Abstract
Treatment of benzil monoazine with sodium methoxide gives 5-methoxy-1,2,5-triphenyl-3,4-diaza-2,4-pentadien-1-one (2), 5-benzoyl-4,5-dihydro-3,4,5-triphenylpyrazol-4-ol (3), and simple cleavage products. Compound 3 is hydrolyzed to 3,4,5-triphenylpyrazole and benzoic acid. It is phototropic, being reversibly converted on exposure to sunlight to a red product, which is readily autoxidized to benzil monoazine. Analogous phototropic products are formed on reaction of p-anisil monoazine and p-tolil monoazine with sodium methoxide. Reduction of the azines to the phototropic products may occur by cleavage to α-keto imine anions followed by their dimerization; such a pathway can also account for the formation of the cleavage products. Alternatively, reduction may proceed by hydride or electron transfer; that hydride transfer need not necessarily be involved is established by the observation that treatment of p-tolil monoazine with potassium t-butoxide also gives the corresponding phototropic product. It is proposed that the phototropic transformations involve intramolecular hydrogen abstraction to give enolic isomers of 3 and its analogs in which the heterocyclic ring has been cleaved.
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