Electronic structure of π-systems. Studies of electronic structures and tautomeric transformations of a series of 4-methyl-8-(R-phenylazo)dihydrofuro[2,3-h]coumarin-9-ones

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Tautomeric transformations of a series of 4-methyl-8-(R-phenylazo)dihydrofuro[2,3-h]-coumarin-9-ones were studied by 1H NMR, IR, electronic absorption spectroscopy, mass spectrometry, and quantum chemistry methods. The obtained results suggest that (p-R-phenyl)-derivatives exist in the quinone hydrazone form, while (o-R-phenyl)derivatives exist as a mixture of quinone hydrazone and azo enol tautomers with domination of the former. The anti-quinone hydrazone tautomers with a strong intramolecular hydrogen bond are more favorable than the syn-quinone hydrazone tautomers.