Thienothiophene

In organic chemistry, thienothiophene is any of several compounds consisting of two fused thiophene rings. They have the molecular formula C₆H₄S₂. Three constitutional isomers have been synthesized: thieno[3,2-b]thiophene, thieno[2,3-b]thiophene, and thieno[3,4-b]thiophene. Other isomers feature S(IV) and are less stable.^[1] Thieno[2,3-b]thiophene was the first member of the series to be isolated. It was obtained in very low yield upon heating citric acid, a source of a six-carbon linear chain, with P₄S₁₀.^[2] More efficient syntheses of this and the other two stable thienothiophenes involve cyclization reactions of substituted thiophenes.^[3]

Thieno[3,2-b]thiophene

Three thienothiophenes, being aromatic and bicyclic, are often compared to naphthalene. They are the topic of academic research. They have no commercial applications nor are they or their derivatives found naturally.

• Isomers of thienothiophene
• 
  Thieno[3,2-b]thiophene
  CAS RN 251-41-2
  m.p. 56.0-56.5 °C
  white solid.
• 
  Thieno[2,3-b]thiophene
  CAS RN 250-84-0
  b.p. 102 °C at 16 mmHg
  colorless oil.
• 
  Thieno[3,4-b]thiophene
  CAS RN 250-65-7
  m.p. 7.0-7.5 °C
  colorless oil.
• 
  2λ⁴δ²-Thieno[3,4-c]thiophene
  CAS RN 24976-21-4
  not isolated.^[4]

References

1. Cava, Michael P.; Lakshmikantham, M. V. (1975). "Nonclassical Condensed Thiophenes". Accounts of Chemical Research. 8 (4): 139–44. doi:10.1021/ar50088a005.
2. Litvinov, V. P.; Gol'dfarb, Y. A. L. (1976). "The Chemistry of Thienothiophenes and Related Systems". Advances in Heterocyclic Chemistry. Advances in Heterocyclic Chemistry. Vol. 19. pp. 123–214. doi:10.1016/S0065-2725(08)60231-7. ISBN 9780120206193.
3. Cinar, M. E.; Ozturk, T. (2015). "Thienothiophenes, Dithienothiophenes, and Thienoacenes: Syntheses, Oligomers, Polymers, and Properties". Chemical Reviews. 115 (9): 3036–3140. doi:10.1021/cr500271a. PMID 25831021.
4. Nakayama, Juzo; Ishii, Akihiko; Kobayashi, Yasunobu; Hoshino, Masamatsu (1988). "Generation and Characterization of the Parent 2λ⁴δ²-Thieno[3,4-c]thiophene". Journal of the Chemical Society, Chemical Communications. 14 (14): 959–60. doi:10.1039/C39880000959.