¹H-NMR Study on H-D Exchange in N, N-Dimethylacetoacetamide Coupled with Keto-Enol Tautomerism in Methanol-d

Abstract

Keto-enol tautomerism and solute-solvent H-D exchange of N, N-dimethylacetoacetamide were followed by the change in H-NMR spectra after dissolving it into methanol-d (8.3% (v/v)). The molar ratio of the keto form passed through a minimum before it approached a final equilibrium value, while the degree of deuterium substitution at the methylene group followed an ordinary first order reaction kinetics. A general reaction scheme (Scheme 2), which includes both keto-enol tautomerism and H-D exchange, was simplified by the use of the experimental conditions so that only five intermediate species were involved (Scheme 3). The experimental data were reasonably reproduced by the theoretical calculations (Eqs.13 and 14) based on the simplified reaction scheme (Figs.2 and 3). The keto-enol tautomerism of N, N-dimethylacetoacetamide in methanol was thus concluded to proceed via solute-solvent proton exchange reaction.