Mechanistic studies.

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  Figure 1

  

  Figure 1.  ¹H NMR investigation of S45–1 (a), 45 (b), 1,2-diethyldisulfane (c), and the reaction of S45–1, CaC₂, and H₂S in NMP at 140 ℃ for 24 h (d).

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  Based on the aforementioned experimental results, a plausible mechanism is proposed in Scheme 8. Initially, CaC₂ reacts with H₂S to generate acetylene and Ca(HS)₂, which can simultaneously release Ca²⁺ and HS⁻. Subsequently, HS⁻ nucleophilically attacks acetylene to form the ethenethiol intermediate Ⅰ, which is prone to tautomerize into more stable ethanethial Ⅱ. Ethanethial, as a key intermediate, reacts with nucleophilic or electrophilic substrates via two distinct cascade strategies to construct different heterocycles. In the first route, 2-methyltetrahydrothiazole is synthesized from a tandem three-component reaction of propargylamine (S33), CaC₂ and H₂S. After ethanethial Ⅱ is produced from CaC₂ and H₂S, it is nucleophilically attacked by the amino group of S33 to generate intermediate Ⅲ under basic conditions. Subsequently, the intermediate Ⅲ undergoes intramolecular electrophilic addition to form 5-benzylidene-2,4,4-trimethylthiazolidine (33). Similarly, in the second route, 2-methylbenzoimidazole is synthesized via a tandem three-component reaction of CaC₂, H₂S and o-phenylenediamine (S45–1) or 2-nitroaniline (S45–4). Initially, 2-nitroaniline is reduced by H₂S to produce o-phenylenediamine, which then attacks intermediate Ⅱ which is generated from CaC₂ and H₂S to produce intermediate Ⅳ. Intermediate Ⅳ is then oxidized by intermediate Ⅱ to form thioamide Ⅴ (which could be detected by HRMS) and ethanethiol. Notably, ethanethiol is easily oxidized to form 1,2-diethyldisulfane. Due to the strong electrophilicity of the carbon atom in the C=S bond of thioamide Ⅴ, another amino group nucleophilically attacks C=S bond in an intramolecular manner, leading to the formation of intermediate Ⅵ. Finally, intermediate Ⅵ efficiently eliminates H₂S to produce 2-methylbenzimidazole (45) as the final product.

  Scheme 8

  

  Scheme 8.  Proposed reaction mechanism.

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  In summary, a novel strategy for the depolymerization of CaC₂ using H₂S has been developed. This strategy paves the way for an efficient and catalyst-free approach to synthesize thioamides. Based on this strategy, two types of tandem reactions have been designed as a proof of concept. These reactions take advantage of the nucleophilic sulfur site and the electrophilic carbon site of thioamides to construct various heterocycles. Specifically, utilizing the nucleophilic sulfur site, a new method has been developed for synthesizing 2-methylthiazole derivatives through the reaction of CaC₂, H₂S with propargylamines contain both a nucleophilic group (-NH₂) and an electrophilic group (C≡C). Additionally, by exploiting the electrophilic carbon site of thioamides, substrates with dual nucleophilic groups can react with CaC₂ and H₂S to establish a tandem reaction for the synthesis of 2-methylbenzimidazole derivatives. Leveraging the reducibility of H₂S, a new multi-step tandem reaction for synthesizing 2-methylbenzimidazole compounds has been established by the reduction of common industrial products such as 2-nitroaniline. Although this synthetic strategy successfully enables the safe, green, and efficient conversion of CaC₂ and H₂S into high-value organic chemicals, it currently has some limitations. The reaction requires high temperatures, and substrates with weak nucleophilicity (e.g., aromatic amines) show limited reactivity under these conditions. Future efforts will focus on optimizing the reaction parameters by selecting appropriate catalytic systems, aiming to achieve milder reaction conditions and broaden the scope of applicable substrates.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Yunzhe Du: Writing – original draft, Methodology, Formal analysis, Data curation. Siliu Cheng: Writing – review & editing, Data curation. Shuyi Li: Writing – review & editing. Junping Niu: Writing – review & editing. Yuan Gou: Data curation. Ligang Yan: Writing – review & editing, Methodology. Tian-Xing Zhang: Writing – review & editing. Ruijun Xie: Writing – review & editing. Limin Han: Writing – review & editing. Tiezheng Jia: Writing – review & editing. Ning Zhu: Writing – review & editing, Methodology, Investigation, Funding acquisition, Conceptualization.

