Abstract
Rates of ketonization of (Z)-2-methoxy-1,2-diphenylvinyl alcohol were measured in aqueous perchloric acid and sodium hydroxide solutions as well as in formic acid and ammonium ion buffers, and the results were used to construct a rate profile for this reaction. These data show this substance to be a remarkably stable enol with a lifetime of 3.6 h at the bottom of its rate profile and a hydrogen ion catalytic coefficient 660 000 times less than that for the enol of acetophenone. Comparison with simple models allows partition of this rate factor into a 440-fold retardation for the β-phenyl substituent and a 1500-fold retardation for the β-methoxy substituent. A global rate of enolization of the keto isomer producing both Z and E enol isomers was also measured, and this leads to pKE > 5.4 as the lower limit of the keto-enol equilibrium constant for the Z enol. This result could be dissected into an enol-content enhancing effect of δ pKE = 3.2 for the β-phenyl group and a surprising enol-content diminishing effect of δ pKE > 0.6 for the β-methoxy group. Key words: enol ketonization, enol stability, keto-enol equilibrium, β-phenyl and β-methoxy substituent effects in keto-enol systems.
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