Ketonization of (Z)-2-methoxy-1,2-diphenylvinyl alcohol. Quantitative evaluation of the remarkable kinetic stability of this enol

Rates of ketonization of (Z)-2-methoxy-1,2-diphenylvinyl alcohol were measu red in aqueous perchloric acid and sodium hydroxide solutions as well as in formic acid and ammonium ion buffers, and the results were used to constru ct 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 660000 times less than tha t for the enol of acetophenone. Comparison with simple models allows partit ion of this rate factor into a 440-fold retardation for the beta-phenyl sub stituent and a 1500-fold retardation for the beta-methoxy substituent. A gl obal rate of enolization of the keto isomer producing both Z and E enol iso mers was also measured, and this leads to pK(E) > 5.4 as the lower limit of the keto-enol equilibrium constant for the Z enol. This result could be di ssected into an enol-content enhancing effect of delta pK(E) = 3.2 for the beta-phenyl group and a surprising enol-content diminishing effect of delta pK(E) > 0.6 for the beta-methoxy group.