Dioxetanes, from singlet oxygen

The ratio of epoxide to dioxetane increased 28-fold on going from methylene chloride to benzene with tetraphenylporphin as sensitizer, such solvents as pinacolone, dioxan, and acetone occupying intermediate positions. Systematic investigation showed that epoxide more often than not accompanies the dioxetane from sensitized oxygenation of norbornene (22) and of biadamantylidene (H. J. Shapiro, quoted in (23) (26)). For each solvent-sensitizer combination, the ratio of epoxide to dioxetane increased in the order biadamantylidene < norbornene binor-bornylidene. This is also the decreasing order of reactivity of these olefins toward singlet oxygen. 

It should be mentioned parenthetically that the several roles which the perepoxlde has been shown not to perform have not served to eliminate this intermediate from a possible central position on the reaction path from singlet oxygen to dioxetane. 

Dioxetanes are obtained from an a-halohydroperoxide by treatment with base (41), or reaction of singlet oxygen with an electron-rich olefin such as tetraethoxyethylene or 10,10 -dimethyl-9,9 -biacridan [23663-77-6] (16) (25,42). 

Classical chemiluminescence from lucigenin (20) is obtained from its reaction with hydrogen peroxide in water at a pH of about 10 Qc is reported to be about 0.5% based on lucigenin, but 1.6% based on the product A/-methylacridone which is formed in low yield (46). Lucigenin dioxetane (17) has been prepared by singlet oxygen addition to an electron-rich olefin (16) at low temperature (47). Thermal decomposition of (17) gives of 1.6% (47). 

Dioxetane decomposition has also been proposed to account for chemiluminescence from other reactions (43), including gas-phase reactions of singlet oxygen with ethylene and vinyl ethers (53). 

The phenyl ring of styrene substrates directs a syn selectivity in their ene reactions with singlet oxygen. This effect was demonstrated by the photooxygenation of / ,/ -dimethyl styrene (20). This substrate, apart from the ene product, produces the 1,2-dioxetane and two diastereomeric diendoperoxides as shown in Scheme 9. 

The 1,2-dioxetane postulated as intermediate was never isolated219. However, indirect evidence of a 1,2-dioxetane as a reaction intermediate was obtained by chemiluminescence resulting from the reaction of 10,10 -d i methyl-9,9 -biacridene (40) with singlet oxygen219-220 (equation 11). Several sources of singlet oxygen were used and, in each case, the reaction resulted in chemiexcitation of /V-methylacridane. 

Mazur, S. and Foote, C.S. (1970) Chemistry of singlet oxygen. IX. A stable dioxetane from photooxygenation of tetramefhoxyefhylene. Journal of the American Chemical Society, 92 (10), 3225-3226. 

Solvent acidity plays a critical role in the reaction of singlet oxygen with /nmv-4-propenylanisol 115, and the authors invoke a proton transfer from methanol or acid to the ionic intermediates to explain the enhanced dioxetane concentrations in methanol and in nonprotic solvents with benzoic acid (Sch. 66) [117]. 

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