Reaction mechanisms, permanganate oxidations

A subsequent detailed analysis of the permanganate oxidation of the tertiary hydrogen atom of 4-phenylvaleric acid in 2.5 M potassium hydroxide solution supports the caged radical mechanism. The reaction order is two overall, A h/ d is ca. 11.5, ring substitution has little elfect on the rate (p 0) and the oxidation proceeds with a net 30-40 % retention of optical configuration. 

The mechanisms of permanganate oxidations have been the subject of a fairly intensive study which has now lasted for almost a century. While many of these studies were carried out in aqueous solutions, much of what was learned is also germane to an understanding of the reactions which occur in phase transfer assisted reactions. Although most of these studies are interrelated they can conveniently be discussed under the following headings products, substituent effects, isotope effects, and solvent effects, with the latter being of particular importance to the phase transfer assisted reactions. 

Additional examples of the oxidative cleavage of double bonds by permanganate to produce aldehydes and ketones are summarized in Table 4. A detailed study of the reaction mechanism has also been reported. ... 

The oxidation of thiols follows a completely different course as compared with the oxidation of alcohols, because the capacity of the sulfur atom to form hypervalent compounds allows it to become the site of oxidation. Thiols are readily oxidised to disulfides by mild oxidants such as atmospheric oxygen, halogens or iron(III) salts (Scheme 6). This type of reaction is unique to thiols and is not undergone by alcohols, it is a consequence of the lower bond strength of the S-H as compared with the O-H bond, so that thiols are oxidised at the weaker S-H bonds, whereas alcohols are preferentially oxidised at the weaker C-H bonds (Scheme 7). The mechanism of oxidation of thiols may be either radical or polar or both (Scheme 6). The polar mechanism probably involves transient sulfenic acid intermediates like (7) and (8). In contrast, thiols react with more powerful oxidants, like potassium permanganate, concentrated nitric acid or hydrogen peroxide, to yield the corresponding sulfonic acids (10). This oxidation probably proceeds via the relatively unstable sulfenic (7) and sulfinic acids (9), which are too susceptible to further oxidation to be isolated (Scheme 8). 

In this section, brief fundamental reaction mechanisms for each AOP are addressed. Included as AOPs are individual and combinational processes in the use of ultraviolet (UV) irradiation, catalyzed titanium dioxide oxidation, Fenton s reagent oxidation, ozonation, peroxone oxidation, and permanganate oxidation. 

Very little information is available about the species which react in the further decomposition of ascorbic add. Apparently, both dehydroascorbic acid and 2,3-diketogulonic acid can be oxidized directly, since both oxalylthreonic acid and the same two free acids have been identified. The former would result from oxidative fission of the chain while the lactone ring was intact. Even less is known of the oxidation mechanism. The oxidation occurs with iodine and with acid permanganate (H12) and also with oxygen or peroxide (R23). Recent studies on the similar decomposition of the enols of aryl pyruvates to aryl aldehydes plus oxalic acid identified these reactions as examples of the direct attack... 

Unsaturated halogenated compounds with a central C = C or C = C bond undergo cleavage on oxidation in the middle of the molecule and form two carboxylic acids without loss of a carbon atom. The reaction mechanism of the oxidation of internal fluoroalkenes by potassium permanganate includes hydroxylation of the C = C bond in 2 by permanganate, accompanied by dehydrofluorination of the geminal fluoroalkane-1,2-diol 3 and formation of an intermediate fluorocarbonyl compound 4 oxidative cleavage of the C-C bond in 4 leads to the final products 5.1... 

The solutes N20, H202, nitrate, nitrite, bromate, iodate, and permanganate oxidize transient univalent metal ions (19, 22, 32). With little doubt, the mechanisms involve electron transfer, although in some cases precise details of the reactions are lacking. 

---