The Region of Mechanistic Transition in Acid-Catalyzed Epoxide Ring Opening. A Mechanistic Switch Mediated by Salt in Aqueous Media

Article abstract of DOI:10.1021/ja00536a032

The mechanism of the acid-catalyzed ring opening of tetramethylethylene oxide has been explored through 1H NMR and UV spectrophotometric product analysis in aqueous media.In dilute aqueous media below pH 4 the hydrolysis proceeds to the extent of 99.92percent forming pinacol; no detectable amount of pinacolone is formed from a possible rearrangement pathway.With increasing amounts of NaClO4 at constant acid concentration there is a dramatic change in the products; rearrangement products appear even at 2.0 M aqueous NaClO4, and rise to the extent of 10percent at 6.0 M NaClO4.Formate buffers in these salt solutions lead to the formation of pinacol monoformate ester concurrent with pinacolone and pinacol; however, limitations in the solubility of the epoxide substrate prohibited rate measurements.The dramatic product change arising from a novel salt effect in aqueous media reveals a mechanistic pathway switch for this acid-catalyzed epoxide ring opening.The initiating step in the mechanism with or without salt is the same, the formation of the protonated epoxide as an intermediate.Nucleophile-assisted opening of the intermediate by water or formate buffer uniquely excludes products derived from skeletal rearrangement.However, high salt concentrations can provide the necessary electrostatic stabilization required for charged intermediates as well as reduce the probability of nucleophilic attack by water.The switch to a carbonium ion intermediate (a species isomeric with the protonated epoxide) is signaled by pinacolone formation.