Photolysis photooxygenation

The intermediacy of nitrobenzyl carbanions in such photolysis is general, and also has been found in the photooxygenation of a series of nitrobenzyl derivatives including 2-methoxy-(m- and p-nitrobenzyl) ethanols95. 

Different isomers of C qO have been prepared by photooxygenation [48], by MCPBA-oxidation [48], by ozonolysis [49] or they were extracted from fullerene soot [11, 50]. Isolation from fullerene soot and analysis of the product of photooxygenation and thermal ozonolysis yields only [6,6]-closed epoxide structures. As already observed for CgoO, ozonolysis and subsequent photolysis of the ozonide C7QO3 gives different [5,6]-open oxidoannulene structures [49]. 

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]. 

In the TPP+BF4 -sensitized photooxygenation of 21, leading to the corresponding thermally stable 1,2-adamantylideneadamantane dioxetane, 23, direct evidence for the chain electron-transfer reaction was achieved by quenching experiments, ns laser flash photolysis, and cyclic voltammetric studies [185]. 

Oxidation photocatalyzed by polyoxometalates [66k] has been applied to the fimctionalization of 1,8-cineole (structure IX-10) [661], widely distributed in the plant kingdom. The photooxygenation of IX-10 gave a mixture of ketones and alcohols which were transformed by the subsequent action of pyridinium chloro-chromate into 5- and 6-keto derivatives in the ratio IX-11 IX-12 = 2.5 1. A laser flash photolysis study of the mechanism has been carried out for the deca-tungstate anion catalyzed reaction [66m]. 

Table 5. Photostationary Ratios d2-77a d2-77a ) in the PET Reactions of d2-77a, Yields of 78a in the PET Photooxygenation of 77a,and Transient Absorption Maxima (Amax) Observed in Laser Flash Photolysis of 77a ... 

Algarra, F., Baldovi, M. V., Garcia, H., Miranda, M. A., and Primo, J., Direct photolysis and electron transfer photooxygenation of enol acetates of 3-phenylpropiophenones, Mowatsh. Chem., 124,209, 1993. 

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