thienothiophene
File:Thieno(3,2-b)thiophene.jpg
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. The other isomer features S(IV) and is less stable.{{Cite journal |last=Cinar |first=Mehmet Emin |last2=Ozturk |first2=Turan |date=2015-05-13 |title=Thienothiophenes, Dithienothiophenes, and Thienoacenes: Syntheses, Oligomers, Polymers, and Properties |url=https://pubs.acs.org/doi/10.1021/cr500271a |journal=Chemical Reviews |language=en |volume=115 |issue=9 |pages=3036–3140 |doi=10.1021/cr500271a |issn=0009-2665|url-access=subscription }}{{cite journal|title=Nonclassical Condensed Thiophenes|author1=Cava, Michael P. |author2=Lakshmikantham, M. V. |doi=10.1021/ar50088a005|journal=Accounts of Chemical Research|year=1975|volume=8|issue=4 |pages=139–44}} 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.{{Cite journal |last=Ozturk |first=Turan |last2=Ertas |first2=Erdal |last3=Mert |first3=Olcay |date=2010-06-09 |title=A Berzelius Reagent, Phosphorus Decasulfide (P4S10), in Organic Syntheses |url=https://pubs.acs.org/doi/10.1021/cr900243d |journal=Chemical Reviews |volume=110 |issue=6 |pages=3419–3478 |doi=10.1021/cr900243d |issn=0009-2665|url-access=subscription |hdl=11511/67266 |hdl-access=free }} More efficient syntheses of this and the other two stable thienothiophenes involve cyclization reactions of substituted thiophenes.
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.
File:Thienothiophene251-41-2.png|Thieno[3,2-b]thiophene
CAS RN 251-41-2
m.p. 56.0-56.5 °C
white solid.
File:Thienothiophene250-84-0.png|Thieno[2,3-b]thiophene
CAS RN 250-84-0
b.p. 102 °C at 16 mmHg
colorless oil.
File:Thienothiophene250-65-7.png|Thieno[3,4-b]thiophene
CAS RN 250-65-7
m.p. 7.0-7.5 °C
colorless oil.
File:24976-21-4S(IV).png|2λ4δ2-Thieno[3,4-c]thiophene
CAS RN 24976-21-4
not isolated.{{cite journal|doi= 10.1039/C39880000959|title=Generation and Characterization of the Parent 2λ4δ2-Thieno[3,4-c]thiophene|author1=Nakayama, Juzo |author2=Ishii, Akihiko |author3=Kobayashi, Yasunobu |author4=Hoshino, Masamatsu |journal=Journal of the Chemical Society, Chemical Communications|year=1988|volume=14|issue=14 |pages=959–60}}