Tetrakis Dewar

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). 

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). 

When uradiated, fluonnated isomers of Dewar benzene yield pnsmane derivatives that rearrange thermally to benzene Photolysis of hexakis(mfluororaethyl)benzvalene ozonide gives tetrakis(tnfluoromethyl)cyclobutadiene and its dimer [J47]... 

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]. 

The [3 + 2] cycloadduct formed on treatment of tetrakis(trifluoromethyl)-Dewar-thiophene with trifluoromethyl diazomethane isomerizes to give an annulated 3-trifIuoromethyl-l//-2-pyrazoline in the presence of acids and bases. Sulfur can be removed from both compounds on reaction with triphenylphosphine (80JA6633). 

Dewar pyrroles, 2,3,4.5-tetrakis-trifluoromethyl-, synthesis from Dewar thiophenes, 60, 35... 

Thus, furan derivatives seem to isomerize to cyclopropenylcarbonyl compounds in photoreactions. However, recently Lemal informed us that Dewar furan was derived from tetrakis(trifluoromethyl)Dewar thiophene (II B). 

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 structure 1 forms a cycloadduct with butadiene. Bromination of this adduct gives a cis dibromide. Cis bromination may be attributed to a steric effect of the two trifluoromethyl groups on the thiirane ring. If desulfurization precedes bromination, then the addition gives a trans dibromide (Scheme 4). The latter pathway provides access to tetrakis(trifluoromethyl)-... 

Reaction of Dewar thiophene 1 with trifluoromethyldiazomethane gives an adduct which was desulfurized and pyrolyzed to tetrakis (trifluoromethyl)-pyrrole, among other products (Scheme ll).34,35... 

Another synthesis of a trifiuoromethylated Dewar pyrrole involves a valence-bond isomer of a diazepine.36 The thermolysis of 1,3,4,5,6-pentakis-(trifluoromethyl)-2,4-diazabicyclo[3.2.0]hepta-2,6-diene gave tetrakis-(trifluoromethyl)pyrrole possibly through a (3,3) sigmatropic recyclization followed by the elimination of trifluoroacetonitrile. The thermolysis of the N-methyl derivative gave a Dewar isomer, which was fairly stable even at 200°C. These reactions are summarized in Scheme 12. 

Some Dewar tetrakis(trifluoromethyl)pyrroles react as dienophiles. Their reactivities depend on the substituent on the nitrogen atom. A few reactions are shown in Table I, with the results of the Dewar thiophene analog for comparison. Interestingly, N-unsubstituted Dewar pyrrole is the most and the N-cyclohexyl compound the least reactive of the three. These results seem to indicate a steric requirement in the transition states. 

Dewar tetrakis(trifluoromethyl)thiophene, (59), has served as a dieneophile (dipolarophile) in several cycloadditions. Diels-Alder reaction of (59) with butadiene <8icci289> and pyrrole (82JA847) has served as the starting point for the synthesis of 1,2,3,8-tetrakis(trifluoromethyl)cyclooctatetraene (60) and tetrakis(trifluoromethyl)Dewar furan (61), respectively, as illustrated in Scheme 20. It is noteworthy that (61) was unattainable by direct photolysis of the substituted furan. Moreover, (59) served as a common precursor for the preparation of tetrakis- (80JA6633) and penta-<83JA3563> (trifluoromethyl)cyclopentadienides (64) and (65), respectively (Scheme 21). Accordingly, photolysis... 

Another attempt to synthesize a Dewar furan by the photolysis of tetrakis(tri-fluoromethyl)furan was made since the photolysis of tetrakis(trifluoromethyl)thio-phene gives a stable Dewar compound (see Eq. 109). However, even this furan only yields the corresponding cyclopropenyl ketone 105) ... 

These intermediates of the photoreaction of thiophenes were not observed directly. The photoreaction of tetrakis(trifluoromethyl)thiophene gives a very stable Dewar isomer 109). Its structure was elucidated by 13C-NMR and X-ray analysis of the Diels-Alder adduct with tetramethylfuran (109)110). 

So far, the formation of valence-bond isomers by photolysis has mainly been discussed. Tetrakis(trifluoromethyl) Dewar thiophene is stable at room temperature and was used for the synthesis of Dewar pyrroles, a Dewar furan, and other interesting compounds from the standpoint of the structural chemistry. In this section, some of these reactions will be discussed. 

The Diels-Alder adduct of tetrakis(trifluoromethyl) Dewar thiophene and butadiene is transformed to tetrakis(trifluoromethyl)cyclooctatetraene127) which is further converted to two semibullvalenes (123) 128). 

The Dewar thiophen structure (232) of the photoproduct of tetrakis(trifluoro-methyl)thiophen (233) has been confirmed.188 The possible intermediacy of a... 

Kobayashi, Y Ando, A. Kawada, K. Ohsawa, A. Kumadaki, I. Organic fluorine compounds. Part 32. A 1,3-dipolar cycloaddition reaction of tetrakis(trifluoromethyl) (Dewar thiophene) and some reactions ofthe cycloadducts. 7. Org. Chem. 1980,45,2962. 

Tetrakis(trifluoromethyl) Dewar thiophene has been converted to the Dewar furan in several steps <82JA847>. The first step in the sequence is the cycloaddition to pyrrole, followed by PhjP-induced desulfurization. 

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