O—H bond, cleavage

Both schemes accommodate the kinetics, the primary isotope effect and the induction factor, which indicates that Cr(IV) is the initial stage of reduction of the oxidant. RoCek s mechanism does not accord with the solvent isotope effect of 2.5 per proton, which has just the value to be expected for acid-catalysis, for the O-H bond cleavage should be subject to a primary isotope effect of about 7. The ester mechanism is not open to this criticim. 

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. 

The most important reactions taking place on the hydroxy groups of alcohols are 0-H bond cleavage and C-O bond cleavage. With the O-H bond cleavage, reactions with strong acids proceed, as do oxidations of primary alcohols to aldehydes, secondary alcohols to ketones and reactions with organic acids (formation of esters). In foods the last three reactions are particularly important, and are usually enzymatically catalysed. Other important reactions are dehydration and the opposite reaction, hydration, which yield unsaturated hydrocarbons from alcohols and isomeric alcohols from unsaturated hydrocarbons, respectively. These reactions are particularly important in terpenic alcohols. In oleochemistry, oxidation and esterification reactions are used for the production of various lipid derivatives. 

Zhang ZR, Bondarchuk O, White JM (2006) Imaging adsorbate O—H bond cleavage methanol on TiO2(110). J Am Chem Soc 128 4198 199... 

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