Suppression Mechanism for Enol–Enol Isomerization of 2‐Substituted Dimedones

Abstract

2-Substituted dimedones,d escribed as having as imple chemical structure, are at ype of cyclic b-diketone with enol-enol interconversion. The influence of the enolenol isomerism interferes with structure analysiso fm any dimedones in solution by NMR spectroscopy. It was demonstrated that the isomerism of the 2-substituted dimedones depends on solvent, time, solute concentration,a nd temperature. Negativea ctivation energy of the isomerization was determinedb yt he NMR line shape and spin-spin relaxation analyses, which is an extraordinary phenomenon found for the first time in the molecular dynamics of NMR. Suppression of the isomerization by heating, dilution of the solute, and time control in severall ow-polarity solvents enabled analysiso ft he enol form in detail. It was clarified that deceleration of the intramolecular chemical exchange rate in the isomerization was responsible for the decrease in the intermolecular hydrogen-bonding effect in the suppression mechanism. Figure 1. E-E isomerism of a) asymmetric and b) symmetric b-diketones.[a] H.

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