  Acknowledgments

  We thank the National Natural Science Foundation of China (Nos. 22379075, 22471114, U23A20528), the Group Project of Developing Inner Mongolia through Talents from the Talents Work Leading Group under the CPC Inner Mongolia Autonomous Regional Committee (No. 2025TEL04), Natural Science Foundation of Inner Mongolia Autonomous Region (No. 2024LHMS02011), the Innovative Research Team in Universities of Inner Mongolia Autonomous Region (No. NMGIRT2212) and Basic Scientific Research Funds of Universities directly under the Inner Mongolia Autonomous Region (Nos. ZTY2025013 and JY20240076) for financial support.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111233.

  
  
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  Scheme 1  The previous and our strategy of CaC₂ depolymerization.

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  Scheme 2  The synthesis methods of thioamides and some strategies for preparing organic fine chemicals using CaC₂ depolymerized by H₂S.

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  Scheme 3  Synthesis of various thioamides from amine with CaC₂ and H₂S. Reaction conditions: Amine (1.0 mmol), CaC₂ (3.0 mmol), H₂S (6.0 mmol), NMP (2.0 mL) at 130 ℃ for 18 h. Isolated yield. ^a S29: 1,3-phenylenedimethanamine (0.5 mmol); ^b S30: piperidin-4-amine (0.5 mmol).

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  Scheme 4  Substrate scope of propargylamine. Reaction conditions: propargylamine (0.2 mmol), CaC₂ (0.6 mmol), H₂S (0.9 mmol), TBD (0.2 mmol), DMSO (2.0 mL) at 110 ℃ for 4 h. Isolated yield.

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  Scheme 5  Substrate scope of o-phenylenediamine derivatives. Reaction conditions: o-phenylenediamine derivatives (1.0 mmol), CaC₂ (3.0 mmol), H₂S (6.0 mmol), NMP (2.0 mL) at 140 ℃ for 24 h. Isolated yield.

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  Scheme 6  Synthetic applications.

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  Scheme 7  Mechanistic studies.

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  Figure 1  ¹H NMR investigation of S45–1 (a), 45 (b), 1,2-diethyldisulfane (c), and the reaction of S45–1, CaC₂, and H₂S in NMP at 140 ℃ for 24 h (d).

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  Scheme 8  Proposed reaction mechanism.

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  Table 1.  Substrate scope of 2-nitroaniline derivatives.^a

  Entry	2-Nitroaniline derivatives			Product	Yield (%)
  	R1	R2
  1	H	H	S45–4	45	95
  2	4-CH3	H	S46–2	46	98
  3	5-CH3	H	S46–3	46	80
  4	3-CH3	H	S47–2	47	94
  5	2-CH3	H	S47–3	47	97
  6	4-CH3, 5-CH3	H	S49–2	49	87
  7	5–OCH3	H	S50–2	50	59
  8	5-Cl	H	S52–2	52	42
  9	4-Cl	H	S52–3	52	94
  10	3-Cl	H	S53–2	53	67
  11	2-Cl	H	S53–3	53	60
  12	4-CF3	H	S55–2	55	92
  13	4-COPh	H	S57–2	57	85
  14	H	CH3	S61–2	61	97
  15	H	Ph	S62–2	62	96
  16	H	Boc	S45–5	45	97
  17			S58–2	58	33
  18			S67–2	67	20
  a Reaction conditions: 2-nitroaniline derivatives (1.0 mmol), CaC 2 (3.0 mmol), H 2S (6.0 mmol), NMP (2.0 mL) at 140 ℃ for 24 h. Isolated yield.

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