Imino–enamine tautomerism and dynamic prototropy in 1-imino-3-amino-1H-indens

The tautomeric structures and dynamic prototropic behavior of the products 1 and 2 obtained in the condensation reaction of 1,3-indandione and 2-pyridyl-1,3-indandione with p-toluidine, respectively, were investigated by 1H NMR spectroscopy and X-ray analysis. In the solid state, compound 1 is in an imino–enamine tautomeric form, whereas in solution it coexists with an imino–imino tautomeric form. Dynamic 1,5-prototropic interconversion of the imino–enamine form was revealed to be very fast at room temperature by temperature-dependent 1H NMR spectra. For 2, the imino–enamine form is the only species present in solution. The hydrogen of the enamine NH is hydrogen-bonded intramolecularly with the nitrogen in the pyridine ring. When the temperature is raised, the NH proton enters into dynamic 1,5-migration, which is accompanied by internal rotation around the pivot bond, which changes the hydrogen-bonding sites. For the condensation product 3 of 2-(2-quinolyl)-1,3-indandione with p-butylaniline, dynamic behavior similar to that found in 2 was observed also in 13C NMR spectra.

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