REVERSIBLE CARBON PROTONATION IN THE HYDROLYSIS OF HETEROCYCLIC ENOL METHYL ETHERS

Article abstract of DOI:10.1016/S0040-4020(01)89931-2

The kinetics of the hydronium-ion catalysed hydrolysis of the following heterocyclic methyl enol ethers have been measured: 3-methoxybenzofuran, 3-methoxybenzothiophene, 3-methoxyindole, 3-methoxy-1-methylindole, 3-methoxyfuran, 3-methoxythiophene, and 2-methoxythiophene.On the basis of the solvent isotope effect k_H/k_D = 3.08 and the failure to detect deuterium exchange when the solvent was CD3CN:D2O (9:1 v/v) it was concluded that the rate limiting step in the hydrolysis of 3-methoxybenzofuran is C-protonation.The effect of the ring-oxygen atom was measured by comparing the rate of hydrolysis of 3-methoxybenzofuran with that of 3-methoxyindene which occurs 2100 times faster.In contrast to the behaviour of 3-methoxybenzofuran the isotope effects, k_H/k_D, for the hydrolyses of 3-methoxyfuran, 3-methoxythiophene, 3-methoxyindole, 3-methoxy-1-methylindole and 2-methoxythiophene are ca 0.4 - 0.5 and deuterium exchange is much faster than hydrolysis when the solvent is CD3CN:D2O (9:1 v/v).It was therefore concluded that with these compound C-protonation is rapid and reversible and that slow step is attack by water on the intermediate cation.