Letter pubs.acs.org/acscatalysis
Ionic Liquid-Catalyzed C−S Bond Construction using CO2 as a C1 Building Block under Mild Conditions: A Metal-Free Route to Synthesis of Benzothiazoles Xiang Gao, Bo Yu, Zhenzhen Yang, Yanfei Zhao, Hongye Zhang, Leiduan Hao, Buxing Han, and Zhimin Liu* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid, Interface, and Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *
ABSTRACT: The construction of the C−S bond using CO2 as a C1 feedstock can be considered as a green route for the synthesis of S-containing compounds and is of great significance. Herein, we report the acetate-based ionic liquids (ILs)-catalyzed synthesis of benzothiazoles via cyclization of 2-aminothiophenols with CO2 and hydrosilane under mild conditions (e.g., 60 °C, 0.5 MPa). It was found that the IL (e.g., 1-butyl-3-methylimidazolium acetate, [Bmim][OAc]) served as a multifunctional catalyst with activating CO2 and hydrosilane to form a formoxysilane intermediate, as well as simultaneously activating 2-aminothiophenols through a hydrogen bond, finally resulting in the production of benzothiazoles. [Bmim][OAc] showed the best performance, and a series of benzothiazoles were obtained in high yields. To the best of our knowledge, this is the first protocol for the synthesis of benzothiazoles using CO2 as a C1 building block under metalfree and mild conditions. KEYWORDS: carbon dioxide, ionic liquid, organosulfur compounds, room temperature, conversion
T
required, and the product yields were low. There is no doubt that exploration of highly efficient and metal-free catalysts for this kind of reaction under mild conditions is of great importance. Ionic liquids (ILs) have been widely used in chemical reactions, material synthesis, separation and fractionation due to their unique properties, such as negligible vapor pressure, high thermal stability, excellent solubility, and easy separation.13 In recent years, applications of ILs in CO2 capture and conversion have attracted considerable attention. Specially, functional ILs, have been extensively used in reversible capture of CO 2, 14 CO 2 hydrogenation, 15 synthesis of ureas,16 oxazolones,17 α-hydroxy ketones,18 formamides,19 and cyclic carbonates10 from CO2. For example, a CO2-reactive protic IL from neutralization of 1,8-diazabicyclo[5.4.0]undec-7-ene with trifluoroethanol could effectively catalyze the reactions of CO2 with 2-aminobenzonitriles at atmospheric pressure and room temperature.20 ILs have shown great potential in CO 2 transformation, which can realize it under mild reaction conditions. Herein, we discovered for the first time that the acetate-based ILs (e.g., 1-butyl-3-methylimidazolium acetate, [Bmim][OAc]) could efficiently catalyze the cyclization of 2-aminothiophenols with CO2 and hydrosilane at room temperature and low pressure (e.g., 0.5 MPa) without any metal catalysts or
he formation of the C−S bond is one of the fundamental transformations in organic synthesis.1 Sulfur-containing organic compounds are generally biocompatible and have great potential as novel pharmaceutical, agricultural, and chemical agents.2 Benzothiazole and its derivatives represent particularly useful organosulfur compounds, which can be used as antiinflammatory agents, enzyme inhibitors, plant growth regulators, antioxidants, vulcanization accelerators, imaging reagents, fluorescence materials, and electroluminescent devices.3 The protocols for the synthesis of benzothiazoles are typically based on transition metal-catalyzed reactions, such as condensation reactions of 2-aminothiophenol with carbonyl or cyano-containing compounds,4 or 2-haloaniline with various sulfur sources.5 Clearly, exploration of green routes by using renewable reagents to synthesize benzothiazoles is highly desirable. As a nontoxic, abundant, renewable, and environmentally friendly carbon source, CO2 can be used as a C1 building block to synthesize many value-added chemicals via the construction of C−H, C−N, C−O, C−C bonds, which offered green routes for the synthesis of these chemicals.6 To date, various valuable chemicals including methanol,7 formic acid,8 amides,9 carbonates,10 and aryl aldehydes11 have been produced using CO2 as a C1 feedstock. Synthesis of organosulfur compounds via the construction of C−S bonds using CO2 as a C1 feedstock is very interesting. The only work on this was the cyclization of 2aminothiophenols with CO2 and hydrosilane to produce benzothiazoles catalyzed by organic base.12 However, high temperature and pressure (e.g., 150 °C and 5.0 MPa) were © XXXX American Chemical Society
