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Synlett 2006(15): 2423-2426  
DOI: 10.1055/s-2006-950426
LETTER
© Georg Thieme Verlag Stuttgart · New York

Palladium-Catalyzed C-H Arylation of Thieno[2,3-b]thiophene: An Approach to Novel 2,5-Diaryl/Heteroaryl Thieno[2,3-b]thiophenes

Sabir H. Mashraqui*, Mohamed Ashraf, Shailesh G. Ghadigaonkar
Department of Chemistry, University of Mumbai, Vidyanagari, Santacruz-E, Mumbai 400098, India
Fax: +91(22)26528547; e-Mail: sh_mashraqui@yahoo.com;
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Publication History

Received 27 June 2006
Publication Date:
08 September 2006 (online)

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Abstract

Direct C-H arylation of thieno[2,3-b]thiophene is reported under Pd(OAc)2/n-Bu4NBr catalysis and provides rapid entry into the first examples of 2,5-diaryl thienothiophenes. The reaction works well with either electron-deficient or electron-rich halo-arenes, giving satisfactory yields of diarylation products. Novel push-pull thienothiophenes bearing electron-deficient aryl groups (λmax 318-373 nm and λcut-off 430 nm) are potentially interesting for their photonic properties.

Key words

palladium catalysis - C-H arylation - thieno[2,3-b]thiophene - 2,5-diarylthieno[2,3-b]thiophenes - UV-visible

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Typical procedure: To a solution of 1 (0.168 g, 1 mmol) and appropriate aryl halide (2.1 mmol) in dry DMF (15 mL) were introduced anhyd K2CO3 (0.600 g, ca. 2 mmol), n-Bu4NBr (0.644 g, 2 mmol) and Pd(OAc)2 (10 mg). The reaction mixture was stirred and heated at 80 °C for the time specified (Table [1] ) under a N2 atmosphere. The reaction mixture was allowed to cool to r.t., poured into H2O, extracted with CH2Cl2 and the organic extract dried over anhyd Na2SO4. The crude material obtained on solvent removal was purified by flash column chromatography on silica gel (hexane-CHCl3- mixtures) to obtain the corresponding diaryl thienothiophenes. 4: Yield 70%; mp 300 °C (dec.); IR (KBr): 2922, 1588, 1512, 1417, 1340, 1106, 932, 850, 750, 737, 694, 524, 485 cm-1; 1H NMR (CDCl3, 300 MHz): δ = 2.60 (s, 6 H, CH3), 7.61 (d, J = 10.5 Hz, 4 H, ArH), 8.31 (d, J = 10.5 Hz, 4 H, ArH); Anal. Calcd for C20H14N2O4S2: C, 58.54; H, 3.41; N, 6.83; S, 15.61. Found: C, 58.34; H, 3.63; N, 6.85; S, 15.49. 10: Yield 65%; mp 293-295 °C; IR (KBr): 3423, 2921, 1722, 1604, 1523, 1433, 1403, 1279, 1180, 1104, 1017, 965, 932, 850, 766, 700 cm-1; 1H NMR (CDCl3, 300 MHz): δ = 2.57 (s, 6 H, -CH3), 2.95 (s, 6 H, -OCH3), 7.54 (d, J = 6 Hz, 4 H, ArH), 8.11 (d, J = 6 Hz, 4 H, ArH); Anal. Calcd for C24H20O4S2: C, 66.05; H, 4.60; S, 14.70. Found: C, 66.10; H, 4.52; S, 14.55. 12: Yield 71%; mp >320 °C; IR (KBr): 3070, 2921, 1655, 1510, 1485, 1432, 1360, 1301, 1269, 1088, 1035, 937, 897, 881, 790, 745, 672, 610, 592, 555, 463 cm-1; 1H NMR (CDCl3, 300 MHz): δ = 2.57 (s, 6 H, CH3), 2.67 (s, 6 H, -COCH3), 7.29 (d, J = 3 Hz, 2 H, thiophene H), 7.60 (d, J = 3 Hz, 2 H, thiophene H); Anal. Calcd for C20H16O2S4: C, 57.69; H, 3.90; S, 31.22. Found: C, 57.54; H, 3.85; S, 31.02. 14: Yield 61%; mp 265-268 °C; IR (KBr): 2958, 2837, 1606, 1570, 1525, 1492, 1461, 1439, 1297, 1275, 1248, 1177, 1110, 1041, 1028, 926, 810, 825, 649, 628, 542, 517 cm-1; 1H NMR (CDCl3, 300 MHz): δ = 2.20 (s, 6 H, -CH3), 3.80 (s, 6 H, -OCH3), 6.90 (d, J = 8.0 Hz, 4 H, ArH), 7.35 (d, J = 8.0 Hz, 4 H, ArH); Anal. Calcd for C22H20O2S2: C, 69.47; H, 5.26; S, 16.84. Found: C, 69.31; H, 5.20; S, 16.64. 18: Yield 47%; mp 308-311 °C; IR (KBr): 3086, 2916, 1440, 1418, 1382, 1216, 1158, 1117, 1045, 1034, 999, 961, 902, 863, 830, 809, 736, 725, 564 cm-1; 1H NMR (CDCl3, 300 MHz): δ = 2.35 (s, 6 H, -CH3), 2.50 (s, 6 H, -CH3), 6.85 (s, 2 H, thienothiophene H), 7.30 (s, 4 H, ArH); Anal. Calcd for C22H18S4: C, 64.40; H, 4.40; S, 31.22. Found: C, 64.28; H, 4.42; S, 31.02.

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The in situ reduction of Pd2+ to Pd0 is conceivable via DMF and or tributylamine mediated processes. Tributylamine in turn could be generated via the Hoffmann elimination of the quarternary ammonium salt under the reaction conditions.