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Cesium loaded on silica as an efficient and recyclable catalyst for the novel synthesis of selenophenes
⁎Corresponding author. Address: Department of Organic Chemistry, Annamacharya Institute of Technology & Sciences, J.N.T. University, Tirupati 517 502, India. Tel.: +91 9441300060; fax: +91 877 2243909. gajulapallilavanya@gmail.com (Palakondu Lavanya)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
A simple and efficient catalytic protocol for the 2-amino-5-substitutedselenophene-3-carbonitrile derivatives (4a–k) via the one-pot condensation of dicyanomethane, selenium and various substituted esters using 1% cesium silica is reported. The present method offers several advantages such as high to excellent yields, short reaction times, recovery and reusability of catalyst, mild reaction conditions and easy workup procedures.
Keywords
Selenophenes
Reusable catalyst
Solvent-free conditions
One-pot reaction
1 Introduction
Solid acids have attracted much attention in organic synthesis owing their easy work-up procedures, easy filtration and minimization of cost and waste generation due to reuse and recycling of these catalysts (Breslow 1980). The application of heteropolyacids as catalytic materials is growing continuously in the catalytic field. These compounds possess unique properties such as well-defined structure, Brønsted acidity, possibility to modify their acid–base and redox properties by changing their chemical composition, high proton mobility, being environmentally benign, and ease of reusability (Clark and Macquarrie, 2002). Because of their stronger acidity, they generally exhibit higher catalytic activity than conventional catalysts. They are used as industrial catalysts for several solid phase reactions (Shaabani and Maleki, 2007).
The synthesis of selenophenes has received significant attention in recent years because of their wide range of biological and pharmaceutical properties such as antiviral (Chu et al., 2000), antibacterial (Chitra et al., 2011), anticancer (Lin et al., 2009), anti-inflammatory (Desai et al., 2010), activities as well as efficacy in photodynamic therapy (Modica-Napolitano et al., 1990), antioxidants (Wang et al., 2012), and anticonvulsant (Wilhelm et al., 2009). Selephenes have also been prepared from various oxidative and catalytic reagents such as IPy2BF4 (Barluenga et al., 2003a,b), halogen (Bellina et al., 2003), TCCA (Sniady et al., 2008), organotellurium electrophiles (Alves et al., 2008a), FeCl3 (Gay et al., 2010), NXS (Sniady et al., 2005), BF3·3OEt2 (Okitsu et al., 2008) and CuI catalysts (Li et al., 2001). Some of these methods suffer from severe drawbacks, including the use of large amounts of expensive reagents and catalysts, poor yield, use of toxic solvents and catalysts, long reaction times, special apparatus, and tedious workup procedures, which necessitate the development of an alternative route for the synthesis of these biologically active molecules.
A great number of these heterocycles have been synthesized and their chemistry has attracted a good pact of attention and activity from a variety of standpoints such as structures, stereochemistry, reactivity’s and applications to organic synthesis (Alves et al., 2008b). Regarding the synthesis of the heterocycles, the transition-metal catalyzed cyclization reaction of simple acyclic precursors is one of the most attractive ways to directly construct complicated molecules under mild conditions (Sibor and Pazdera, 1996). During the past years, there has been an extraordinary accumulative attention to the development of environmentally benign protocols and the great challenge for chemists is to apply cost-effective, green, mild and alternative methodologies (Maddila and Jonnalagadda, 2012a,b). In this sense, cesium silica has performed as a versatile alternative, due to its low price, low toxicity and environmentally benign properties. Considering these features, many findings concerning cesium-silica catalyzed organic transformations have been reported.
Cesium silica is one of the most abundant catalysts in the world and has been widely studied during the last several years because it is a biodegradable material and a renewable resource. Its unique properties make it an attractive alternative to conventional organic or inorganic supports in catalytic applications. Recently, cesium silica has emerged as a promising solid-support acid catalyst for acid-catalyzed reactions, such as the synthesis of bicyclic β-lactam (Nivsarkar and Kaushik, 2005), methyl methacrylate (Mamoru, 2005), Keggin-type heteropoly compounds (Rafieea et al., 2009), 3-(a-hydroxyaryl)indoles (Elayaraja and Karunakaran, 2012), Knoevenagel condensation (Zhang et al., 2004), and α,β-unsaturated carbonyl compounds (Veloso et al., 2011). It is therefore of interest to examine the behavior of cesium silica as a catalyst for the synthesis of 2-amino-5-substitutedselenophene-3-carbonitrile derivatives. To the best of our knowledge, condensation of different substituted esters, selenium and dicyanomethane in the presence of a catalytic amount of Cs/SiO2 for the synthesis of selenophenes has not been reported in the literature. Herein we report the use of Cs/SiO2 for the synthesis of 2-amino-5-substitutedselenophene-3-carbonitrile derivatives (Scheme 1).
