Pyruvic acid, hydrogen abstraction

The mechanism of this reaction is obscure. One suggested mechanism, analogous to the vapor phase reaction, involves concerted decarboxylation of the pyruvic acid to yield a triplet hydroxy carbene which can either dimerize or attack another molecule of pyruvic acid to yield the observed product.91 Dimerization seems to be the less likely process since the carbene can rearrange to acetaldehyde or react with water. Further, this mechanism predicts that acetoin will be formed when pyruvic acid is irradiated in any solvent that does not possess readily abstractable hydrogen atoms, such as benzene, a solvent in which no reaction is observed. One possible explanation of this discrepancy is that the solvation of the pyruvic acid is extremely different in benzene and in water. However, the specific role that the water plays in the reaction has not been determined. 

Rate constants for reaction of triplet ( , ) biacetyl with the same hydrogen donors are also included, being generally two or three orders of magnitude less. CIDNP studies on the photoreduction of pyruvic acid by propan-2-ol show the occurrence of abstraction [reaction (12)], followed by the transfer of a hydrogen atom from the polarized ketyl radical to ground-state pyruvic acid [reaction (13)],23 exactly as for the corresponding photoreduction of benzophenone. 

The formation of acetaldehyde in the reaction shown in Eq. (8.15) involves oxidation of the CH3CHOH radical, which is formed by decarboxylation of the lactate anion. By contrast, the formation of pyruvic acid in the reaction shown in Eq. (8.16) involves hydrogen abstraction from CH3CH(0H)C02H by the excited state of the uranyl ion to give the CH3C(0H)C02H radical. Subsequent oxidation of this radical leads to the formation of pyruvic acid. These pathways are supported by EPR measurements at 77... 

The intramolecular abstraction of a hydrogen atom from the y-carbon of a pho-toexcited carbonyl group is called the Norrish II process (Scheme 31). This reaction has been applied to the oxidation of the anomeric carbon by irradiation of 2-oxo propyl glycoside 187 [118] or glycosyl pyruvic esters 188 [119], Binkley [119] similarly prepared the kojic acid diacetate 189 by photochemically induced oxidation of the glucosyl pyruvic ester 190. 

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