Acetyltrimethylsilane Keto-Enol System. Determination of the Keto-Enol Equilibrium Constant and Acid Dissociation Constants of the Keto and Enol Forms in Aqueous Solution

Article abstract of DOI:10.1021/jo00062a004

The enol isomer of the prototype α-silyl-substituted ketone, acetyltrimethylsilane, was generated by flash photolytic photooxidation of α-(trimethylsilyl)ethanol in aqueous solution and its rates of acid- and base-catalyzed ketonization in that medium were determined.These, in combination with rates of enolization of the ketone determined by iodine scavenging, provided the keto-enol equilibrium constant pK(E)=4.89, the dissociation constant of the enol ionizing as an oxygen acid pK(a)^E=11.54, and the dissociation constant of the ketone ionizing as a carbon acid pK(a)^K=16.44.Comparison of these results with corresponding values for simple alkyl-substituted carbonyl compounds shows that α-silyl substitution raised K_E markedly, lowers K(a)^E modestly, and raised K(a)^K somewhat more strongly.Possible causes of these effects are discussed.