Dewar pyridines, synthesis

Kobayashi, Y., Ohsawa, A., Baba, M., Sato, T., and Kumadaki, L, Studies on organic fluorine compounds. XX. Synthesis and reactions of a substituted Dewar pyridine, Chem. Pharm. Bull., 24, 2219, 1976. 

Proof for the existence of benzene isomers in irradiated benzene has been obtained in several ways. These will not be discussed in detail, but they may be classified broadly as physical and chemical. Nuclear magnetic resonance has been used by Wilzbach and Kaplan to identify benzvalene.39 Prismane has also been identified by NMR and by vapor-phase chromatography. The Dewar form has been synthesized in several steps which start with ris-1,2-dihydrophthalic anhydride. Photochemically this compound yields bicyclo(2,2,0)hexa-5-ene-2,3-dicarboxylic aqid anhydride. This was followed by catalytic reduction and oxidative decarboxylation to give the Dewar form of benzene.39 The method of synthesis alone provides some basis for structure assignment but several other bits of supporting evidence were also adduced. Dewar benzene has a half-life of about 48 hr at room temperature in pyridine solution and its stability decreases rapidly as the temperature is raised. 

A Dewar isomer of unsubstituted pyridine was identified by Wilzbach, but it was very unstable.63 Other Dewar isomers of six-membered heterocycles, isolated and fairly stable, are substituted with perfluoroalkyl groups. However, 1,2-dihydroheterocyclic compounds such as a-pyrone or a-pyridone derivatives are easily photoisomerized to bicyclic Dewar isomers, which are useful for the synthesis of cyclobutadienes. The photochemistry of 1,2-dihydroheterocyclic compounds will be discussed, followed by those of fully aromatic compounds. 

In their synthesis of Dewar benzene (10) van Tamelen and Pappas effected oxidative decarboxylation of the anhydride (9) with lead tetraacetate in pyridine at 43-45°. 

CMe CF CF-CFs + EtaN-CiCMe-CFa-CF-CFs ss] can be found in Chapter 2 (pp. 62,72) and production of the Dewar valence-isomer of pentakis(pentafluoro-ethyl)pyridine (53) is also discussed elsewhere (p. 93). An interesting synthesis of the fluoroenamine (54 X = CF CF2) has emerged from studies on aza[I3]-... 

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