ACID-CATALYZED HYDROLYSIS OF CIS-ANETHOLE AND TRANS-ANETHOLE OXIDES -DISCRETE CARBOCATION INTERMEDIATES AND SYN ANTI HYDRATION RATIOS

Rate and product studies of the hydronium ion-catalyzed hydrolysis rea ctions of trans-anethole oxide (12b) and its geometric isomer, cis-ane thole oxide (13b), were carried out. Acid-catalyzed hydrolysis of tran s-anethole oxide is 50 times faster than that of its cis isomer and th is difference in reactivity is attributed to steric interactions betwe en the cis-beta-CH3 and the aryl group in the transition state for hyd rolysis of cis-anethole oxide that are not present in the transition s tate for the acid-catalyzed hydrolysis of trans-anethole oxide. Carboc ation 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 a nd 13b, and identical azide product mixtures from their reactions in s olutions 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 nucleophi les. Molecular modeling calculations suggest that there are three mini mum energy conformations of this carbocation intermediate. Results are interpreted in terms of a mechanism in which rotation about the C(alp ha)-C(beta) bond of the intermediate is rapid relative to the rate at which it reacts with solvent or other nucleophiles. Mechanisms involvi ng concerted addition of solvent are ruled out.