Three-Electron Oxidations. 17. The Chromium(VI) and Chromium(V) Steps in the Chromic Acid Cooxidation of 2-Hydroxy-2-methylbutyric Acid and 2-Propanol

Article abstract of DOI:10.1021/ja00531a022

The chromic acid cooxidation of 2-hydroxy-2-methylbutyric acid (HMBA) and 2-propanol is a two-stage reaction.The stoichiometry of the first stage is HMBA + i-PrOH + 2Cr(VI) -> CH3CH2COCH3 + CO2 + CH3COCH3 + Cr(V) + Cr(III); that of the slower second stage is HMBA + Cr(V) -> CH3CH2COCH3 + CO2 + Cr(III).Kinetic, product, and isotope-effect studies of the first stage are consistent with a three-electron mechanism in which a termolecular complex formed of HMBA, i-PrOH, and HCrO4^- decomposes to acetone, carbon dioxide, a free radical, and Cr(III).Depending on the reaction conditions, either the decomposition or the formation of the termolecular complex may be rate limiting.The change in rate-limiting step results in a strong acidity dependence of the deuterium isotope effect in the cooxidation of 2-deuterio-2-propanol.The chromium(V) oxidation appears to proceed through a 3:1 complex of 2-hydroxy-2-methylbutyric acid and chromium(V) and results in the exclusive oxidation of the hydroxy acid even in a 20-fold excess of 2-propanol; in contrast to chromium(VI), chromium(V) thus show a remarkable selectivity toward hydroxy acids.