Hydrolysis of α-acetoxystyrenes. Kinetics and investigations of 18O exchange

The alkaline hydrolyses of α-acetoxystyrenes 1a-f and 1-acetoxy-1-ethoxy-2-phenylethylene (2) have been shown to proceed by the same mechanism which has been demonstrated for the hydrolysis of alkyl and aryl acetates. Among the pieces of experimental evidence leading to this conclusion are the inverse solvent deuterium isotope effects (0.74 + 0.07 for 1c, 0.80 +- 0.09 for 2), the kinetics of hydrolysis, which are first order in hydroxide ion, and the absence of general base catalysis of hydrolysis. In mildly acidic solution, however, the hydrolysis of 2 proceeds exclusively by a mechanism involving a rate-determining proton transfer to the leaving group double bond, a mechanism which was previously demonstrated for α-acetoxystyrenes in strongly acidic solution. Carbonyl labeled α-acetoxystyrene-18O as synthesized, and 18O exchange from the carbonyl position during alkaline hydrolysis was investigated; no 18O exchange was observed. This behavior is similar to that observed for aryl esters, and contrasts with that observed for hydrolysis of esters with less acidic leaving groups. These observations support our contention that acetophenone enols are about as acidic as phenols, a conclusion which, along with the fraction enol in acetophenone, leads to a carbon pK/sub a/ for acetophenone of 15.8 +- 1.0