Dewar isomers

Photochemical isomerization of partially fluorinated benzenes also leads to relatively stable Dewar isomers [164, 265], however, at elevated temperatures, they rearrange to benzene derivatives [166]... 

The calculated relative energies of all the possible intermediates involved in the photochemical isomerization are collected in Fig. 4 (OOOJOC2494). In this case the irradiation can involve the excited singlet state, and then the formation of Dewar isomer is possible. As in 2-methylfuran, the fission of a O—Cq, bond in the triplet state of the molecule is not so favored as in furan. The corresponding biradicals... 

In the photochemical isomerization of isoxazoles, we have evidence for the presence of the azirine as the intermediate of this reaction. The azirine is stable and it is the actual first photoproduct of the reaction, as in the reaction of r-butylfuran derivatives. The fact that it is able to interconvert both photochemically and thermally into the oxazole could be an accident. In the case of 3,5-diphenylisoxazole, the cleavage of the O—N bond should be nearly concerted with N—C4 bond formation (8IBCJ1293) nevertheless, the formation of the biradical intermediate cannot be excluded. The results of calculations are in agreement with the formation of the azirine [9911(50)1115]. The excited singlet state can convert into a Dewar isomer or into the triplet state. The conversion into the triplet state is favored, allowing the formation of the biradical intermediate. The same results [99H(50)1115] were obtained using as substrate 3-phenyl-5-methylisoxazole (68ACR353) and... 

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 imidazole derivatives such as 81 gave isomeric compounds 82 (Scheme 30) (67TL5315 69T3287). Dewar isomers are invoked to justify the observed photochemical behavior. 

Computational results are reported for the isomerization of 1,4,5-trimethyl-imidazole (99MI233). They show that the isomerization occurs through the Dewar isomer arising from the excited singlet state. The formation of the triplet state is energetically favored however, the biradical intermediate cannot be produced because it has higher energy than the excited triplet state. 

Theoretical calculations explain the photochemical behavior of phenylthiazoles (Fig. 14) (99MI233). The RCRE mechanism cannot be invoked because the radical intermediates have higher energies than the corresponding triplet states. Furthermore, the formation of the Dewar isomer is favored in comparison with the formation of the zwitterionic intermediate. Nevertheless, the reaction conditions used by Kojima and Maeda could allow for an endothermic reaction giving this type of intermediate. The same results were obtained using 2,5-diphenylthiazole. 

The authors justified their result considering that the bond index [Br = (coefficient r LUMO) (coefficient s LUMO) ] on the LUMO accounted for the formation of the Dewar isomer on the central ring of the trithiazole. By contrast, calculations show that the Dewar isomer should be formed on the third ring of the trithiazole (Fig. 17) (99MI233). 

Calculations allow one to justify the observed behavior (Fig. 19) (99MI233). In the case of 3- and 5-phenylisothiazole, the reaction should implicate a Dewar isomer, because the excited triplet isothiazole derivative cannot be converted into the corresponding biradical. 

Oxadiazole was obtained through the first excited singlet state. When the reaction was carried out in the presence of a triplet sensitizer, 99 was not detected but the quinazolinone 100 was obtained (Scheme 41) [91JCS(P2)187]. Compound 99 cannot be obtained via the Dewar isomer. The author supposed the formation... 

Tetra-r-butylpyridazine (34) is converted into its Dewar isomer (35) when irradiated in pentane with UV light of wavelength > 300 nm. Irradiation of this product at shorter wavelengths, or thermolysis, gives rise to further reaction (91TL57). Irradiation of 4-amino-2,6-dimethylpyrimidine gives the acyclic amino imine via the Dewar pyrimidine as shown in Scheme 2a. The photoisomerization of perfluoropyridazines to pyrazines is considered also to involve Dewar diazine intermediates. 

The first observation of the thermal transformation of a strained paracyclophane into its Dewar isomer has been reported.56 Hexahalobispropellane (41), on treatment with potassium t-butoxide, has been shown to afford the phenol (43) along with (44). The formation of both these compounds has been rationalized57 by invoking the intermediacy of (42) (see Scheme 10). 

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). 

Irradiation of a benzothiophene derivative 63 gave the corresponding Dewar isomer 64 (95TL3177). 

Disrotatory closure of the substituted benzene to produce a Dewar benzene is photo-chemically allowed, as is, of course, the reverse process. However, because benzene is conjugated, it absorbs UV light at longer wavelengths than the Dewar benzene isomer. Therefore, it is possible to selectively excite the benzene chromophore and produce the less stable Dewar isomer. In this particular case the rm-butyl groups favor the reaction because they destabilize the benzene isomer somewhat, owing to steric hindrance. Because the two adjacent im-butyl groups in the Dewar isomer do not lie in the same plane, this steric strain is decreased in the product. Because of this steric effect and the forbidden nature of the conversion back to benzene, the Dewar isomer is relatively stable. However, when it is heated to 200°C, it is rapidly converted to the benzene isomer, probably by a nonconcerted pathway. 

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... 

Earlier work on the photolytic or thermal rearrangement of polyhalogenated pyridazines to corresponding pyrazines has been continued,14,161,774,1690 but the fascinating results offer little of preparative value. It has been reported that 300-nm irradiation of 3,4,5,6-tetra-tert-butylpyridazine (66) gave a quantitative yield of the Dewar isomer (3,4,5,6-tetra-tert-butyl-l,2-diazabicyclo [2.2.0]hexa-2,5-diene 67] that subsequently afforded 2,3,5,6-tetra-tert-butylpyrazine (68) in 18% yield on 254-nm irradiation.1464... 

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 . 

The conversion of 9-t-butylanthracene into its Dewar isomer (1) and the back reaction were first reported in 1980, and have been further investigated... 

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