Pyridazines, Dewar isomers

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... 

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... 

If this mechanism applied to perfluoro(4,5-diisopropylpyridazine), perfluoro(2,3-diisopropylpyrazine) should be obtained. However, the 2,5-substituted isomer is obtained together with its Dewar isomer 163). In a more detailed study, a 2,5-bonded Dewar pyridazine was isolated which isomerized to the Dewar pyrazine. Therefore, another mechanism involving isomerization of the Dewar pyridazine to the Dewar pyrazine which does not proceed via the prismane has been proposed (151)164). 

Some aspects of the pyridazine-to-pyrazine isomerization have already been mentioned in this review. The work described in ref. 7 concerning isolation of the intermediate diaza-Dewar isomers in this conversion has been extended, and the... 

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