Trisulfur Radical Anion as the Key Intermediate for the Synthesis of Thiophene via the Interaction between Elemental Sulfur and NaOtBu Guoting Zhang, a Hong Yi, a Hong Chen, a Changliang Bian a Chao Liu a, * and Aiwen Lei a,b * a College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072, P. R. China. E-mail: aiwenlei@whu.edu.cn; Tel: (+86)-27-68754672; b National Research Center for Carbohydrate Synthesis, Jiangxi Normal University, Nanchang, Jiangxi 330022, P. R. China 1. General Information........2 2. General Procedures for Synthesizing Symmetric 1,3-diynes 2 3. General Procedures for Synthesizing Unsymmetric 1,3- Diynes...2 4. General Procedures for Synthesizing 2,5-Disubstituted Thiophenes........3 5. General Procedures for Synthesizing 2-Substituted Thiophenes....3 6. Mechanism Study....3 7. Characterization of Products...5 8. NMR Spectra of Products.. 9 1
1. General Information All manipulations were carried out by standard Schlenk techniques. Unless otherwise noted, analytical grade solvents and commercially available reagents were used to conduct the reactions. Thin layer chromatography (TLC) employed glass 0.25 mm silica gel plates. Flash chromatography columns were packed with 200-300 mesh silica gel in petroleum (boiling point is between 60-90 C). Gradient flash chromatography was conducted eluting with a continuous gradient from petroleum ether and ethyl acetate. All new compounds were characterized by 1 H NMR, 13 C NMR and HRMS. The known compounds were characterized by 1 H NMR and 13 C NMR. GC-MS spectra were recorded on a Varian GC-MS 3900-2100T. The 1 H and 13 C NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer with tetramethylsilane as an internal standard. The chemical shifts (δ) were given in part per million relative to internal tetramethyl silane (TMS, 0 ppm for 1 H), CDCl 3 (77.3 ppm for 13 C). High resolution mass spectra (HRMS) were measured with a Waters Micromass GCT instrument and accurate masses were reported for the molecular ion + Hydrogen (M + H). EPR spectra were recorded on a Bruker X-band A200 spectrometer. 2. General Procedures for Synthesizing Symmetric 1,3-Diynes All kinds of substituted symmetric 1,3-diynes were prepared from oxidative coupling of corresponding terminal alkynes according to the reported procedure. 1 3. General Procedures for Synthesizing Unsymmetric 1,3-Diynes under Air The CuI (5.0 mol %) and NiCl 2 6H 2O (5.0 mol %) were dissolved in THF(4.0 ml) and TMEDA (20 mol %), and the solution was stirred for 2 min at room temperature. Phenylacetylene (25 mmol) and 2-methylbut-3-yn-2-ol (5.0 mmol) were added subsequently and the reaction mixture was stirred under air for 12 hours at room temperature. After completion of the reaction, as indicated by TLC, the mixture was concentrated in vacuo and the residue was purified by column chromatography on silica gel. All remaining unsymmetric diynes were prepared using a procedure similar to that used to synthesize 3a. 1 2
4. General Procedures for Synthesizing 2,5-Disubstituted Thiophenes A mixture of 1, 4-diphenylbuta-1, 3-diyne 1a (0.30 mmol), sulphur powder (0.90 mmol), and NaOtBu (1.8 mmol) in 2.0 ml DMF/HOtBu (v:v = 3:1) was stirred under an N 2 atmosphere at 25 ºC for 5.0 h. After completion of the reaction, as indicated by TLC and GC-MS, the mixture was quenched with diluted hydrochloride (4 ml, 2.0 M), and the solution was extracted with ethyl acetate (3 15 ml). The organic layers were combined, and dried over sodium sulfate. The pure product was obtained by flash column chromatography using n-hexane on silica gel to afford 2a in 92% yield. All remaining 2,5-Disubstituted thiophenes were