Chemoselectivity concentrations, effect

Baiker and co-workers [31, 32] investigated the role of phase behavior in the interpretation of the chemoselective oxidation of octyl alcohols in CO2 and the enantioselective hydrogenation of ethyl pyruvate in supercritical ethane. They found that the effects of temperature and pressure and the concentration of the reaction gases (H2 or O2) had a large impact upon the selectivities. Only through careful consideration of the number of phases and their behavior through different conversion levels cotxld the results be properly interpreted. 

The first approach consists of those systems that utilize molecular hydrogen as the reducing agent. The reaction conditions, such as solvent, acidity/basicity, catalyst type and concentration, hydrogen pressure, and stirring rate have a great effect on the efficiency, stereochemistry, and chemoselectivity of these hydrogenation reactions. 

The authors noticed no C-H/C-D isotope effect for the reaction of 13 with methanol and ferf-butanol, but saw a KIE k Jk = 1.4) for the O-H/O-D bond, suggesting that the stronger O-H bond is activated preferentially over the weaker C-H bonds (Pig. 12). In addition, the authors observed the formation of acetone upon the oxidation of tert-butanol. Upon comparison of rate constants (which have been normalized to account for the amount of hydrogens available for abstraction), tert-butanol reacts 50 times faster than cyclohexane. The authors propose a proton-coupled electron transfer event is responsible for the observed selectivity this complex represents a rare case in which O-H bonds may be homolyzed preferentially to C—H bonds. In further study, 13 was shown to oxidize water to the hydroxyl radical by PCET [95]. Under pseudo-first-order conditions, conversion of 13 to its one-electron reduced state was found to have a second-order dependence on the concentration of water, in stark contrast to the first-order dependence observed for aUphatic hydrocarbons and alcohols. Based on the theimoneutral oxidation of water (2.13 V v. NHE in MeCN under neutral conditions [96]) by 13 (2.14 V V. NHE in MeCN under neutral conditions) and the rate dependence, the authors propose a proton-coupled electron transfer event in which water serves as a base. While the mechanism for O-H bond cleavage of alcohols and water is not well understood in these instances, the capacity to cleave a stronger O-H bond in the presence of much weaker C-H bonds is a tremendous advance in metal-oxo chemistry and represents an exciting avenue for chemoselective substrate activation. 

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