Acid-Catalyzed Hydrolysis of a Phenylketene Dithioacetal. Reversible Protonation of the Carbon-Carbon Double Bond

Article abstract of DOI:10.1021/ja00351a044

Reactions of 1,1-bis(methylthio)-2-phenylethene (1) were followed in acidic CD3CN-D2O (50-90 vol percent) solutions by 1H NMR spectroscopy.The H-D isotope exchange at the 2-position (to give 2-deuterio derivative 1-d) occurs concurrently with hydrolysis, which gives S-methyl phenylthioacetate.Relative rates of the isotope exchange to the hydrolysis increase from 1.0 to 13.4 on going from 50 to 90 vol percent content of CD3CN.Rates of the hydrolysis were also measured by UV spectroscopy in 10 vol percent CH3CN-H2O (D2O) at 30 deg C.Pseudo-first-order plots for the reaction of 1 in CH3CN-D2O curved as expected from the concurrent isotope exchange, the relative rate of exchange to hydrolysis being 2.1 in this medium.The deuterated substrate 1-d is about 1.4 times as reactive as 1; this large apparent secondary kinetic isotope effect is ascribed mainly to the reduced reversibility of protonation of 1-d rather than to true secondary effects.The hydrolysis rate increases rapidly with increasing buffer concentration to reach a constant maximum value at about 30percent increase.Added 2-mercaptoethanol accelerates the disappearance of 1.All the results are accommodated by a mechanism involving a largely reversible protonation of the double bond and a mostly rate-determining hydration of the carbocation intermediate.