Photooxygenation reactions oxygen

Trans alkenes show similar regioselectivity on their photooxygenation reactions. However, their reactivity towards singlet oxygen is much less than that of the corresponding cis aUtenes. ... 

Photochemical reactions of organic substrates with molecular oxygen have been extensively studied, with respect to both their preparative and mechanistic aspects. This article will be restricted to a certain type of these reactions which we may call type II (direct and indirect) photooxygenation reactions in solution. This classification is based on the following definitions. 

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. 

Hochstrasser and Porter4 reported on Primary Processes in Photo-Oxidation. A short review was given on direct photooxygenation reactions in solution as well as in the solid state. Gas-phase reactions in the presence of oxygen were also discussed. 

Consequently, the formation of a labile addition product, Sensrad. .. 02, was postulated. This should transfer its oxygen to a substrate A via a collision complex Sensrad... 02... A, which collapses into the ground-state sensitizer and an energy-rich AOa. The latter stabilizes to A02 by dissipating its excess energy as heat.26 Since photooxygenation reactions were shown to occur even at very low temperatures with relatively high quantum yields, the identity of Sensrad with a sensitizer in the excited triplet state was assumed.61,68-70... 

Until recently, the hypothesis that the termination reaction of type II photooxygenation reactions occurs between the substrate and an excited light absorber-oxygen complex seemed to be well established. The typical products obtained from cyclic 1,3-dienes and olefins (see Fig. 1 and Sect. IV) could only be made by photochemical reactions. 

It turns out that, except for the latter case, there was no kinetic need to postulate an intermediate complex formation. We may, therefore, replace the complex discussed in the original papers by singlet oxygen. This leads to the following reaction sequence found to occur in type II direct and indirect photooxygenation reactions ... 

Carbon disulfide quenches the fluorescence of anthracene quite efficiently,145,149 but seems to have little effect on its triplet lifetime.147 Diphenylanthracene in benzene fluoresces with a quantum yield of 0.8 and shows a high sensitivity to the oxygen concentration in photooxygenation reactions. With about 1 vol% of CS2 present, AC>2 is practically independent of [02] (> 10"5 mole/liter). In jjoth cases, where carbon disulfide was either used as solvent or was added to an otherwise strongly fluorescent solution, the quantum yields of photooxygenation followed... 

In the termination step [eq. (13), p. 12] of type II photooxygenation reactions, singlet oxygen reacts with a suitable substrate to an oxygenation product, which may be stable or unstable. In the latter case, after-reactions such as rearrangements, decompositions, or reactions with the solvents can occur, which sometimes may obscure the mechanism of the termination step. 

Cyclopentadienes, 1,3-cyclohexadienes, 1,3-cycloheptadienes, as well as furan and aklyl-substituted furans, have been investigated as substrates of photosensitized oxygenation reactions, while aromatic compounds such as anthracenes and tetracenes as well as aryl-substituted carbo-and heterocyclic pentadienes were studied in direct and indirect (photosensitized) photooxygenation reactions. 

D.L. Kearn and A.U. Khan, Sensitized photooxygenation reactions and the role of singlet oxygen , Photochem. Photobio., 10, 1969, 193. 

The electron transfer sensitized photooxygenation reactions have been the subjects of numerous investigations and of several reviews [181-183]. In the presence of oxygen, CP is oxygenated during the DCA photosensitized reaction via the reaction of CP"+ and 0 2", which is formed by electron transfer from DCA to 02, to give the corresponding dioxolanes (Scheme 20) [173] ... 

The reaction of a substance with oxygen under the influence of light. When oxygen remains in the product this latter process is also called photooxygenation. Reactions in which neither the substrate nor the oxygen are electronically excited are sometimes called photoinitiated oxidations. 

Yamaguchi, K., Yabushita, S., Fueno, T., Houk, K. N. Mechanism of photooxygenation reactions. Computational evidence against the diradical mechanism of singlet oxygen ene reactions. J. Arrr. Chem. Soc. 1981, 103, 5043-5046. 

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