Tautomerism of 2-(pyridine-2-yl)-1,3-diazaspiro[4.4]non-1-en-4-one

• Spirocyclic imidazolinone is in solution as a mixture of tautomers.
• The ratio of the tautomers is solvent- and phase-dependent.
• The interconversion between the tautomers is slow on the NMR time scale.

The work deals with both solution state (using 1H, 13C and 15N NMR) and solid state (using X-ray) structure of 2-(pyridine-2-yl)-1,3-diazaspiro[4.4]non-1-en-4-one. The compound exists in the solution predominantly as 5-oxo tautomer where the amount of 4-oxo form depends on the solvent used. Mutual interconversion between the tautomers is slow on the NMR time scale and is studied by means of both EXSY and variable temperature NMR. In the solid state, the compound is solely as 5-oxo tautomer.

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