Synthesis of 2-amino-5-substitutedselenophene-3-carbonitrile derivatives.
2 Results and discussion
2.1 Catalyst characterization results
The powder XRD diffraction patterns (Fig. 1) of the prepared catalyst show the presence of CsO2, Cs3O and Cs2O phases, with d-spacing values of 2.36, 1.88 and 3.43 ˚A for 2θ angles of 38, 48 and 26° respectively. The d-spacing phases correlate with the ICDD File numbers 03-065-2662, 01-085-0437 and 01-074-1918 for CsO2, Cs3O and Cs2O phases respectively.
XRD spectra of 1% Cs loaded on SiO2 support.
The presence of Cs on SiO2 surface is noticeable in Fig. 2. The Cs is totally occupying the pores and it is distributed over silica surface. The Cs particles are closer together but much of the surface is still visible and more densely consistent with BET results. A possible reason could be that silica has a large surface area and therefore requires more Cs particles to occupy its pores. The SEM images confirm the crystalline nature of the prepared catalyst. SEM-EDX shows that the cesium is evenly distributed on SiO2 surface, and SEM-EDX results are in correlation with ICP-OES elemental analysis (Table 1).
SEM image of 1% Cs loaded on SiO2 support.
| Catalyst | Cs wt.% (ICP) | Surface area (m2/g) | Pore volume (cm3/g) | Pore size (A°) | Cs wt.% (EDX) |
|---|---|---|---|---|---|
| 1% Cs-SiO2 | 0.95 | 208 | 0.56 | 107.49 | 0.95 |
The TEM image of Cs/SiO2 showed an agglomeration of particles. Agglomeration of particles is the result of exposure of the samples to a beam with the high energy resulting in the loss of hydroxyl groups. On high magnification, the particles appear as irregular a needle-like shape which was also observed in the literature. The darker parts of the images show the presence of cesium dispersed evenly on the SiO2 surface which appear as elongated rod-like particles in the TEM images with particle sizes around 50–100 nm (Fig. 3).
TEM image of 1% Cs loaded on SiO2 support.
ICP showed that nominal amount of cesium is present in the catalyst. The catalyst showed type-IV H1 isotherm, which shows the mesoporus nature of the catalysts (Fig. 4). The Cs loaded on SiO2 support material exhibited 208 m2/g surface area which is less than SiO2 support. Surface area decreased with Cs loading on SiO2 which might be attributed to the clogging of the narrow pores of the SiO2 support with the cesium making it inaccessible to nitrogen molecules, leading to a decrease in the surface area. The particulate properties of Cs/SiO2 catalyst are shown in Table 1. The texture of the catalyst was dependent on the Cs concentration on SiO2 which decreases the surface area as well as the pore volume.
N2-adsorption–desorption isotherm of Cs-SiO2.
2.2 Effect of the catalyst
The efficiency of the cesium silica reagent compared to various sulfur analog acidic catalysts was also examined (Table 2). In this study it was found that cesium silica is a more efficient and superior catalyst (Entry 1) over other acidic catalysts (Entries 2–4) with respect to reaction time and yield of the desired selenophene. It was also observed that the yield of the selenophene was only 15% in the absence of the cesium silica reagent (Entry 5).