Received: August 24, 2015 Revised: September 28, 2015
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DOI: 10.1021/acscatal.5b01874 ACS Catal. 2015, 5, 6648−6652
Letter
ACS Catalysis
acetate-based ILs were effective for this reaction, which may be originated from the unique role of the acetate anion in the reaction. Other hydrosilanes, including bis(trimethylsiloxy), triethylsilane (Et3SiH), diethylsilane (Et2SiH2), poly(methylhydrosiloxane) (PMHS), bis(dimethylsilyl)amine (TMDS), methyldimethoxysilane (Me(OMe)2SiH), and diethoxymethylsilane (Me(OEt)2SiH), were also examined in the reaction of CO2 with 2-aminothiophenol. As illustrated in Table S1, (EtO)3SiH displayed the best performance, and Me(OEt)2SiH and Me(OMe)2SiH were also very effective, superior to other hydrosilianes. In addition, the influence of hydrosilane amount on the formation of benzothiazole was also investigated (Table S1, entries 8−11). The reusability of [Bmim][OAc] was tested as well, and the yield of benzothiazole almost stayed unchanged as the IL was reused five times (Figure S2), indicating that the IL was stable and recyclable for this reaction. Encouraged by the above results, a range of differently substituted 2-aminothiophenols were examined to react with CO2 and (EtO)3SiH catalyzed by [Bmim][OAc]. Unlike 2aminothiophenol, these substrates had very low solubility or were even not soluble in [Bmim][OAc], so the reactions were carried out in N,N-dimethylformamide (DMF), and the results are listed in Table 2. It was demonstrated that all the substrates could react with CO2 and (EtO)3SiH, producing corresponding benzothiazoles in good to excellent yields. 2-Aminothiophenols with electron-donating or electron-withdrawing groups including 5-methyl, 5-methoxy, 5-ethoxy, 5-fluoro, 5-chloro, and 5bromo were all tolerated (Table 2, entries 2, 3, 5−7, 8). The reactions also worked on 4-substituent 2-aminothiophenols; however, they showed less activity than the 5-substituent substrates (Table 2, entries 4, 8 vs 3, 7, respectively). When the substituent group was nitro-, a strong electron-withdrawing group, corresponding 5-nitrobenzothiazole was obtained with a yield of 48% (Table 2, entry 10). Our further study indicated that the [Bmim][OAc]-catalyzed system could also be applied to diverse o-phenylenediamines reacting with CO2 and (EtO)3SiH under mild reaction conditions (Scheme 1). Various substituted o-phenylenediamines with electron-donating groups, for example, CH3 and OCH3, to electron-withdrawing groups, for example, F, Cl, Br, and NO2, were all tolerated, affording the corresponding benzimidazoles in excellent yields. To explore the reaction mechanism, the interactions of [Bmim][OAc] with CO2, 2-aminothiophenol, and (EtO)3SiH were examined by NMR analysis, respectively. From the 1H NMR spectra of [Bmim][OAc] and its mixture with 2aminothiophenol (Figure 1), it was found that the signals of H atoms from NH2 (1) and SH (2) of 2-aminothiophenol in the mixture shifted downfield from δ 4.92 to 6.39, and δ 5.44 to 6.62, respectively; meanwhile, the signals of H atoms (3) from the methyl of acetate anion in the mixture shifted downfield from δ 1.56 to 1.80, and the signals assigning to C−H (4, 5, 6) of [Bmim][OAc] in the mixture shifted upfield obviously. These shifts demonstrated the formation of strong hydrogenbonding interaction between [OAc]− of the IL and the substrate. Considering that the ILs without [OAc]− anion showed no catalytic activity, it can be deduced that the hydrogen bonding of [OAc]− with the substrate was very important for the synthesis of benzothiazoles. The 1H, 29Si, and 15N NMR analyses for [Bmim][OAc], (EtO)3SiH, and their mixture were carried out. The 1H NMR
