Thiophene, Dewar

JOC1537). The mechanisms of these transformations may involve homolytic or heterolytic C —S bond fission. A sulfur-walk mechanism has been proposed to account for isomerization or automerization of Dewar thiophenes and their 5-oxides e.g. 31 in Scheme 17) (76JA4325). Calculations show that a symmetrical pyramidal intermediate with the sulfur atom centered over the plane of the four carbon atoms is unlikely <79JOU140l). Reactions which may be mechanistically similar to that shown in Scheme 18 are the thermal isomerization of thiirane (32 Scheme 19) (70CB949) and the rearrangement of (6) to a benzothio-phene (80JOC4366). 

The rearrangement (automerization) of Dewar thiophene 5-oxide (61), observed by NMR, occurs so much more rapidly than that of the corresponding episulfide that special mechanisms have been invoked. The one which involves a zwitterionic intermediate (Scheme 108) is favored over a pseudopericyclic sulfur-walk mechanism in which the electrons of the carbon-sulfur o--bond and the pair of electrons on sulfur exchange places as the sulfur atom migrates around the ring (80JA2861). 

The stable Dewar thiophene (68) is obtained by irradiation of 2,3,4,5-tetrakis(trifluoromethyl)thiophene (Scheme 150) (72ClC272l). Dewar thiophenes are proposed as intermediates in the photochemical isomerizations of substituted thiophenes (Scheme 17). 

In contrast to (14), several examples of Dewar thiophenes are known. The first example was (17), prepared by irradiation of (16) (72CJC2721, 75CPB2773). The degenerate Sulfur walk to (17 ) occurs at a rate of 55 s at 157 "C (77JA629), appreciably faster than thermal... 

Dewar thiophenes i.e. 22 and 23) are intermediates in the photoisomerization of cyanothiophenes. Their presence has been demonstrated by trapping and by direct NMR observation (79CC881, 79CC966). The rapid sulfur walk i.e. 22- 23) fully explains the substituent scrambling in the room temperature irradiations (i.e. 21 - 24). 

Strained pertrifluoromethyl-substituted valence tautomers of aromatic systems, such as tetrakis(trifiuoromethyl)Dewar thiophene [87] and hexalas(tnfluorQ-methyl)benzvalene [Diels-Alder reactions with various cyclic and acyclic dienes (equations 76 and 77). 

Dewar pyrrole [756] and Dewar thiophene stabilized by the presence of fluormated substituents have been successfully isolated, and their chemical pro perties have been studied [757, 1S8, 159, 160, 161] The olefinic bond m these... 

The irradiation of the thiophene in gas phase yields ethylene, allene, methyl-acetylene, carbon disulfide, and vinylacetylene. No Dewar thiophene or cyclo-propene derivatives were isolated (69CJC2965). The irradiation in liquid phase gave the Dewar thiophene which can be trapped as a Diels-Alder adduct with furan (85JA723). The Dewar thiophene and cyclopropene-3-thiocarbaldehyde can be obtained by irradiation in argon matrices at 10 K (86JA1691). 

Also in this case calculation results fit the experimental data (Fig. 7) [99H(50)1115]. In fact, the singlet excited state can evolve, giving the Dewar thiophene (and then isomeric thiophenes) or the corresponding excited triplet state. This triplet state cannot be converted into the biradical intermediate because this intermediate shows a higher energy than the triplet state, thus preventing the formation of the cyclopropenyl derivatives. 

The results of theoretical calculations on 2-phenylthiophene fit the experimental results (Fig. 8) [99H(50) 1115]. In fact, in this case, the formation of the triplet state of 2-phenylthiophene cannot allow the formation of the biradical intermediate allowing the formation of the Dewar thiophene. 

The irradiation of 2- and 3-cyanothiophene gave interesting results in agreement with the scheme described above (Scheme 19). The photoisomerization reaction involved only the excited singlet state and Dewar thiophenes were isolated when the reactions were carried out at -68°C and shown to be intermediates in the isomerization reactions [79JCS(CC)881 79JCS(CC)966]. 

Photolysis of 2,3-bis(trifluoromethyl)thiophene gave an isomer mixture of 2,3- and 3,4-bis(trifluoromethyl) thiophenes and Dewar thiophenes. The 2,5- analogue gave 2,4-bis(trifluoromethyl)thiophene via an intermediate that was not the Dewar thiophene. This indicated that provided they are orientated correctly, two trifluoromethyl groups are sufficient to isolate a Dewar thiophene (84TL1917). 

Dewar furan and Dewar thiophene (162, X = 0,S) have been obtained only under conditions of low-temperature photolysis along with... 

The parent Dewar thiophene (24) has been generated and trapped (85JA723). When a solution of thiophene in furan (mole ratio, 1 10) was irradiated at 229 nm at 25°C, the two 1 1 adducts (25) and (26) were formed. 

Several further experimental results lend support to the mechanisms postulated to explain the formation of the photoproducts. Thus irradiation of thiophene and substituted thiophenes in the presence of -propylamine leads to pyrroles, presumably via cyclopro-penylthiocarbonyl, or Dewar thiophene, intermediates (75T785). An extremely interesting development is the isolation of l,2,3,4-tetrakis(trifluoromethyl)-5-thiabicyclo[2.1.0]pent-2-ene, or perfluoro(tetramethyl Dewar thiophene) (2) by vapor-phase irradiation of perfluorotetramethylthiophene (81ACR76) the yield is about 58%. The half-life of thermal rearomatization of (2) at 160 °C is 5.1 h. The fluxional behaviour of the Dewar thiophene... 

The Dewar thiophene (2), possessing a strained double bond substituted with highly electronegative groups, undergoes cycloaddition with dienes (77JCS(P1)2355) and azides... 

Photolysis of perfluoro(tetramethylthiophene) (43) in the vapor phase gives pcrfluoro(l,2,3.4-tetramethyl-5-thiabicyclo[2.1.0]pent-2-ene) (44).4S Dewar thiophene 44 is thermally stable, reverting to 43 slowly at 160 C.46 Triphenylphosphane catalyzes the isomerization of Dewar thiophene 44 to thiophene 43 at room temperature.46... 

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