Zur theorie der α-ketosa¨uren: Mechanismus der enolisierung einiger brenztraubensa¨ureamide

Abstract

The mechanism of enolisation of pyruvamide is discussed by the influence of substituents on the kinetic CH 3-acidity, by general base-catalysis of enolisation, by the enthalpy and entropy of activation and primary kinetic and kinetic solvent deuterium isotope effects respectively. A Bro¨nsted coefficient β = 0·71 has been obtained in the general base catalysis of pyruvdiethylamide enolisation. The effect of car☐ylsubstituents on the kinetic CH 3-acidity is produced not only by an inductive mechanism. The importance of solvent structure is demonstrated by a strong negative entropy of activation for the H 2O-catalysed reaction. In the H 2O-catalysed enolisation of pyruvdiethylamide a large kinetic deuterium solvent isotope effect k o H 2O /k o D 2O = 2·39) was obtained at 25°C. In contrast, when hydroxid is the catalyst, the primary kinetic deuterium isotope effect is unusually low (k H/k D = 3·5). Thus, in comparison to other keto compounds, a different mechanism of enolisation for the pyruvic acid derivatives must be postulated. Some aspects of this mechanism are discussed in the paper.