Formation and Hydrolysis of Enamine in Aqueous Solution

A mixture of sodium phenylalanine and ethyl acetoacetate in aqueous solution underwent enamine formation as determined by nuclear magnetic resonance- and ultraviolet-spectroscopic measurements. An equilibrium state was observed in the aqueous solution between the enamine and the starting materials. Although the observed equilibrium constant, Kobs, showed dependency on the pH value of the solution, the equilibrium constant, K, was not influenced by the pH value. The effect of pH values on Kobs was related to the pKa value of phenylalanine, suggesting that only the unionized form of the amino moiety of phenylalanine could take part in the enamine formation.