| Entry | Catalyst | Yield (%) |
|---|---|---|
| 1 | Cesium silica | 96 |
| 2 | Silica sulfuric acid | 91 |
| 3 | P-toluene sulfuric acid | 84 |
| 4 | HClO4-silica | 65 |
| 5 | No catalyst | 15 |
2.3 Effect of the catalyst concentration and temperature
The effects of catalyst concentration and the temperature on the reaction were also studied, and the results are shown in Table 3. It is observed that 0.050 g of catalyst and room temperature are enough to obtain excellent yield. Further increase in temperature to 60 °C, has no significant change in the rate of reaction. Therefore, the reaction temperature was kept at RT to obtain good to excellent yields. To optimize the amount of cesium silica as a catalyst, the reaction was carried out by varying the amount of the catalyst and maximum yield was obtained with 0.050 g of catalyst. It was observed that at normal conditions, the reaction proceeded smoothly and completion of the reaction was accomplished. The generality of this reaction was examined using several types of substituted esters. In all cases, the reactions gave the corresponding products in good to excellent yields.
| Entry | Catalyst (g) | Temperature (°C) | Time (min) | Yield (%) |
|---|---|---|---|---|
| 1 | 0.03 | RT | 80 | 43 |
| 2 | 0.03 | 60 | 80 | 51 |
| 3 | 0.05 | RT | 35 | 94 |
| 4 | 0.05 | 60 | 60 | 62 |
| 5 | 0.1 | RT | 120 | 60 |
2.4 Chemistry
Cesium silica is one of the important condensation catalysts for the selective construction of heterocyclic ring systems, especially in the synthesis of 2-amino-5-substitutedselenophene-3-carbonitrile derivatives. The catalyst decreases the production of chemical waste without using some highly toxic reagents in the synthesis of these products. All the reactions were carried out at room temperature by the addition of Cs/SiO2 to the mixture of substituted esters, selenium powder and dicyanomethane by using solvent free condition. The process was monitored by thin-layer chromatography (TLC). The results showed that the reactions were completed within 35–54 min of stirring, and the desired products were obtained in excellent yields (Table 4).
| Entry | R1 | R2 | Product | Time (min) | Yield% | M.p (°C) |
|---|---|---|---|---|---|---|
| 1 | H | H | 4a | 38 | 87 | 210–212 |
| 2 | EtCOO | Me | 4b | 42 | 90 | 231–233 |
| 3 | Me | H | 4c | 48 | 89 | 197–198 |
| 4 | Me | Me | 4d | 51 | 84 | 206–208 |
| 5 | Ph | H | 4e | 50 | 83 | 243–244 |
| 6 | H | Ph | 4f | 44 | 91 | 184–186 |
| 7 | H | 4-Me-Ph | 4 g | 35 | 82 | 192–194 |
| 8 | H | 4-Br-Ph | 4 h | 52 | 86 | 206–207 |
| 9 | H | 4-Et-Ph | 4i | 49 | 90 | 238–240 |
| 10 | Br | 4-Br-Ph | 4j | 45 | 83 | 218–219 |
| 11 | H | 2,4-OMe-Ph | 4 k | 54 | 89 | 178–180 |
We examined the effect of the amount of catalyst in this reaction. The best results were obtained using 0.050 g of catalyst (96%). Using lower amounts of catalyst resulted in lower yields, and in the absence of catalyst the yield of the product was found to be nil.
To demonstrate the need of the cesium silica for these reactions, an experiment was conducted in the absence of cesium silica. The yields obtained are very poor, side products are formed, and reactants are not involved completely in this reaction. Observably, cesium silica is an important component of the reaction. The effectiveness of the cesium silica compared to various acidic catalysts was also studied (Table 2). In this study, it was found that cesium silica is a more effective and superior catalyst over other acidic catalysts with respect to reaction yield. It was also observed that the yield was only 15% in the absence of the cesium silica catalyst.
We also investigated the reusability of the catalyst. For this purpose after completion of the model reaction, the cesium supported silica was separated from the reaction mixture by simple filtration, washing with CH2Cl2, and drying in a vacuum oven at 60 °C for 5 h prior to reuse in subsequent reactions. The recovered catalyst can be reused at least three additional times in subsequent reactions without significant loss in product yield.
