The Mechanism of C-H Bond Oxidation by Aqueous Permanganate

The permanganate ion (MnO4-) has been widely used as a reagent for water treatment for over a century. It is a strong enough oxidant to activate carbon hydrogen bonds, one of the most important reactions in biological and chemical systems. Our current textbook understanding of the oxidation mechanism in aqueous solution involves an initial, rate-limiting hydride abstraction by permanganate followed by reaction of the carbocation with bulk water to form an alcohol. This mechanism fits well into the classic oxidation sequence of alkane -> alcohol -> aldehyde -> carboxylate, the central paradigm for both abiotic and biotic alkane oxidation in aqueous environments. In this study, we provide three lines of evidence through (1) a broken symmetry density functional theory approach, (2) isotope labeling experiments, and (3) kinetic network modeling to demonstrate that aqueous permanganate can circumvent prior alcohol formation and produce aldehydes directly via a reaction path that bifurcates after the initial transition state. In contrast to classic transition state theory, the rate-limiting step is found to not determine product distribution, bearing critical implications for pathway and rate predictions. This complex reaction network provides new insights into the oxidation mechanisms of organic compounds involving transition metal complexes as well as enzyme or metal oxide surface active sites.