additives, producing a series of benzothiazoles in good to excellent yields. [Bmim][OAc] served as a multifunctional catalyst, which could activate 2-aminothiophenols via hydrogen bonding interaction, and simultaneously activate CO2 and hydrosilanes, respectively, to react with each other to form formoxysilanes that were the key intermediates for the synthesis of benzothiazoles. To the best of our knowledge, this is the first example to synthesize benzothiazoles using CO2 as a C1 feedstock under metal-free and mild conditions. Moreover, [Bmim][OAc] could catalyze the cyclization of diverse ophenylenediamines with CO2 to form benzomidazoles under mild conditions. In addition, the IL can be reused for five times without activity loss. The reaction of CO2 with 2-aminothiophenol and triethoxysilane ((EtO)3SiH) was first carried out in various ILs at 40 °C and 0.5 MPa, and the results are listed in Table 1. This reaction Table 1. Reaction of 2-Aminothiophenol with CO2 and Hydrosilane under Different Conditionsa
entry
catalystf
T/°C
convb/%
yieldb/%
1 2 3d 4 5 6 7 8 9 10e
-[Bmim][OAc] [Bmim][OAc] [Emim][OAc] [Bmmim][OAc] [DBU][OAc] [tBu4P][OAc] [TMG][OAc] [Bmim][OAc] [Bmim][OAc]
40 40 40 40 40 40 40 40 30 30
0 >99 76 >99 74 >99 >99 >99 75 89
0 99/90c 75 90 73 92 88 79 71 86
a
Reaction conditions: 2-aminothiophenol (1 mmol), IL (1 mmol), (EtO)3SiH (Si−H 10 equiv), CO2 (0.5 MPa), 24 h. bDetermined by 1 H NMR (DMSO-d6, 400 MHz) using mesitylene as an internal standard. cYields of isolated products. dCatalyst (0.1 mmol). eReaction time: 40 h. f([Emim][OAc]: 1-ethyl-3-methylimidazolium acetate; [Bmmim][OAc]: 1-butyl-2,3-dimethylimidazolium acetate; [DBU][OAc]: 1,5-diazabicyclo[4.3.0]non-5-ene acetate; [TMG][OAc]: tetramethylguanidine acetate).
did not proceed without catalyst (Table 1, entry 1). To our delight, the acetate-based ILs with different cations were effective for catalyzing this reaction (Table 1, entries 2−8). Especially, [Bmim][OAc] showed the best activity, affording benzothiazole in a yield of 99% under the experimental conditions (Table 1, entry 2). As 0.1 equiv of ionic liquid was used, the yield of benzothiazole decreased to 75% (Table 1, entry 3). This IL was active for catalyzing this reaction even at room temperature, producing benzothiazole in a yield of 86% as reaction time prolonged to 40 h (Table 1, entry 9). The influences of the temperature, CO2 pressure, and reaction time on the reaction were investigated, and it was indicated that the reaction could occur under mild conditions (see Figure S1). The differences in the catalytic activity of the tested acetatebased ILs may result from the influences of their cations, suggesting that both cations and anions of the ILs had joint effects on the reaction. Other ILs, including [Bmim][Br], [Bmim][BF4], [Bmim][ClO4], 1-butyl-3-methylimidazolium trifluoroacetate ([Bmim][TA]), and 1-butyl-3-methylimidazolium trifluoromethansulfonate ([Bmim][Tfo]), had no catalytic activity for this reaction. These findings indicated that the 6649
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ACS Catalysis Table 2. Synthesis of Various Benzothiazolesa
Figure 1. 1H NMR spectrum of [Bmim][OAc], 2-aminothiophenol (0.1 mmol) without and with [Bmim][OAc] (0.1 mmol). (DMSO-d6, 298 K).
signal assigning to Si−H (a) of (EtO)3SiH in the mixture shifted downfield from δ 4.18 to 4.61; meanwhile, the signals of H atoms (1, 2, 3) from [Bmim]+ shifted upfield and the signals of H atoms (4) from [OAc]− shifted downfield obviously (Figure 2). In addition, the 29Si NMR signal assigning to Si
a
Reaction conditions: reactant (0.5 mmol), [Bmim][OAc] (0.5 mmol), (EtO)3SiH (Si−H 10 equiv), DMF (1 mL), CO2 (0.5 MPa), 60 °C, 24 h. bDetermined by 1H NMR (DMSO-d6, 400 MHz) using mesitylene as an internal standard. cReactant (1 mmol), [Bmim][OAc] (1 mmol), (EtO)3SiH (Si−H 10 equiv), CO2 (0.5 MPa), 40 °C, 24 h. dYields of isolated products.