2.5 Synthesis of 2-amino-5-substitutedselenophene-3-carbonitrile derivatives (4a–k)
A mixture of dicyanomethane 3 (1.2 mmol), substituted esters 1a–k (1 mmol), selenium powder (1.5 mmol), and 5% cesium silica (0.050 mg) was reacted at room temperature for 35–54 min. as shown in Table 2. Completion of the reaction was indicated by TLC monitoring. After completion of the reaction, unreacted selenium was filtered. Then, ethyl acetate (10 ml) was added, and the reaction mixture was washed with water (15 ml). The organic layer was dried over anhydrous sodium sulfate and concentrated to dryness, and crude solid product was recrystallized from ethanol to give compounds (4a–k) in high yields. The suggested mechanism of the cesium silica catalyzed mechanism is depicted (Proposed Mechanism
). The products (4a–k) were confirmed by the FT-IT, 1H NMR and Mass spectral studies.
2.5.1 2-Aminoselenophene-3-carbonitrile (4a)
IR (υ cm–1, KBr): 3286, 3158, 1660, 1514, 1030; 1H NMR (300 MHz, DMSO–d6): δ 6.84 (d, 1H, J = 8.4 Hz, CH), 6.51 (d, 1H, J = 8.1 Hz, CH), 5.48 (brs, 2H, NH2); 13C NMR (75 MHz, DMSO–d6): δ 160.73, 127.15, 124.81, 114.13, 87.92; (ESI−MS) m/z: 172 [M+H]+. Anal. Calcd. for C5H4N2Se: C, 35.11; H, 2.36; N, 16.38. Found: C, 35.09; H, 2.39; N, 16.42.
2.5.2 Ethyl 5-amino-4-cyano-3-methylselenophene-2-carboxylate (4b)
IR (υ cm–1, KBr): 3300, 3165, 1671, 1518, 1028; 1H NMR (300 MHz, DMSO–d6): δ 5.51 (brs, 2H, NH2), 4.27 (q, 2H, J = 7.4 Hz, CH2), 2.47 (s, 3H, CH3), 1.40 (t, 3H, J = 6.8 Hz, CH3); 13C NMR (100 MHz, DMSO-d6): δ; (ESI−MS) m/z: 256 [M – H]+. Anal. Calcd. for C9H10N2O2Se: C, 42.04; H, 3.92; N, 10.89. Found: C, 41.99; H, 3.96; N, 10.91.
2.5.3 2-Amino-5-methylselenophene-3-carbonitrile (4c)
IR (υ cm–1, KBr): 3290, 3155, 1668, 1524, 1260, 1029; 1H NMR (300 MHz, DMSO-d6): δ 6.53 (s, 1H, CH), 5.46 (brs, 2H, NH2), 1.64 (s, 3H,CH3); 13C NMR (75 MHz, DMSO-d6): δ 160.78, 149.18, 134.24, 113.97, 88.05, 18.87; (ESI−MS) m/z: 186 [M+H]+. Anal. Calcd. for C6H6N2Se: C, 38.94; H, 3.27; N, 15.14. Found: C, 38.97; H, 3.31; N, 15.11.
2.5.4 2-Amino-4,5-dimethylselenophene-3-carbonitrile (4d)
IR (υ cm–1, KBr): 3285, 3160, 1661, 1528, 1030; 1H NMR (300 MHz, DMSO–d6): δ 5.50 (brs, 2H, NH2), 2.45 (s, 3H, CH3), 1.62 (s, 3H, CH3); 13C NMR (75 MHz, DMSO-d6): δ 161.33, 146.12, 114.48, 107.90, 88.72, 18.51, 14.51; (ESI−MS) m/z: 198 [M+H]+. Anal. Calcd. for C7H8N2Se: C, 42.22; H, 4.05; N, 14.07. Found: C, 42.25; H, 4.08; N, 14.11.
2.5.5 2-Amino-5-phenylselenophene-3-carbonitrile (4e)
IR (υ cm–1, KBr): 3296, 3170, 1666, 1515, 1265, 1020; 1H NMR (300 MHz, DMSO-d6): δ 7.42–7.34 (m, 5H, ArH), 6.62 (s, 1H, CH), 5.67 (brs, 2H, NH2); 13C NMR (75 MHz, DMSO-d6): δ 161.37, 135.31, 128.22, 127.23, 126.78, 126.45, 125.73, 114.09, 87.94; (ESI−MS) m/z: 248 [M+H]+. Anal. Calcd. for C11H8N2Se: C, 53.46; H, 3.26; N, 11.33. Found: C, 53.49; H, 3.23; N, 11.28.
