Dewar pyridazines

Tri-/-butylazete 627 has been prepared both by photolysis as well as by thermolysis of Dewar pyridazine 664 (Equation 254) <1995LA169>. 

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 cycloaddition reaction of cyclobutadienes with azodicarboxylates gives 3,6-bonded Dewar pyridazines. An example of this type of reaction has already been reported before94. A further example is described by the reaction sequence (147)158,159). The formed cycloadduct a hydrolyzed, decarboxylated, and oxidized to 3,6-bonded Dewar pyridazine which loses nitrogen to form cyclobutadiene (147). 

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). 

To conclude, it can be said that the photoreaction of perfluorinated pyridazines proceeds via 1,4-bonded Dewar pyridazines while their thermolysis occurs via the benzvalene analogs which are generated by a radical mechanism. 

Isopropyl-4,5,6-fm-butylpyridazine, which exists in the twist conformation, is transformed upon photolysis into the corresponding 1,2-Dewar-pyridazine, which is stable (91AG1495). Irradiation of the ketone 179 with UV light produces about 10% of 180 (92JA1838). Photooxidative decomposition of 3,3,6,6-tetraalkyl-substituted perhydropyridazines was investigated and it was found that decomposition is stereospecific and that the 1,4-biradical determined the stereochemical outcome and not the 1,4-cation radical. Cyclobutane and 1-butene derivatives were products identified (93JA4925). 

A remarkable sequence, observed by Eisenbarth and Regitz is initiated by the action of heat on the esters 509 (R = MeO or Ph) and yields, first, the phosphorylated diazines 510, irradiation of which causes isomerization to a Dewar pyridazine (511). 

Photoisomerization of perfluoro(4,5-diisopropylpyridazine) is postulated to proceed through Dewar diazabenzenes (25) and (26) to perfluoro(2,5-diisopropylpyrazine) (27), which is in equilibrium with the isomeric perfluoro(2,6-diisopropylpyrazine) (28) after prolonged irradiation in the liquid phase (Scheme 9) (75JCS(P1)1130). Benzo-fused pyridazines do not isomerize readily under photochemical conditions. An exception is perfluorocinnoline which rearranges to perfluoroquinazoline. 

Diazocines are isolated, as byproducts, in another photochemical reaction which starts from fluorinated pyridazines. On irradiation of 6 a Dewar diazabenzene derivative is formed, via an electrocyclic ring closure, which looses a fluorinated nitrile to give the azacyclobutadiene system 7. This reactive intermediate then leads vide supra) to the 1,5-diazocine 8.50... 

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... 

Another detailed treatment for pyridine, pyridazine, pyrimidine, pyrazine, 1,2,4,5-tetrazine, and a number of five-membered aromatic heterocycles is based on AMI semiempirical calculations <89H(28)ll35>. Dewar and Holder s values for the aromatic energies are all lower than Wiberg s benzene (28.3 kcal mol ), pyridine (25.6 kcal mol ), pyridazine (22.7 kcal mol ), pyrimidine (25.0 kcal mol ), pyrazine (24.6 kcal mol ), and 1,2,4,5-tetrazine (12.4 kcal mol ) again the tetrazine system seems to be least stabilized. 

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... 

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. 

In contrast to this reaction, photolysis of pyridazines yields mainly pyrazines. The photochemical reorganization of the 1,2-diazine framework presumably involves intermediates of the Dewar benzene type 1 and 2. 

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