Thienothiophenes structure

Thiophene and thienothiophene units have been frequently used in thienoquinoid or condensed forms in the design of new molecular architecture (16). Incorporation of the thiophene or thienothiophene unit as a rc-bridge linker could stabilize the quinoidal structure in the reduced forms (Figures 13 and 14). These two-types of new dications (222+ and 232+) stabilize the presumed two-electron reduction state by contribution of the thienoquinoid substructures (22 and 23) instead of the quinoidal form in the dication 212+ (77). 

In 1949 Cox and co-workers ° from X-ray structural analysis and quantum-mechanical calculations suggested that thieno[3,2-b]thiophene (2) possesses a ground state intermediate form between the extremes with completely localized double bonds and that with complete delocalization of TT-electrons. The discrepancy between the theoretical and experimental (Fig. 1) values (0.05 A) for the central C,—Cg bond was noted, and the thienothiophene (2) molecule was shown to be planar and to have a center of symmetry. 

Mass spectrometry was also used to analyze mixtures of deuterated and nondeuterated thiophenes and thienothiophenes 1 and 2, as well as to confirm the structure of l,3,5,6-tetraphenylthieno[3,4-c]thiophene (149) (see also Bugge ). 

A complete analysis of the IR spectra of thienothiophenes 1 and 2 in the gaseous, liquid, and crystalline states was carried out by Kimel feld et a/. The following isotopically substituted compounds were also studied 2-deuterothieno[2,3-h]thiophene (l-2d), 2-deuterothieno[3,2-I)]-thiophene (2-2d), 2,5-dideuterothieno[2,3-h]thiophene (l-2,5-d2), and 2,5-dideuterothieno[3,2-h]thiophene (2-2,5-dj). The IR spectra of oriented polycrystalline films of all compounds were measured in polarized light, and Raman spectra of liquid thienothiophenes 1, l-2d, and 1-2,5-dj, of crystals of thienothiophenes 2 and 2-2,5-d2 and melts of thienothiophenes 2 and 2-2d were analyzed. The planar structure of point-group Cj, for thienothiophene 1 in the liquid and gaseous states was assumed. Then the thirty vibrations of compounds 1 and l-2,5-d2 can be divided into four symmetry classes Aj (11), Bj (10), A2 (4), and B2 (5) the vibrations of molecule (l-2d) (C, symmetry) are divided into two classes A (21) and A" (9). 

The structures of the acetylation products of 1 and 2 were also proved by the following conversions. The oximes derived from the acetyl thienothiophenes gave the corresponding acylamino compounds on Beckman rearrangement. The latter were identical with the derivatives obtained by reduction (Hj/Ni) in acetic anhydride of the 2-nitro-substituted thienothiophenes 1 and 2 [Eq. (61)]. 

Bugge brominated thienothiophenes 1 and 2 with IV-bromo-succinimide in glacial acetic acid to 2-bromothieno[2,3-6]thiophene (66%) and 2-bromothieno[3,2-6]thiophene (55%). The structure of 2-bromothieno[2,3-6]thiophene was confirmed by the replacement of bromine by lithium at —70° followed by carbonation to thieno[2,3-6j-thiophene-2-carboxylic acid 2-bromothieno[3,2-fe]thiophene was independently prepared by the treatment of 2-lithiothieno[3,2-6]thiophene with one equivient of bromine at —70°. The 2-bromo derivatives of thienothiophenes 1 and 2 decompose within several hours at 20°, but remain uncWged for weeks at —15°. 

Thienothiophenes, like thiophenes and benzo[h]thiophenes, easily undergo reductive desulfurization with Raney nickel. The method was fvst applied in this series by Challenger et to determine the structures of 2-acetylthieno[3,2-h]thiophene and 2-acetylthieno[2,3-hl-thiophene the former gave 2-octanone (229), and the latter 5-methyl-heptan-2-ol (230) [Eqs. (75) and (76)]. 