Figure 2. 1H NMR spectrum of pure (EtO)3SiH; pure [Bmim][OAc]; the mixture of [Bmim][OAc] (0.1 mmol) and (EtO)3SiH (1 mmol), 40 °C, 24 h; the reaction solution from (EtO)3SiH reacted with CO2 in the presence of [Bmim][OAc]. Reaction condition: [Bmim][OAc] (1 mmol), (EtO)3SiH (Si−H 10 equiv), CO2 (0.5 MPa), 40 °C, 24 h. (DMSO-d6, 298 K).
Scheme 1. Synthesis of Benzimidazolesa
atom in (EtO)3SiH shifted upfield from δ −59.24 to −81.81 as (EtO)3SiH mixed with the IL (Figure S3). A slight chemical shift of N atoms in IL was also observed in the 15N NMR spectrum of the mixture (Figure S4). All these shifts demonstrated that this IL activated the Si−H bond of (EtO)3SiH, which could weaken the Si−H bond of (EtO)3SiH and make it more favorable for the insertion of CO2. [Bmim][OAc] was exposed to CO2 at 40 °C and 0.5 MPa for 24 h and was examined after CO2 release by 13C NMR analysis. A new 13C signal appeared at δ = 154.8 (Figure S5), attributing to the carbonyl carbon of carbonate, which suggested that CO2 was activated by the cation [Bmim]+ of the IL to form a carbonate intermediate, in agreement with that previously reported.21 Furthermore, the solution of [Bmim][OAc], (EtO)3SiH, and CO2 was prepared at 0.5 MPa and 40 °C and examined by 1H NMR analysis after it was stirred for 24 h and CO2 was released. In the spectrum of the mixture, a new
a
Reaction conditions: reactant (0.5 mmol), [Bmim][OAc] (0.5 mmol), (EtO)3SiH (Si−H 10 equiv), DMF (1 mL), CO2 (0.5 MPa), 60°C, 24 h. The yield was determined by 1H NMR (DMSO-d6, 400 MHz) using mesitylene as an internal standard.
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ACS Catalysis signal appeared at δ = 8.31 (Figure 2, b), indicating the formation of formoxysilane intermediate, in agreement with that previously reported.22 This means that the IL catalyzed the reaction of CO2 with (EtO)3SiH to form the formoxysilane intermediate. After 2-aminothiophenol was added into the above solution, excitingly, benzothiazole was obtained in a yield of 94%. From the above findings, it is clear that the IL served as a multifunctional catalyst, activating CO2, (EtO)3SiH, and the substrate simultaneously, and that the formoxysilane from CO2 and (EtO)3SiH was the key intermediate, which could further react with 2-aminothiophenol easily, producing the final product. On the basis of the experimental results and previous reports,9a,12,21a,22 a possible mechanism was proposed, as shown in Scheme 2. CO2 was activated by the IL to form intermediate
ACKNOWLEDGMENTS
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A. The Si−H bond of (EtO)3SiH was activated by the IL, thus making the insertion of A much easier to form formoxysilane intermediate C; meanwhile, X (NH, S)-o-substituted aniline was activated via hydrogen bond by the anion ([OAc]−) of IL, and the nucleophilic N atom attacked the carbon atom of intermediate C to form intermediate E. Subsequently, F was obtained through the intramolecular nucleophilic cyclization of E, followed by dehydration, thus yielding the product G. In summary, a new strategy to synthesis of benzothiazoles has been developed via the reaction of 2-aminothiophenols with CO2 and triethoxysilane catalyzed by acetate-based ILs under mild conditions, and a series of benzothiazoles were obtained in high yields. [Bmim][OAc] was the most effecive catalyst that can simultaneously activate CO2, triethoxysilane and 2-aminothiophenols. We believe that this simple, metal-free route to produce benzothiazoles has the potential of various applications, and more reactions forming the C−S bond may be discovered using CO2 as a C1 source and ILs as the catalyst.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b01874.
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We thank the National Natural Science Foundation of China (Nos. 21533011, 21125314, 21403252, 21503239).
Scheme 2. Possible Reaction Mechanism
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Letter
Experimental procedures and characterization data (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 6651
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