The mechanism of oxidation of cyclohexanone under acid conditions. Part 4. Primary and solvent isotope effects on oxidations of ketones and enols: an indirect determination of the relative enol contents of isotopically substituted ketones in water and deuterium oxide, and of the solvent isotope effects for the electron transfer processes

Abstract

Two non-bonded electron transfer oxidants, trisbipyridylruthenium(III) and hexachloroiridium(IV), are known to abstract an electron from the enol of cyclohexanone, and can be used under conditions where the oxidation step is rate limiting. The solvent and substrate deuterium isotope effects have been studied under these conditions, and they provide evidence which can be used to distinguish between attack on enol and attack directly on the ketone even under circumstances where direct kinetic evidence cannot be used to make this distinction. In these, and similar systems, the apparent existence of a primary isotope effect can be highly misleading if the reagents are not in isotopic equilibrium, since the isotope effects in the enolisation process change the enol content of the solution by a factor of up to 7. The electron transfer step is found to be retarded by deuterium oxide by a factor of ca. 1.9.