Photooxygenation inhibition

Fig. 28 A. Intrazeolite photooxygenation of a -chlorosulfide that requires a hydroperoxy sulfonium ylide and B. The formation of an extended cation complexed persulfoxide that can potentially inhibit hydroperoxy sulfonium ylide formation.

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]. 

The rates of chemical reaction (k ) and physical quenching (kq) of I02 with 37 and were determined by competitive Inhibition of photooxygenation of 1,3-dlphenyllsobenzofuran. 

TABLE 9. Methylene blue (10"3 M) Sensitized Photooxygenation of 2,3-dimethyl-2-butene (5 x 10-2 m) in CH2Cl2> inhibited by M and 70 ... 

The next two entries in Table 1 are compounds prepared by photooxygenation of furoic acid in methanol or ethanol. These normethyl compounds are strong growth inhibitors/ showing complete inhibition at 1 mM with 37% inhibition for the methyl derivative (2ac) and 52% for the ethyl derivative (2ad)at 0.1 mM. 

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