Oxidations of Individual OH Groups

1 By Valence Change of an Oxyacid Ester or Related Species. Many of the two-electron oxidations of alcohols of use to preparative chemists involve formation of an ester-type intermediate of general formula CH 0-X , followed by formation of the carbonyl group in what is essentially an El 

In strongly acidic solution there is evidence for a slow, direct reaction of a neutral species, possibly the acyclic gum-diol. 

Oxidations by dimethyl sulfoxide and an electrophile involve the formation of a sulfoxonium ion, which then collapses to a carbonyl group. The 

The well-known oxidations of primary and secondary alcohols with Cr species proceed through chromate esters. The definitive mechanistic expeii-ments demonstrating that previously observed chromate esters were indeed on the reaction pathway showed that either formation or decomposition of the ester could be rate-determining. The rates of oxidation of cyclohexanol and the secondary hydroxyl group of a very steiically hindered steroid in aqueous acetic acid were measured as a function of the solvent composition. The former increased radically as the acetic add concentration increased whereas the latter remained invariant. The former exhibited a primary deuterium kinetic isotope effect of 5, whereas there was no KIE on the oxidation of the crowded steroid. Therefore, the rate-determining step in the oxidation of the cyclohexanol was decomposition of the chromate ester and in the oxidation of the steroid it was its formation, with the ester an obligate intermediate in both reactions. 

The details of hydrogen transfer from carbon are unclear. Analogy with other reactions of this type, particularly those of TEMPO, would suggest a cyclic transition state, characterised by primary hydrogen effects of 2, under basic conditions and a general base-catalysed process under acidic conditions, characterised by isotope effects around 5. However, the cyclic mechanism was also proposed for the standard oxidation conditions of aqueous acetic acid, originally on the basis of a rate decrease in aqueous acetic acid as sodium acetate was added, despite such an addition possibly deprotonating the 

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