prepared using a procedure similar to that used to synthesize 2a. 5. General Procedures for Synthesizing 2-Substituted Thiophenes A mixture of 2-methyl-6-phenylhexa-3,5-diyn-2-ol 3a (0.30 mmol), sulphur powder (0.60 mmol), NaOtBu (1.2 mmol) and tetrabutylammonium hydroxide (10% in Water, 0.06mmol, about 72μL) in 0.50mL HOtBu and 1.5 ml DMF was stirred under an N 2 atmosphere at 100 ºC for 5.0 h. After completion of the reaction, as indicated by TLC and GC-MS, the mixture was quenched with diluted hydrochloride (4 ml, 2.0 M), and the solution was extracted with ethyl acetate (3 15 ml). The organic layers were combined, and dried over sodium sulfate. The pure product was obtained by flash column chromatography using n-hexane on silica gel to afford 4a in 35% yield. All remaining 2-substituted thiophenes were prepared using a procedure similar to that used to synthesize 4a. 6. Mechanism Study 6.1 Deuterated Experiment A mixture of 1, 4-diphenylbuta-1, 3-diyne 1a (0.30 mmol), sulphur powder (0.90 mmol), and NaOtBu (1.8 mmol) in 2.0 ml d 6 -DMSO was stirred under an N 2 atmosphere at 25 ºC for 5.0 h. After completion of the reaction, as indicated by TLC and GC-MS. The organic layers were combined, and dried over sodium sulfate. The pure product was obtained by flash column chromatography on silica gel in 90% yield. 3
Scheme 1. Deuterated Experiment 100 % 238 75 50 25 0 121 77 117 135 204 236 45 51 90 167 177 193 266 281 290 316323 341 50.0 75.0 100.0 125.0 150.0 175.0 200.0 225.0 250.0 275.0 300.0 325.0 350.0 % 100 238 75 50 25 0 239 212 219 223 228 234 252 266 268 244 249 256 263 215.0 220.0 225.0 230.0 235.0 240.0 245.0 250.0 255.0 260.0 265.0 Figure 1 GC-MS of the deuterated experiment Figure 2 1 H NMR (400M, CDCl 3) of the deuterated experiment 4
6.2 EPR Experiment 1. EPR Studies of Interaction between Elemental Sulfur and Base A dried schlenk tube equipped with a stir bar was loaded with sulphur powder (0.50 mmol), and base (1.0 mmol) in 2.0 ml solvent was stirred under an N2 atmosphere at 25 ºC. After 30 mins, the solution sample was taken out into a small tube and analyzed by EPR. EPR spectra was recorded at room temperature on EPR spectrometer operated at 9.419 GHz. Typical spectrometer parameters are shown as follows, scan range: 1000 G; center field set: 3361 G; time constant: 163.84 ms; scan time: 30.72 s modulation amplitude: 5.0 G; modulation frequency: 100 khz; receiver gain: 1.00 10 5 ; microwave power: 18.53 mw. 2. EPR Studies of Interaction between Elemental Sulfur and Na2S A dried schlenk tube equipped with a stir bar was loaded with sulphur powder (0.50 mmol), and Na2S (0.5 mmol) in 2.0 ml DMF was stirred under an N2 atmosphere at 25 ºC. After 60 mins, the solution sample was taken out into a small tube, frozen by liquid nitrogen and then analyzed by EPR. EPR spectra was recorded at room temperature on EPR spectrometer operated at 9.419 GHz. Typical spectrometer parameters are shown as follows, scan range: 1000 G; center field set: 3361 G; time constant: 163.84 ms; scan time: 30.72 s modulation amplitude: 5.0 G; modulation frequency: 100 khz; receiver gain: 1.00 10 5 ; microwave power: 18.53 mw. 7. Characterization of Products 2,5 -diphenylthiophene (2a): 65.1 mg (92%), white solid; 1 H NMR (400 MHz, CDCl 3) δ = 7.62 (d, J = 7.6 Hz, 4H), 7.39 (t, J = 7.4 Hz, 4H), 7.26 7.29 (m, 4H); 13 C NMR (100 MHz, CDCl 3) δ = 143.6, 134.3, 128.9, 127.5, 125.7, 124.0. 2 5