2.5.6 2-Amino-4-phenylselenophene-3-carbonitrile (4f)
IR (υ cm–1, KBr): 3305, 3161, 1677, 1521, 1030, 776; 1H NMR (300 MHz, DMSO-d6): δ 7.34–7.21 (m, 5H, ArH), 6.91 (s, 1H, CH), 5.46 (brs, 2H, NH2); 13C NMR (75 MHz, DMSO-d6): δ 161.54, 151.42, 131.53, 129.35, 128.84, 128.04, 118.08, 114.22, 88.12; (ESI−MS) m/z: 248 [M+H]+. Anal. Calcd. for C11H8N2Se: C, 53.46; H, 3.26; N, 11.33. Found: C, 53.49; H, 3.23; N, 11.28.
2.5.7 2-Amino-4-p-tolylselenophene-3-carbonitrile (4g)
IR (υ cm–1, KBr): 3317, 3182, 1659, 1524, 1027; 1H NMR (300 MHz, DMSO-d6): δ 7.26–7.18 (m, 4H, ArH), 6.87 (s, 1H, CH), 5.63 (brs, 2H, NH2), 2.21 (s, 3H, CH3); 13C NMR (75 MHz, DMSO-d6): δ 160.97, 151.35, 143.77, 140.11, 131.24, 129.32, 116.27, 114.12, 87.95, 18.61; (ESI−MS) m/z: 262 [M+H]+. Anal. Calcd. for C12H10N2Se: C, 55.12; H, 3.86; N, 10.73. Found: C, 55.15; H, 3.91; N, 10.77.
2.5.8 2-Amino-4-(4-bromophenyl)selenophene-3-carbonitrile (4h)
IR (υ cm–1, KBr): 3295, 3180, 1672, 1531, 1018; 1H NMR (300 MHz, DMSO-d6): δ 7.43–7.38 (m, 4H, ArH), 6.93 (s, 1H, CH), 5.69 (brs, 2H, NH2); 13C NMR (75 MHz, DMSO–d6): δ 161.63, 151.56, 136.10, 135.82, 132.44, 123.67, 118.28, 114.54, 88.37; (ESI−MS) m/z: 328 [M+2H]+. Anal. Calcd. for C11H7BrN2Se: C, 40.52; H, 2.16; N, 8.59. Found: C, 40.47; H, 2.20; N, 8.53.
2.5.9 2-Amino-4-(4-ethylpheyl)selenophene-3-carbonitrile (4i)
IR (υ cm–1, KBr): 3306, 3175, 1673, 1528, 1021, 760; 1H NMR (300 MHz, DMSO-d6): δ 7.12–7.04 (m, 4H, ArH), 6.89 (s, 1H, CH), 5.65 (brs, 2H, NH2), 2.71 (q, 2H, J = 7.8 Hz, CH2), 1.18 (t, 3H, J = 6.4 Hz, CH3); 13C NMR (75 MHz, DMSO-d6): δ 161.58, 150.24, 147.71, 136.08, 132.52, 129.74, 117.36, 114.60, 88.29, 27.36, 14.13; (ESI−MS) m/z: 276 [M+H]+. Anal. Calcd. for C13H12N2Se: C, 56.73; H, 4.39; N, 10.18. Found: C, 56.77; H, 4.34; N, 10.15.
2.5.10 2-Amino-5-bromo-4-(4-bromopheyl)selenophene-3-carbonitrile (4j)
IR (υ cm–1, KBr): 3291, 3170, 1670, 1517, 1008; 1H NMR (300 MHz, DMSO-d6): δ 7.40–7.34 (m, 4H, ArH), 5.71 (brs, 2H, NH2); 13C NMR (75 MHz, DMSO-d6): δ 16.60, 151.89, 135.19, 133.21, 129.35, 123.60, 114.76, 109.27, 88.92; (ESI−MS) m/z: 406 [M+2H]+. Anal. Calcd. for C11H6Br2N2Se: C, 32.63; H, 1.49; N, 6.93. Found: C, 32.59; H, 1.52; N, 6.89.
