Photooxygenation electron-poor

DCA in such a photooxygenation Similar oxygenations of more electron-poor substrates 123 (e.g., Ar = Ar = Ph) give the cyclic peroxides 124 in poor yields (15-30%), and the corresponding benzophenones as the major product . Alternatively, 3,3,6,6-tetraphenyl-l,2-dioxane and related tetraaryl-analogues 124 can be effectively prepared in 80-90% yield by irradiation of pre-formed charge transfer complexes of 123 with SbCls in methylene chloride at —78°C in the presence of oxygen ... 

SCHEME 35. Threo/erythro diastereoselectivity in the photooxygenation of electron-poor aUyUc alcohols... 

Clennan and coworkers reported that the intrazeolite environment is unable to influence the regiochemistry in the photooxygenation of electron-poor alkenes such as a,/3-unsaturated carbonyl compounds 133-135, relative to the photooxygenation in solution. The regioselectivity results are presented in Table 21. 

TABLE 21. Regiochemistry in the intrazeolite photooxygenation of electron-poor alkenes 133-136"... 

Cumyl propyl peroxide, determination, 708 Curing processes, 692, 707 Curtin-Hammett principle, electron-poor alkene photooxygenation, 877 Cyanine dye, TBARS assay, 667 Cyanohydrins, bis(trimethylsilyl) peroxide reactions, 797... 

In fact, the addition of 1,4-dimethoxybenzene (DMB) and/or several similar compounds, at concentrations as low as 10 4 M, to a mixture of aryl-olefins and DCA almost completely inhibits the reactions. Concentration dependence and flash photolysis studies confirm that the primary electron-transfer process occurs between the singlet excited sensitizer and DMB (E01 = 1.34 V vs SCE) with the generation of the corresponding radical ion pair. As a consequence, quantum yields lower than 1, even at infinite substrate concentration, are measured [95]. In this regard, valuable confirmations came from the cyanoaromatic photoinduced electron-transfer oxygenation of alkynes [99], Farid and Mattes reported that the photooxygenation of diphenylacetylene DPA (E° = 1.85 V vs SCE) 25, leading to a mixture of benzil 26 and benzoic acid 27, was efficiently sensitized by DCA ( = 0.15), but poorly by TCA ( > < 0.001) [Eq. (12)] [99]. 

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