Photooxygenation reactions groups

Figure 1 gives examples of the substrates mentioned above. They are divided into two groups (1) substrates which have been found to give rise to the same addition products, A02, in direct and indirect photooxygenation reactions, and (2) substrates which have been studied so far only as oxygen acceptors in type II indirect photooxygenation reactions. 

Fig. 1. Examples of substrates A suitable for type II direct (group 1) and indirect (groups 1 and 2) photooxygenation reactions.

Beside xanthene dyes, methylene blue,127 porphyrins and por-phins,128-130 zinc-tetraphenylporphin,128 a-hydroxyanthraquinones in alkaline alcoholic or pyridine solutions,112,131 vitamin A, /9-carotene, hypericin, rubrene, 3,4-benzpyrene, 20-methylcholanthrene, chlorophyll, and many other compounds have been found to be sensitizers of photooxygenation reactions.70,132,133 Some groups of dyes, however, such as porphyrins and porphins containing transition metals (e.g., Co), azo dyes, cyanine dyes, and triphenylmethane dyes either did not show any or only very poor sensitizer capabilities in the photooxygenation of a-terpinene.128,134... 

The direct photooxygenation of rubrene was also studied by two other groups of investigators.35-37 Schumacher36 confirmed the participation of two molecules of rubrene in this reaction. However, he postulated the formation of an unstable, electronically excited rubrene-... 

The [4 + 2] photooxygenation of dienes and benzene rings leads to endo oxides. With chiral naphtyl alcohols 191,192, and 193 were formed with a dias reomeric excess up to 90%, when the reaction was carried out at — 30°C in CD (Scheme 50). In order to explain the selectivity in aprotic solvents, it was propo that an intramolecular hydrogen bond can be developed between the hydrox group and 102 during the approach of the dienophile from one or the other si of the naphthalene ring, favoring one of the two diastereoisomeric transit states. 

To further account for this latter statement, some years later, the same group reported that the DCA-sensitized photooxygenation of TS, nearly inert vs singlet oxygen, was highly modified upon the addition of benzoquinone (BQ) to the reaction mixture, attributing the change in the reaction course to the efficient interception of superoxide ion by BQ (exothermic by 0.47 V) [126]. 

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