2.5.11 2-Amino-4-(2,4-dimethoxypheyl)selenophene-3-carbonitrile (4k)
IR (υ cm–1, KBr): 3308, 3168, 1669, 1532, 1041; 1H NMR (300 MHz, DMSO-d6): δ 7.11–7.05 (m, 3H, ArH), 6.97 (s, 1H, CH), 5.73 (brs, 2H, NH2), 3.72 (s, 6H, 2-OMe); 13C NMR (75 MHz, DMSO-d6): δ 161.51, 159.62, 159.08, 151.80, 133.31, 115.42, 114.68, 109.50, 103.21, 88.77, 55.17, 54.23; (ESI−MS) m/z: 308 [M+H]+. Anal. Calcd. for C13H12N2O2Se: C, 50.83; H, 3.94; N, 9.12. Found: C, 50.78; H, 3.90; N, 9.16.
3 Experimental
3.1 Spectral instrumentation
Melting points were determined on an Electrothermal type 9100 melting point apparatus. IR spectra were recorded using a 4300 Shimadzu spectrophotometer with KBr plates. The 1H NMR spectra were recorded on a varian 300 MHz spectrometer for 1H NMR. Carbon nuclear magnetic resonance (13C NMR) spectra were recorded on a Bruker 75 MHz spectrometer. The mass spectra of the new compounds were recorded on a triple quadrupole mass spectrometer Varian 1200 L/MS/MS with electrospray interface (ESI), coupled with a high performance liquid chromatography with a Varian Prostar 240 SDM ternar pump. The instrument was operated in positive ions mode. The products (4a–k) were isolated with column chromatography and characterized by comparison of physical and spectral data with those of known samples.
3.1.1 Catalyst characterization
The X-ray diffraction patterns were recorded with a Bruker D8 Advance instrument. The data was collected over the range 10–90° with a step size of 0.5 per second. The BET surface areas were determined using a Micrometrics Gemini 2360 multi-point BET surface area analyzer. The powdered sample (∼0.05 g) was degassed overnight at 200 °C using a Micromeritics FlowPrep 060 instrument. The inductively coupled plasma (ICP) was performed using a Perkin Elmer Optical Emission Spectrometer Optima 5300 DV to determine the elemental composition of the materials and adsorption of the Cs on the three support materials. The SEM image was viewed in a Phillips XL30 Electron Scanning Microscope. Transmission electron microscopy (TEM) images were viewed using a Jeol JEM-1010 Electron Microscope. The images were captured and analyzed using iTEM software.
3.2 Catalyst preparation
The wet impregnation method was used in the preparation of metal oxide supported catalysts. 1 % Cs/SiO2 catalyst was prepared by dissolving appropriate amount of cesium nitrate (Cs2(NO3)2) in distilled water (20.0 mL) and it is added drop wise onto 10 g of silica (SiO2, Aldrich) stirring for 3 h using a magnetic stirrer at room temperature. The water in the solution was evaporated by heating at 80 °C for 12 h (Maddila et al., 2013a; Chetty et al., 2012a). Catalysts are further dried in an oven at 130–140 °C for 12 h. Then the catalyst is calcined in the presence of air, at 550 °C for 3 h to obtain the 1% w/w catalysts (Chetty et al., 2012b; Maddila et al., 2013b).
4 Conclusion
In conclusion, a new heterogeneous catalyst based on cesium supported silica has been developed and efficiently catalyzed the one-pot synthesis of selenophene derivatives under solvent-free conditions. The catalyst was efficient for a broad variety of dicyanomethane, esters and selenium, leading to the corresponding selenophenes in good to excellent yields (up to 91%). In the catalyst of Cs/SiO2 there are cesium moieties which are covalently connected to the silica. These groups provide active catalytic sites on the surface of silica. Moreover, a new strategy for the synthesis of new materials based on silica using reaction of different reagents for the preparation of a wide range of functional groups and catalytic sites on the surface of silica was disclosed. The successfully recyclability of the catalyst with no loss in its activity, use of nontoxic; using friendly process and easy work up procedure are the merits of this procedure. The Cs/SiO2 catalyst provides great promise toward further useful applications in other heterocyclic reactions in the future.
Acknowledgement
The authors are thankful to the authorities of Annamacharya Institute of Technology & Sciences, J.N.T.University, Tirupati, India and School of Chemistry, University of KwaZulu-Natal, Westville campus, Durban, South Africa for the facilities and encouragement.
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