Acid-Catalyzed Phenylcyclohexene Oxide Hydrolysis:  Role of Para-Phenyl Substituent on Syn:Anti Hydration Ratio

Abstract

Rate and product studies of the hydronium ion-catalyzed hydrolysis of 1-phenylcyclohexene oxide and its p -methoxy and p -methyl derivatives in 1:9 dioxane−water solutions were carried out under conditions in which the diol products are stable. It is shown that the cis:trans diol ratio from hydrolysis of this series of epoxides does not systematically increase with the electron-donating ability of the para-substituent, contrary to conclusions in the literature. The equilibrium constants and rates for acid-catalyzed approach to equilibration of cis - and trans -1-( p -methoxyphenyl)cyclohexane-1,2-diol and of cis - and trans -1-( p -methylphenyl)cyclohexane-1,2-diol were measured, and the cis - diol was determined to be the more stable isomer in each case. An intermediate in the acid-catalyzed hydrolysis of 1-( p -methoxyphenyl)cyclohexene oxide is trapped, after its rate-limiting formation, by azide ion. For a series of 1-arylcyclohexene oxides, results are interpreted in terms of a mechanism in which there is a discrete carbocation intermediate, and products are determined solely by the partitioning reactions of this intermediate, with the pathway leading to the more stable product being energetically favored.