Photooxidation of olefins sensitized by .alpha.-diketones and by benzophenone. A practical epoxidation method with biacetyl

Abstract

Photooxidation of nine olefins in benzene solution with oxygen and benzil leads to epoxides as the main product in yields ranging from 5% in 60 min for trans-stilbene to 98% in 15 min for norbornene. From cis or trans acyclic olefins the epoxide is always trans. Competing processes include formation of allylic hydroperoxides from olefins (2.3-dimethyl-2-butene and 1,2-dimethylcyclohexene) that are quite reactive toward singlet oxygen, and energy transfer with cis,trans isomerization of olefins with lower lying triplet states (stilbene). Recovery of b e n d is high, showing that it is neither consumed stoichiometrically in the process nor oxidized directly in effective competition with the photoepoxidation. The action of biacetyl as a photosensitizer is similar to that of benzil but is attended by much more destruction of the diketone and yields epoxide with a wider range of olefins. Seven of the 18 olefins in Table I11 are converted to epoxides in yields of 90% or more, making this a useful preparative method. Benzil and biacetyl are compared with the T,T* photosensitizer fluorenone, which can produce singlet oxygen but gives no epoxidation, and with the n-r* sensitizer benzophenone, in which cycloaddition to an olefin to give oxetane is an important competing process and becomes the major course of the reaction with norbornene. The nature of the mechanism is discussed.Dye-sensitized photooxidation reactions have been extensively investigated and the results are consistent with the singlet oxygen mechanism. Photooxidation with n-x* triplet sensitizers, however, presents a less simple picture. A free-radical mechanism is indicated in benzophenone-sensitized oxidation of 2-pr0panol.~ Gollnick and co-workers have pointed out that n-A* triplet sensitizers can also generate singlet ~x y g e n .~ We report here the results of photooxidation of various olefins with benzil and biacetyl as sensitizers and show that the present photooxidation is quite different from the singlet oxygen oxidation and also from radical-chain autoxidations.Photooxidation with Benzil. The irradiation4 of a benzene solution of 1,2-dimethylcyclohexene (1,0.5 M) and benzil (0.1 M) with a Hanovia 450-W medium-pressure mercury lamp a t 0" for 15 min, oxygen being bubbled through during the irradiation, gave allylic hydroperoxide (2a) and epoxide (3) in 58 and 12% yield, respectively, together with 30% of recovered olefin. Benzil was recovered almost quantitatively. Table I shows the contrast between these results and either the dye-sensitized oxidation5-' or the autoxidation Important differences between dye and benzil sensitizations appear also in the oxidation of aromatic olefins. Benzil-sensitized oxidation of five aromatic olefins gave epoxides as shown in Table 11, whereas it is known that these aromatic olefins are converted very slowly to ketones by long exposure to singlet PhRC=O + R'R"C=O "very slow" ~x y g e n .~ A characteristic feature is the stereochemistry of epoxide formation: only trans-epoxide was obtained independently of the geome...

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