Keto–enol tautomerism in β-ketoesters: CH3C(O)CHXC(O)OY (X = H, Cl; Y = CH3, C2H5). Vibrational analyses, NMR spectra and quantum chemical calculations

The tautomeric equilibria of the β-ketoesters [CH3C(O)CH2C(O)OCH3 (I), CH3C(O)CHClC(O)OCH3 (II), CH3C(O)CH2C(O)OCH2CH3 (III) and CH3C(O)CHClC(O)OCH2CH3 (IV)] were studied by NMR spectroscopy and, in the case of I and II, by quantum chemical calculations (ab initio and density functional methods). In addition, liquid-state Fourier transform infrared and Raman spectra were analysed for all four compounds. They revealed the existence of two tautomers, diketo and enol forms. In the NMR spectra only signals for the diketo form could be observed for I and III. However, the calculations for I and II predicted the enol structures to be the most stable species. Copyright © 2001 John Wiley & Sons, Ltd.