Photochemistry of some cyclopropyl conjugated 1,2-diketones. Part 2. Fragmentation reactions and reduction by aldehydes

Abstract

The photochemistry of some cyclopropyl conjugated and some related 1,2-diketones in benzene as solvent in the absence and presence of an aliphatic aldehyde as hydrogen donor has been studied. In benzene no photodecomposition occurs with λ > 403 nm, but with radiation of lower wavelength (>300 nm) the 1,2-diketones, with the exception of dicyclopropylethanedione, undergo photodecomposition. The products arise from initial bond rupture between the two carbonyl groups, subsequent decarbonylation of part of the acyl radicals (and not of the relatively stable cyclopropylcarbonyl radicals), and eventual combination of the various types of radicals present. Upon irradiation of a mixture of biacetyl and dicyclopropylethanedione, acyl interchange was observed. The photoreaction of 1,2-diketones in the presence of aliphatic aldehydes led to the formation of dihydrodimers (2), αα-diacylalkanols (3), and esters (4). The mechanisms of formation of these products from the semidione radical formed from triplet excited 1,2-diketone by hydrogen abstraction from the aldehyde, are discussed.