Photodecomposition of acetoins

Abstract

The photodecomposition of α-substituted acetoins (la–e), both by direct irradiation and by α-diketone sensitization, has been studied. Direct irradiation at 313 nm gives acetyl derivatives (2a–e) in good yield. The reactions are not quenched by triplet quenchers. Quantum yields for the formation of 2-acetyl-1-methylpylpyrrole (2a) are low relative to those for acetophenone (2e) and markedly decrease as solvent polarity increases. α-Diketones such as biacetyl and benzil photosensitize the decomposition of acetoins (1a–d) to (2a–d), and are thereby converted into reduction products. The decomposition of α-phenylacetoin (1e) is not sensitized by the α-diketones. The sensitized decomposition of acetoins (1a–d) can be completely quenched by triplet quenchers. Quantum yields for disapperance of biacetyl in the reaction with (1a) and rate constants for biacetyl phosphorescence quenching by (1a) are both enhanced on going from benzene to acetonitrile as solvent. Mechanisms are discussed on the basis of kinetic and deuterium-labelling experiments.