Kinetics and Mechanism of the Epoxidation of Allylic Alcohols by Hydrogen Peroxide with Methyltrioxorhenium as Catalyst.

Abstract

The title reaction was carried out in aqueous acetonitrile (5% and 20% water) and in nitromethane. Provided acid was not added, the product was mostly the epoxide, accompanied by small amounts of the triol that results from acid-catalyzed ring opening of the epoxide. With added acid, only the triol was detected. The kinetic study afforded a striking result: only the diperoxorhenium compound was an epoxidation catalyst; the monoperoxo compound CH(3)Re(O)(2)(eta(2)-O(2)) was inactive; no other substrates, including nonallylic alkenes, show that reactivity difference. This, together with the realization of high diastereoselectivity for 2-cyclohexen-1-ol in chloroform, suggests a particular mode of hydrogen bonding: an O atom of one peroxo group engages in oxygen transfer, in a transition state that is stabilized by hydrogen bonding between the allylic OH group and O atom of the other peroxo group. Calculations supported that conclusion.