Reaction of singlet oxygen with enolic tautomers of 1-aryl-2-methyl 1,3-diketones

The Effect of Solvent Polarity on the Product Distribution in the Reaction of Singlet Oxygen with Enolic Tautomers of 1-(2‘,4‘,6‘-Trialkylphenyl)-2-methyl 1,3-Diketones

“…The proportion of 6 increased with decreasing proportion of the sum yield of the enedione 4 and the epoxy ketone 5 as the solvent polarity increased. These products were fully characterized as described previously . The 1 H NMR spectrum of 4 showed two singlets at δ 5.8−5.9 and 6.4−6.6 due to two olefinic protons and that of 5 showed methylene protons of the epoxy ring at δ 2.9−3.2.…”

“…The methine proton of isopropyl group appeared at δ 2.1−2.2, fairly high field compared to that of the isopropyl ketone. The 1 H NMR spectra of 4c , 5c , and 6c showed the methine proton of the isopropyl ketone group at δ 3.26, 3.02, and 4.38, respectively . On the basis of the NMR data together with chemical behavior, the structure of the initially formed product may be assigned as 9c .…”

“…We have recently reported that the reaction of singlet oxygen with enolized 1,3-diketones 3a − d using Methylene Blue (MB) as the sensitizer at room temperature in acetonitrile gave the enedione 4a − d , the epoxy ketone 5a − d , and the hydroperoxy ketone 6a − d . In the photooxygenation of 3b − d in methanol under the same conditions, the initially formed enedione 4b − d reacted with methanol to give the corresponding Michael adducts 7 (Scheme ) . In that paper, we proposed that the enedione 4 and the epoxy ketone 5 would arise from the initially formed hydroxyperoxide 8 (Scheme ).…”

“…In the photooxygenation of 3b-d in methanol under the same conditions, the initially formed enedione 4b-d reacted with methanol to give the corresponding Michael adducts 7 (Scheme 2). 7 In that paper, we proposed that the enedione 4 and the epoxy ketone 5 would arise from the initially formed hydroxyperoxide 8 (Scheme 3). The hydroxyperoxide 8 decomposes to afford 4 with loss of hydrogen peroxide.…”

“…Further purification by column chromatography afforded pure samples of enediones 4 , epoxy ketones 5 , and hydroperoxy ketones 6 . The physical properties of these products have been already reported …”