Acid-Catalyzed Hydrolysis of cis- and trans-Anethole Oxides: Discrete Carbocation Intermediates and Syn/Anti Hydration Ratios

Article abstract of DOI:10.1021/jo00062a006

Rate and product studies of the hydronium ion-catalyzed hydrolysis reactions of trans-anethole oxide (12b) and its geometric isomer, cis-anethole oxide (13b), were carried out.Acid-catalyzed hydrolysis of trans-anethole oxide is 50 times faster than that of its cis isomer and this difference in reactivity is attributed to steric interactions between the cis-β-CH3 and the aryl group in the transition state for hydrolysis of cis-anetole oxide that are not present in the transition state for the acid-catalyzed hydrolysis of trans-anethole oxide.Carbocation intermediates in the hydrolysis of both 12b and 13b are trapped, subsequent to their rate-limiting formation, by azide ion.Identical diol product mixtures from the acid-catalyzed hydrolysis of both 12b and 13b, and identical azide product mixtures from their reactions in solutions at low pH containing sodium azide, suggest that both 12b and 13b react to form a common discrete carbocation intermediate and that products are derived from reaction of this intermediate with nucleophiles.Molecular modeling calculations suggest that there are three minimum energy conformations of this carbocation intermediate.Results are interpreted in terms of a mechanism in which rotation about the C_α-C_β bond of the intermediate is rapid relative to the rate at which it reacts with solvent or other nucleophiles.Mechanisms involving concerted addition of solvent are ruled out.