Dimedone, tautomerism

The potential of carbon-13 NMR in the analysis of keto-enol tautomerism has been demonstrated for 2,4-pentanedione (acetylacetone) and dimedone [293]. Quantitative evaluation of equilibrium concentrations is possible by application of the inverse gated decoupling technique illustrated in Fig. 2.23. 

Did you notice when we were looking at the NMR spectrum of dimedone (p. 522) that the two CH2 groups in the ring seemed to be the same, though they are different (a and b) and the delocalization we have just looked at does not make them the same This must mean that the enol is in rapid equilibrium with another identical enol. This is not delocalization—a proton is moving—so it is tautomerism. 

The hydrogen-bonded enol structure looks unsymmetrical, but in fact, as with dimedone, the two identical enol structures interconvert rapidly by proton transfer, that is, by tautomerism. 

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