Substituent effects, permanganate

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. 

Substituent effects. As the results summarized in Figure la indicate, changing substituents has an almost negligible effect on the rate of oxidation of unsaturated compounds by aqueous permanganate. 

Freeman chose substrates D-I to study the influence of steric bulk on the free energy of activation for the permanganate dihydroxylation. It is difficult to separate these substituent effects on the reaction rate, because it is not clear how to determine the relative contributions of steric and electronic effects to the overall observed effect. However, some comparisons are useful. For example, in the case of trans-crotonic acid D and 4,4-dimethyl-fra/rv-2-pentenoic acid I the large bulk of the t-butyl group compared to that of a methyl group should result in a dominance of the steric over the electronic substituent effect. 

Although permanganate ions are not generally used to effect oxidative coupling of phenols, it has been shown that, in the presence of a catalyst in an organic solvent, 2-methoxyphenols are coupled oxidatively under very mild conditions to produce the dimeric products (>50%) [48], Unsaturated substituents are not oxidized under the mild conditions. 

Permanganate oxidation of alkenes. Substituent and solvent effects. Difficulties with MP2 calculations ... 

The steric effects involved in the cis-hydroxylation of 5,6-dihydro-2H-pyrans substituted at positions 6 and/or 2 with aqueous potassium permanganate were studied by Mochalin and co-workers. It was found that the reaction course is dependent on the steric hindrance exerted by the bulky substituent at C-2 both OH groups enter preferentially trans with respect to that substituent. In consequence, products of (x-lyxo or -erythro configuration are obtained in the case of di- or mono-substituted substrates. However, in the case of 5,6-dihydro-2-methoxy-2H-pyran, both possible stereoisomeric products, methyl 4-deoxy-a-and p-DL-eo t o-pentopyranosides, were formed in approximately equal amounts. 

Oxygen Nucleophiles. A reagent such as permanganate oxidizes toluene to benzoic acid, whereas benzylic oxidation by palladium acetate results in benzyl alcohol derivatives. The oxidation is favored by electron-releasing substituents in the phenyl ring. Catalytic amounts of palladium acetate and tin diacetate, in combination with air, effects an efficient palladium-catalyzed benzylic oxidation of toluene and xylenes. For the latter substrates, the Q, Q -diacetate is the main product.A mixed palladium diacetate-copper diacetate catalyst has also been found to selectively catalyze the benzylic acyloxylation of toluene (eq 64). ... 

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