ACETYLTRIMETHYLSILANE KETO ENOL SYSTEM - DETERMINATION OF THE KETO-ENOL EQUILIBRIUM-CONSTANT AND ACID DISSOCIATION-CONSTANTS OF THE KETO AND ENOL FORMS IN AQUEOUS-SOLUTION

The enol isomer of the prototype alpha-silyl-substituted ketone, acetyltrimethylsilane, was generated by flash photolytic photooxidation of alpha-(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 alpha-silyl substitution raises K(E) markedly, lowers K(a)E modestly, and raises K(a)K somewhat more strongly. Possible causes of these effects are discussed.