Dewar pyrrole

Both (17) (74TL2841) and (18) (76JA4325) undergo cycloadditions at the double bond without disruption of the thiirane ring (80JOC2962, 80JA6633). This type of reaction has been used to convert (17) to the Dewar pyrrole (20) via the tricyclic thiirane (19) (77JA7350). 

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... 

In 1970, Hiraoka reported that 2-cyanopyrrole, irradiated in methanol with a low-pressure mercury arc for 20 h, gave a mixture of 3-cyanopyrrole and pyrrole-2-carbaldehyde [70JCS(CC)1306]. l-Methyl-2-cyanopyrrole (38) also gave this reaction (Scheme 15) [71JCS(CC)1610]. In this case, the author isolated the product of the isomerization 39, the product of the shift in C-2 of the IV-methy 1 group 40, and a third product that was assumed to be derived from the addition of methanol to the Dewar pyrrole 41. The reaction depends on the temperature used in fact, no reaction occurred when the reaction was performed at -68°C. This result is in agreement with the presence of a thermal-activated step [78JCS(CC)131]. More... 

All these data seem to be in agreement with a mechanism depicted in Scheme 16, where the thermal-activated step is the 1,2-sigmatropic shift between the Dewar pyrroles. 

When pyrrole is irradiated, only decomposition products were obtained. Theoretical data can fit this statement (Fig. 6). In fact, the direct irradiation populates the excited singlet state, which can be converted into the Dewar pyrrole or into the corresponding triplet state. Clearly, the intersystem crossing to the triplet state allows the system to reach the lowest energy state. The excited triplet state can give the biradical intermediate, and this intermediate can give either the decomposition... 

In contrast, when the irradiation is performed on 2-cyanopyrrole, the isomeric products are observed. In fact, in this case, the corresponding Dewar pyrrole shows a lower energy than in the previous case, allowing the formation of the isomeric products (Fig. 6). When 2-methylpyrrole is used as substrate, the formation of the triplet state is favored, but this triplet state cannot evolve through the formation of the biradical intermediate. 

The triazoline adducts from benzvalene (Scheme 21)162 and diphospha-benzvalene (Scheme 22) photolyze to yield novel tetracyclic aziridine ring systems165 that are valence isomers of azepines,162 whereas that from De-war thiophene (Scheme 20) gives a novel tricyclic aziridine that desulfurizes with triphenylphosphine to yield the trifluoromethylated Dewar pyrrole (Scheme 153).15 9,160 The stabilization of these strained molecules is attributed to the perfluoroalkyl effect.159... 

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

Dewar thiophene (9.113A) and, from this, Dewar pyrrole derivatives have been isolated [246]. In contrast, photolysis of furan derivatives only promoted cyclopropenyl ketone rearrangements [271] (Figure 9.113). 

The isolation of Dewar pyrroles came from the investigation of the reactivity of Dewar thiophene l.29 It was found to be a dipolarophile too. Some reactions of its cycloadducts with azides are summarized in Scheme 10. 

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. 

A possible approach to Dewar pyrroles is shown in Eq. (15). However, the dechlorination of the product was not reported.37... 

The isolation of a Dewar pyrrole in the photolysis of a pyrrole has not been reported, but the isolation of the trapped intermediate as its adduct with methanol or furan28 and the photoreaction of N-phenyltetrakis(trifluoro-methyl)pyrrole32 strongly support the intervention of Dewar pyrroles in the photoreaction. However, not enough data are available to say which substituents will give a Dewar pyrrole. 

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. 

The dramatic difference in reactivity between the two Dewar pyrroles certainly reflects a buttressing effect between the N-cyclohexyl and the... 

After a brief survey of the history of valence-bond isomers of aromatic compounds, new syntheses and the reactions of these isomers reported in the last decade are reviewed. In the second chapter, the valence-bond isomers of homoaromatic compounds, especially benzene derivatives, are described and in the third chapter those of heterocyclic compounds. Photoreactions of perfluoroalkylated aromatic compounds afford valence-bond isomers in high yields. These isomers are very stable and useful for the synthesis of highly strained compounds. Therefore, the emphasis is put on the chemistry of trifluoromethylated benzvalenes, Dewar thiophenes, and Dewar pyrroles. 

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 1,3-dipolar cycloaddition of azides to the Dewar thiophene proceeds smoothly. The obtained adducts are converted to the corresponding Dewar pyrroles. Some reactions of the adducts with azides are shown in scheme (120) 120-124>. 

Stepwise elimination of a sulfur atom and nitrogen molecule from the adduct gives the Dewar pyrroles 120,121). The stability of the desulfurized products depends... 

N-Phenyl Dewar pyrrole rearranges spontaneously to a cyclobutaindole while other Dewar pyrroles are stable at room temperature and thermally or photo-chemically isomerized to the pyrroles 124). 

It is interesting that the systems related to 11—pertrifluoromethyl Dewar pyrrole (19) and pertrifluoromethyl Dewar furan (20)-do not appear to undergo comparable walk rearrangements. Upon heating, these compounds isomerize to the corresponding substituted pyrrole and cyclopropenyltrifluoromethylketone, respectively. 

Fluorothiiranes are readily desulfurized by a variety of reagents. Triphenylpho-sphine, for example, reacted at RT with aminosulhde 102 to yield Dewar pyrrole 103. °° For the desulfurization of azo compound 104, a nonbasic thiophile was required because of the extremely facile azo-to-hydrazone isomerization. An imi-dazolethione accomplished this catalytically and cleanly to give 105 plus elemental sulfur, presumably via zwitterion 106. ° Thiirane 30 was transformed into perfluor-oisobutylene (107) by both amines and antimony pentafluoride. ... 

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