Kinetics and mechanism of the oxidation of alcohols by ferrate ion

A kinetic study of the reduction of ferrate ion under basic conditions has been completed. The observation that a typical aliphatic ether, tetrahydrofuran, is oxidized at a rate comparable to that of aliphatic alcohols, such as cyclopentanol, indicates that the reaction between ferrate and alcohols is likely initiated by attack of the oxidant at an α-C—H bond, a conclusion that is consistent with the occurrence of primary deuterium kinetic isotope effects (2.8–4.3 at 25 °C) when α-hydrogens are replaced by deuterium. Because only acyclic products are obtained from the oxidation of cyclobutanol by ferrate, it may be concluded that free radical intermediates are involved in the reaction. The insensitivity of the reaction rates to substituent effects during the oxidation of substituted mandelic acids indicates that substantial charges are not built up in the transition state. All of these observations are most readily accommodated by a mechanism in which the reaction is initiated by a 2 + 2 addition of an Fe=O bond to the α-C—H of an alcohol to give an organometallic intermediate that subsequently decomposes by homolytic cleavage of the resulting C—Fe bond. Comparisons are made with the reactions between alcohols and other high-valent transition metal oxides.