Kinetics of oxidation of epimeric piperidin-4-ols by vanadium(V)

Abstract

The kinetics of oxidation of six epimeric pairs of piperidin-4-ols by vanadium(V) in the presence of sulphuric and perchloric acids in aqueous acetic acid have been investigated. The corresponding deuteriated piperidin-4-ols have been prepared in most cases and their rates of oxidation by V^V have been measured. The reaction is first order in both oxidant and substrate at constant acid concentration. The order with respect to [H₃O⁺] is found to be unity at constant ionic strength in perchloric acid. The primary kinetic isotope effect is suggestive of the involvement of the C–H or C–D bond of the hydroxy-bearing carbon in the rate-determining step of the oxidation. A mechanism involving a free radical intermediate has been proposed. However, a different rate-determining step is suggested for c-2,c-6-diphenyl-t-3,t-5,N-trimethylpiperidin-r-4-ol which has no isotope effect. The reactivities of piperidin-4-ols towards oxidation have been discussed on the basis of conformational differences. Activation parameters have also been determined.