2,5-di-p-tolylthiophene (2b); 44.4 mg (56%), white solid; 1 H NMR (400 MHz, CDCl 3) δ = 7.50 (d, J = 8.0 Hz, 4H), 7.21 (s, 2H), 7.17 (d, J =7.6 Hz, 4H), 2.36 (s, 6H); 13 C NMR (100 MHz, CDCl 3) δ = 143.3, 137.3, 131.7, 129.6, 125.5, 123.5, 21.2. 2 2,5-bis(4-pentylphenyl)thiophene (2c); 79.0 mg (70%), white solid; 1 H NMR (400 MHz, CDCl 3) δ = 7.52 (d, J = 8.2 Hz, 4H), 7.20 (s, 2H), 7.16 (d, J = 8.0 Hz, 4H), 2.89 2.26 (m, 4H), 1.70 1.53 (m, 4H), 1.32 1.34 (m, 8H), 0.89 (t, J = 6.8 Hz, 6H); 13 C NMR (100 MHz, CDCl 3) δ = 143.3, 142.4, 131.9, 129.0, 125.5, 123.5, 35.7, 31.6, 31.2, 22.7, 14.2. 2 2,2':5',2''-terthiophene (2d); 55.1 mg (74%), white solid; 1 H NMR (400 MHz, CDCl 3) δ = 7.19 (dd, J = 5.1, 1.0 Hz, 2H), 7.15 (dd, J = 3.6, 0.9 Hz, 2H), 7.06 (s, 2H), 7.00 (dd, J = 5.1, 3.6 Hz, 2H); 13 C NMR (100 MHz, CDCl 3) δ = 137.1, 136.1, 127.9, 124.5, 124.3, 123.7. 3 2,5-bis(4-bromophenyl)thiophene (2e); 74.4mg (63%), white solid; 1 H NMR (400 MHz, CDCl 3) δ = 7.47-7.53 (m, 8H), 7.28 (s, 2H); 13 C NMR (100 MHz, CDCl 3) δ = 142.7, 133.0, 132.1, 127.0, 124.5, 121.5. 2 2,5-bis(4-chlorophenyl)thiophene (2f); 68.6mg (75%), white solid; 1 H NMR (400 MHz, CDCl 3) δ = 7.53 (d, J = 8.5 Hz, 4H), 7.35 (d, J = 8.5 Hz, 4H), 7.25 (s, 2H); 13 C NMR (100 MHz, CDCl 3) δ = 137.14, 136.21, 127.93, 124.51, 124.34, 123.72. 2 6
2,5-bis(4-fluorophenyl)thiophene (2g); 62.0mg (76%), white solid; 1 H NMR (400 MHz, CDCl 3) δ = 7.55 7.59 (m, 4H), 7.19 (s, 2H), 7.08 (t, J = 8.4 Hz, 4H); 13 C NMR (100 MHz, CDCl 3) δ = 162.3 (d, J C,F = 245.8 Hz), 142.5, 130.5 (d, J C,F = 3.4 Hz), 127.3 (d, J C,F = 7.9 Hz), 124.0, 115.9 (d, J C,F = 21.7 Hz). 2 2-(5-phenylthiophen-2-yl)pyridine (2h); 46.9mg (66%), white solid; 1 H NMR (400 MHz, CDCl 3) δ = 8.64 8.55 (m, 1H), 7.74 7.64 (m, 4H), 7.55 (d, J = 3.9 Hz, 1H), 7.45 7.38 (m, 2H), 7.35 7.27 (m, 2H), 7.17 7.14 (m, 1H), 13 C NMR (100 MHz, CDCl 3) δ = 152.5, 149.6, 146.2, 143.8, 136.7, 134.2, 129.0, 127.8, 125.8, 125.4, 124.0, 121.9, 118.5. 4 2-phenylthiophene (4a); 16.8mg (35%), white solid; 1 H NMR (400 MHz, CDCl 3): δ = 7.60 7.51 (m, 2H), 7.29 (t, J = 7.6 Hz, 2H), 7.25 7.14 (m, 3H), 6.98 (dd, J = 5.0, 3.7 Hz, 1H); 13 C NMR (100 MHz, CDCl 3) δ = 144.4, 134.4, 128.9, 128.0, 127.5, 126.0, 124.8, 123.1. 5 2-(p-tolyl)thiophene (4b); 25.1 mg (48%), white solid; 1 H NMR (400 MHz, CDCl 3): δ = 7.51 (d, J = 8.0 Hz, 2H), 7.34 7.31 (m, 1H), 7.25 7.22 (m, 1H), 7.18 (d, J = 7.9 Hz, 2H), 7.06 (dd, J = 4.9, 3.8 Hz, 1H), 2.36 (s, 3H); 13 C NMR (100 MHz, CDCl 3): δ = 144.6, 137.4, 131.6, 129.6, 128.0, 125.9, 124.3, 122.6, 29.8. 6 2-(4-pentylphenyl)thiophene (4c); 26.8 mg (36%), white solid; 1 H NMR (400 MHz, CDCl 3): δ = 7.52 7.50 (m, 2H), 7.34 7.31 (m, 2H), 7.28 7.26 (m, 2H), 7.06 (dd, J = 4.9, 3.8 Hz, 1H); 13 C NMR (100 MHz, CDCl 3): δ = 143.1, 133.2, 132.9, 129.1, 128.2, 127.1, 125.2, 123.5. 6 7
2-([1,1'-biphenyl]-4-yl)thiophene (4d); 29.0 mg (41%), white solid; 1 H NMR (400 MHz, CDCl 3): δ = 7.71 7.66 (m, 2H), 7.64 7.59 (m, 4H), 7.44 (t, J = 7.6 Hz, 2H), 7.35 (dd, J = 6.5, 4.0 Hz, 2H), 7.28 (dd, J = 5.1, 0.9 Hz, 1H), 7.09 (dd, J = 5.0, 3.6 Hz, 1H); 13 C NMR (100 MHz, CDCl 3): δ = 144.1, 140.5, 140.2, 133.4, 128.9, 128.2, 127.6, 127.4, 127.0, 126.3, 124.9, 123.2. 7 2-(4-methoxyphenyl)thiophene (4e); 34.8 mg (61%), white solid; 1 H NMR (400 MHz, CDCl 3): δ = 7.53 (d, J = 8.9 Hz, 2H), 7.22 7.17 (m, 2H), 7.04 (dd, J = 5.0, 3.6 Hz, 1H), 6.93 6.88 (m, 2H), 3.82 (s, 3H); 13 C NMR (100 MHz, CDCl 3): δ = 159.2, 144.3, 128.0, 127.3, 127.2, 123.9, 122.1, 114.3, 55.4. 8 N,N-dimethyl-4-(thiophen-2-yl)aniline (4f); 26.8 mg (44%), white solid; 1 H NMR (400 MHz, CDCl 3): δ = 7.58 7.50 (m, 2H), 7.20 (d, J = 4.3 Hz, 2H), 7.13 7.03 (m, 1H), 6.83 6.72 (m, 2H), 3.02 (s, 6H); 13 C NMR (100 MHz, CDCl 3): δ = 145.0, 145.3, 127.9, 126.9, 123.0, 122.8, 120.9, 112.6, 40.6. 9 8
8. NMR Spectra of Products 9
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