Photo-oxidation of alpha-keto-carboxylic acids and related compounds
Basic photochemistry, the photoreactions of carbonyl compounds and the mechanisms involved in photo-oxygenation reactions are surveyed. The photochemical reactions of alpha-keto-acids and esters are reviewed. Fran photophysical and decarboxylation studies of alpha-keto-acids and esters in degassed solution, the direct photo induced decarboxylation reaction is shown to occur via a bimolecular electron transfer reaction as opposed to a Norrish Type I fragmentation. The addition of triplet quenchers enhances the decarboxylation reaction via a mechanism suggested to involve radical processes. The proposed mechanisms are substantiated by using electron acceptors to sensitize decarboxylation. The finding that t-butyl pyruvate is photostable in degassed solution and has a much longer lifetime than other alkyl pyruvates, together with triplet quenching studies, shows that alkyl pyruvates fragment from the triplet state via Norrish Type II process. Fran photophysical, quenching and product studies in degassed solution, the Norrish Type II reaction of long chain alpha-keto-acids and esters occurs from the excited singlet state. alpha-Keto-acids and esters undergo direct photo-oxidative decarboxylation, leading to higher yields of carbon dioxide than under degassed conditions. Decarboxylation of pyruvic acid occurs from both the C-1 and C-2 positions. Singlet oxygen does not appear to be involved and a mechanism mediated via electron transfer, with per-acid intermediates, is postulated. In accord with the proposed mechanism, the decarboxylation can be sensitized by electron acceptors. The dye-sensitized photo-oxygenation of alpha-keto-acids and esters proceeds via a mechanism involving dye-acid/ester interaction rather than a singlet oxygen mediated process. Kinetic studies and solvent isotope effects lend credence to the proposed mechanism. The reported results confirm that photochemical alpha-cleavage of the bond linking the carbonyl groups in 1, 2-dicarbonyl compounds is an unfavourable process.