Tetrakis Dewar thiophene, isolation

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... 

Pyrrole is believed to be more aromatic than furan, with an aromatic stabilization energy estimated to be 100-130 kj mole-1, thus only with such extremely powerful dienophiles as tetrakis(trifluoromethyl)Dewar thiophene were the IEDA adducts isolated. Therefore, several attempts to achieve an IEDAR with pyrroles using high pressure have been made [9-14]. 

Irradiation of tetrakis(trifluoromethyl)thiophene gave a valence-bond isomer which Heicklen first proposed to have a Dewar structure,9 then a cyclopropenyl thioketone,10 and still later11 a Dewar form. The structure was shown to be Dewar form 1 by 19F and 13C NMR and by the study of its reactions (Eq. 8).12 14 This is the first isolated example of a Dewar viner of a five-membered heterocycle. 

Dewar thiophenes are thiiranes fused to cyclobutene rings. While the reversion of the tetrakis(trifluoromethyl) derivative to the normal thiophene structure requires a high temperature (r 1/2 160 , C Hg] 5.1 hr), the transformation can be achieved at room temperature by phosphines (86) such as Ph3P. A 1 1 adduct can be isolated when PhjPCl is added to the Dewar thiophene. However, the harder PhPCl2 and PCI3 and pentacovalent phosphorus compounds are totally ineffective for the isomerization. 

A number of approaches to tetrakis(trifluoromethyl)-pyrroles was developed using tetrakis(trifluoro-methyl)-Dewar-thiophene (183) [71]. The 1,3-dipolar cycloaddition with azides led to the tricyclic thiiranes 184. Subsequent desulfurization by treatment with PPha afforded the cyclobutenes 185 in good to quantitative yields. The result of thermolysis of 185 was strongly depended on the substituent on the amine nitrogen. Pyrroles 188 were formed in high yields (cases a and d), while only cyclopropene 189, or a mixture of 188 and 189 (cases b and c) were isolated. Pyrrole 188a was also synthesized by the reaction of 183 with aniline in 19 % yield [72]. 

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