Three-Electron Oxidations. 18. Carbon-13 and Deuterium Isotope Effects in the Cooxidation of 2-Hydroxy-2-methylbutyric Acid and 2-propanol. Evidence for a Two-Step Mechanism

Article abstract of DOI:10.1021/ja00407a036

The mechanism of the chromic acid cooxidation of 2-hydroxy-2-methylbutyric acid (HMBA) and 2-propanol (ROH) was investigated by using deuterium and carbon-13 isotope effects.The carbon isotope effect associated with the oxidative decarboxylation of HMBA was determined from the mass ratio 44:45 (12CO2:13CO2), the deuterium isotope effect from kinetic measurements.When the acidity is increased from = 0.02 or 0.05 M to = 1.86 M, the deuterium isotope effect, k_H/k_D, increases from 1.1 to 5.7 while the carbon isotope effect, (12)k/(13)k, decreases from 1.027 to 1.0048.The magnitude of the carbon-13 isotope effect is dependent on the concentration of HMBA.The change in deuterium isotope effect indicates that the rate-limiting step changes from formation of a Cr(VI)-HMBA-ROH complex at low acidities to its decomposition into oxidation products at high acidities.This interpretation is buttressed by the observation that the less readily oxidized and more rapidly esterified methanol exhibits a large deuterium isotope effect throughout the entire acidity range.The existence of a significant carbon isotope effect at low acidities, where complex formation is rate limiting, strongly suggests that the oxidative decomposition of the Cr(VI)-HMBA-ROH complex take place in two steps.The first step results in the oxidation of the alcohol and formation of a Cr(IV)-HMBA intermediate; this intermediate must be sufficiently long lived to equilibrate with free HMBA before undergoing oxidative decomposition in the second step.The overall three-electron reduction of chromium(VI) by HMBA and ROH thus consists of a two-electron oxidation of the carbon-hydrogen bond of the alcohol followed by a one-electron oxidation of the carbon-carbon bond of the hydroxy acid.The observation of a carbon-13 isotope effect in the chromium(V) oxidation of HMBA is consistent with a mechanism of reversible complex formation between Cr(V) and HMBA and rate-limiting oxidation.In the cooxidation of oxalic acid (OX) and 2-propanol significant deuterium and carbon-13 isotope effects were observed throughout the entire acidity range, leaving open the possibility that in this case the oxidative decomposition of the complex Cr(VI)-OX-ROH may take place in a single three-electron transfer step.