MECHANISTIC STUDY ON THE PHOTO-OXIDATION OF α-DIKETONES. INTERACTION OF TRIPLET α-DIKETONES WITH OXYGEN

Article abstract of DOI:10.1016/S0040-4020(01)96593-7

The mechanism for photo-oxidation of α-diketones in the presence of olefins has been studied, focusing on the interaction of triplet diketones with O2.Two types of reactions occur competitively.One is the formation of ¹O2 by energy transfer to O2 and the other is the addition of O2 to triplet diketone, which yields acylperoxy radicals leading to the radical epoxidation of olefins.Ratios of the two reactions were determined from the yields of ¹O2 products and epoxides.For most diketones, quantum yields for ¹O2 formation were considerably high, in the range 0.3-0.8; but the yields for epoxides were in a wider range of 0.001-0.5.While the ratios of ¹O2 formation and O2 addition to triplet diketones ranged from 29:71 for biacetyl to 1:99 for mesitil, the ratios remained constant by changing solvents or temperature.The latter O2 addition reaction decreased in the order of MeCOCOMe > PhCOCOMe > PhCOCOPh but the effect of m- and p-substituents on benzils was not significant, indicating the addition of O2 as a neutral biradical.In exceptional cases, the O2 addition was not effective for sterically inaccessible mesitil and some cyclic α-diketones.The mechanism of ¹O2 formation was discussed in comparison to other carbonyl compounds.