To confirm the structures of the products formed in the reaction between thienothiophenes 1 and 2 and phenyl radicals, the corresponding 2-phenyl- and 3-phenyl-substituted thienothiophenes 1 and 2 were synthesized by independent methods (see Section II,B). In addition, 2-phenyl-substituted thienothiophenes 1 and 2 were prepared by photolysis of the corresponding 2-iodo compounds, under conditions previously used with the iodothiophenes [Eqs. (86), (87)]. 

In a similar fashion, the structure as well as the stereochemistry of the exo/endo adducts obtained by the trapping of various nonclassical thienothiophenes have been assigned on the basis of1H NMR. For example, N- phenylmaleimide was shown to have added exclusively to the substituted ring rather than the unsubstituted one in compound (10) to give adducts (26) and (27) on the basis of H NMR evidence (Scheme 5) (75ACR139). 

The major products resulting from the acetylation of thienothiophenes (7) and (3) in the presence of Lewis acids are the C-2 substituted isomers (67 and 69 Schemes 16 and 17). GC-MS analysis, however, indicated the presence of j3-substituted isomers (68) and (70) as well (76AHC(19)123). Chemical proof of structures (67) and (69) was provided by standard transformations. For example, ketone (69) upon desulfurization with Raney nickel furnished 2-octanone (Scheme 17). 

Although thienothiophene (8) should be vulnerable to electrophilic substitution at C-2, C-4 and C-6, on the basis of the resonance structures it becomes clear why substitution occurs at C-4 and C-6 rather than at C-2. 

In 2-alkyl-substituted thienothiophenes I and 2 the free a-position is attacked.44,57 7,219 However, acetylation of 2-ethylthieno[2,3-61-thiophene (20) produced, together with 2-acetyl-5-ethylthieno[2,3-61-thiophene, small quantities of some other compound which was ascribed219 the structure of 3-acetyl-2-ethylthieno[2,3-61thiophene by analogy with the acetylation products of 2-ethylthiophene.24 Acetylation of 2,5-dialkylthienothiophenes 1 and 2, like 2,5-dialkyl-thiophenes,247 proceeds easily in the presence of stannic chloride. For example 2,5-diethylthieno[2,3-6]thiophene (with AcCl/SnCl/QH ) forms 3-acetyl-2,5-diethylthieno[2,3-6]thiophene in 80% yield. 7... 

Iodination and chlorination have been considerably less studied. Data on iodination are limited to those of Challenger and co-workers, who showed that thienothiophenes 119 and 220 with iodine and mercuric oxide give rather unstable 2-iodo derivatives, the structures of which were confirmed by their conversion into the corresponding 2-carboxylic acids.20 The formation of 2,5-diiodothieno[3,2-6]thiophene by iodination of 2 was also observed.22... 

Gleiter and coworkers64 have studied a number of planar heteroaromatic systems involving S—N bonds within the ring structure. Thieno[3,4-c]isothiazoleis a nonclassical condensed thiophene, and its triphenyl derivative, triphenylthieno [3,4-c] isothiazole (106), is a 1071 system containing two different masked 1,3-dipolar systems. It was synthesized from 4-phenyl-1,3,2-oxathiazolylium-5-olate (104) and dibenzoylacetylene via the direct precursor isothiazole (105) and its photoelectron spectrum compared with that of the related thieno [3,4-c] thiophene (107)34. The spectrum is characterized by an isolated first band at 6.9 eV which is verified by PPP-CI calculations to involve the 7c( au ) orbital confined to the thienothiophene ring. 

Compounds characterized by three modes of fusion of the thiophene rings, viz., [2,3-Z ] (structure A), [3,2-Z ] (B) and [3,4-Z ] (C), have attracted the most attention. These isomers are refered to as conventional or classical thienothiophenes because structures of the type D ([3,4-c]-fusion) are much less accessible. 

In the last 25 years. X-ray diffraction analysis has been particularly widely used (along with other methods) to determine the structures of thienothiophenes. For example, the crystal structures of the following compounds were established by X-ray diffraction 3,4-dibromothieno[2,3-Z)]thiophene (86MI1), 2,2,5,5-tetramethyl-... 

The 7i-electron structure of thiophene, isomeric thienothiophenes and other fused sulfur-containing compounds (58 compounds, including those annulated with the... 

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