AN HMO STUDY OF THE AZO-HYDRAZONE TAUTOMERISM IN DIAZONIUM COUPLING PRODUCTS OF 5-ISOXAZOLONES

The HMO method has been used to study tautomerism in 3-substituted 4-arylazo-5-isoxazolones (2), the possible tautomeric structures being 2a-2d. Also the common anions of these tautomers, 6, have been theoretically studied. In addition to the free molecules, HMO calculations have been carried out for structures with hydrogen bonds to protic solvents, for tautomers with intramolecular hydrogen bonds, and for dimers formed via intermolecular hydrogen bonds. In all these situations, the hydrazone form 2a has been shown to be most stable. Also the inductive effect of electron-donating and electron-withdrawing substituents in the arylazo moiety upon the stability of various tautomers has been investigated. The results obtained on the basis of HMO calculations are in full agreement with the experimental data of Summers and co-workers. Several m- and p-substituted 4-arylazo-3-methyl-5-isoxazolones have been synthesized and their spectral data and melting points reported.

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