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

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 (λ_max 318-373 nm and λ_cut-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

References and Notes

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17

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-CHCl_3- 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.

18

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.