Thiophene Dewar isomer

In agreement with the previously reported theoretical study, the results of semi-empirical calculations showed that the formation of the Dewar isomer is favored [99H(50)1115]. Probably, the observed formation of the azirine derives from a thermal isomerization of the first photoproduct, in line with that described in the case of furan and thiophene derivatives (Fig. 11). 

The irradiation of 2,3-di(trifuoromethyl)thiophene (60) gave a mixture of products where the authors found both isomeric thiophenes and an 8 2 mixture of Dewar isomers 61 and 62 (Scheme 6) (83H(20)174 84TL1917). 

The irradiation of the 2,5-di(trifluoromethyl)thiophene gave the corresponding 2,4 isomer, while 3,4-di(trifluoromethyl)thiophene gave the 2,4 isomer only in traces. 2,3,4-Tri(trifluoromethyl)thiophene gave the 2,3,5 isomer and a mixture of the corresponding Dewar isomers (83H(20)174). 

Bis(trifluoromethyl)thiophene, on irradiation, gives an equilibrium mixture of the two Dewar thiophenes 28 and 29 (8 1), along with the rearranged 2,5-, 2,4-, and 3,4-bis(trifluoromethyl)thiophenes. The Dewar isomer 29 seems to be... 

The initially formed singlet excited state 1 can convert either to the corresponding triplet state 2 by intersystem crossing, or to the Dewar isomer 4. In the former case, homolytic cleavage of the S-C(5) bond in 2 can lead to the biradical 3 and ultimately result in ring-opened or ring-contracted products. The Dewar isomer 4 is responsible for the formation of the isomeric thiophene 6 via 5 obtained by a sulfur walk . 

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). 

This is the first example of the isolation of a Dewar isomer of a five-membered heteroaromatic compound. Many interesting reactions of this Dewar thiophene have been reported. Some of them will be discussed in Section 3.2. 

The stabilizing effect of the trifluoromethyl groups on the highly strained Dewar compound is still not clear. The Dewar isomer is only formed if the thiophene is substituted by trifluoromethyl groups or other stabilizing groups. The photoreaction of 2-cyanothiophene affords 3-cyanothiophene. If this reaction is carried out in the presence of furan, the adduct of 3-cyano Dewar thiophene and furan is formed. This result suggests that the intermediate of the photoisomerization of 2-cyanothiophene is a Dewar compound (110)111... 

Furthermore, the photoreaction of 2-cyano-3-methyl- and 3-cyano-4-methylthio-phene gives the same Dewar isomer, 3-cyano-4-methyl Dewar thiophene, which can be isolated at room temperature. The Diels-Alder reaction with furan affords a mixture of two adducts. This implies that an equilibrium exists between the three Dewar thiophenes through the migration or walk of the sulfur. The equilibrium is shifted to the 3-cyano-4-methyl Dewar thiophene due to the electronic pull-push effects of the two substituents, although this isomer is less reactive than 3-eyano-2-methyl Dewar thiophene in the Diels-Alder reaction U2). 

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 thiophenes 46 and 47 are in equilibrium 47 Diels-Alder reaction of this mixture with 2.5-dimethylfuran leads to complete conversion of the mixture, but with the formation of only one product 51, derived from the reaction of isomer 47 47... 

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... 

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. 

In spite of its highly strained Dewar structure 1 is stable and may be kept in a refrigerator for several years. It isomerizes to the aromatic isomer at high temperature (half-life in benzene at 160 C was 5.1 h). The treatment of the Dewar thiophene with triphenylphosphine did not cause the elimination of sulfur, but instead accelerated the isomerization to the aromatic isomer. This catalytic acceleration is limited to tervalent phosphorus compounds but phosphorous trichloride or phenyldichlorophosphine are ineffective. An adduct of diphenylchlorophosphine with the Dewar thiophene was postulated to be a ff-complex with a P—S bond. It decomposed spontaneously to the thiophene and the phosphine as shown in Eq. (9).15 Recently,... 

The formation of pyrroles by the photolysis of furan or thiophene in the presence of a primary amine was considered to involve valence-bond isomers of pyrroles but the latter were not isolated. However, the photoreaction of cyanopyrrole was examined at the same time as that of cyanofuran (Section I,A), and much earlier than that of cyanothiophenes (Section I,B). Irradiation of 2-cyano- 1-methylpyrrole in methanol was reported to give l-cyano-3-methoxy-5-methyl-5-azabicyclo[2.1.0]pentane, which may have been formed from the Dewar compound (Eq. 14).4 The structure of 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. 

Since a review on Dewar heterocyclic compounds has been published recently102), in this chapter only some typical reactions of valence-bond isomers of heteroaromatic compounds will be discussed. The emphasis will be placed on the chemistry of trifluoromethylated Dewar thiophene and pyrroles, both of which are rare examples of stable valence-bond isomers of heterocyclic compounds. Since the isolation of valence-bond isomers of heteroaromatic compounds is mostly limited to the per-fluoroalkylated isomers, those postulated as intermediates are also included. 

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. 

In addition to the photochemical isomerization of 2-substituted thiophenes to their 3-substituted isomers which was extensively studied and reported in the 1960s (Scheme 4) <66JA5047, 67JA3487, 67JA3495,67JA3498,67JA3501,70JOC2737), photochemically-derived intermediate Dewar thiophenes may be isolable if properly substituted. These compounds may react as the alkene component in [4 2]... 

The Dewar thiophen (12), isolated after photolysis of the parent, is in thermal equilibrium with the other three isomers that have the methyl and cyano-groups on adjacent carbons, as evidenced by the photochemical interconversion of the four... 

---