Dewar intermediate

Phenyloxazole, irradiated in benzene at 80°C, gave two products 61, derived from a ring contraction, and 4-phenyloxazole (62), obtained in very low yields (Scheme 25) [73JCS(CC)539 77JCS(P1)239]. The authors thought that 4-phenyloxazole was ascribable to a Dewar intermediate. 

The authors generally considered the above results consistent with an ICI mechanism. In this contest, iodine favors the intersystem crossing or the opening of the Dewar intermediate, as reported in the Scheme 33 (72BSF2673). 

Dewar thiophene 1 is the first authentic example of such a structure. Its high stability suggests that the electronic structure may be strongly perturbed by the trifluoromethyl groups. Therefore, the isomerization of other thiophenes may not always pass through the Dewar intermediate. 

On the other hand, the photoisomerization of oxazoles to isoxazoles involves not only the interchange of the 2 and 3 positions, but that of the 2 and 4 positions. To explain the latter reaction, the intervention of a Dewar compound was proposed (Scheme 13).41 The walk of the oxygen atom is characteristic of this mechanism. A quantum mechanical calculation suggested that the O—C-2 bond was weak (for the azirin) and the bond order between C-2 and C-5 was high (for the Dewar intermediate) in the excited state. 

In order to determine whether the thiazole ring can be photorearranged the photoreactions of diphenylthiazoles were examined. A variety of isomers do result. Since the formation of all the products was not explained by the two intermediates shown in Eq. (20), the walk of the sulfur atom in the Dewar intermediates were proposed, as shown in Scheme (15). In this case, the intervention of still other valence-shell extended intermediates could not be ruled out.46... 

The photoreaction of pyridine in cyclohexane gives a product which seems to be formed through a Dewar intermediate (Scheme 34).87... 

The mass spectra of cyanopyridines which were isotopically labeled showed that they isomerize to one another, probably through Dewar intermediates (Scheme 39).98... 

Peculiarly, the 3,6-bonded Dewar pyridazine appears to be more stable than pyridazine. This conclusion could originate from the large approximations and the difference in the structure between the aromatic and the Dewar forms. Thus, the order in the Dewar isomers of one diazine might be correct. The photochemical and thermal isomerization of perfluoroalkylated diazines have been investigated mainly by Chamber s group, and Dewar intermediates were proposed for photoisomerizations. The photoisomerization of tetrafluoropyridazine to the pyrazine was given a mechanism shown in Eq. (34).101... 

The photoreaction of 1,2,4,5-tetrazine seems to occur through a Dewar intermediate (Scheme 44).111... 

Depending on the formed intermediates, the photoreactions of five-membered heteroaromatic compounds may be classified into two types, these involving formation of cyclopropenyl ketones and Dewar intermediates. However, there are some reactions where both types of intermediates participate. Kojima and his coworker showed that both isomers should be taken into account in the photoreaction of an 1,3-oxazole compound (116)ll6). 

Examples of the photoisomerization of pyrazoles to imidazoles have been reported by Nishiwaki et al. In the photoisomerization of perfluoroalkyl-pyridazines to the corresponding pyrazines, Chambers et al. have shown that an intermediate diaza Dewar-benzene is involved rather than a diazaprismane. Ogata and Takagi have observed a novel photoisomerization of 2-pyridylaceto-nitrile to u-aminobenzonitrile, possibly via a Dewar-intermediate. 

Fig. 5.1. 1,2-shift ofCH3 group through Dewar intermediate. 

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