Photooxygenation type-2 reaction

In a different type of reaction, alkenes are photooxygenated (with singlet O2, see 14-8) in the presence of a Ti, V, or Mo complex to give epoxy alcohols formally derived from allylic hydroxylation followed by epoxidation, for example, ... 

In order to rationalize the complex reaction mixtures in these slurry reactions the authors suggested that irradiations of the oxygen CT complexes resulted in simultaneous formation of an epoxide and dioxetane36 (Fig. 34). The epoxide products were isolated only when pyridine was co-included in the zeolite during the reaction. Collapse of the 1,1-diarylethylene radical cation superoxide ion pair provides a reasonable explanation for the formation of the dioxetane, however, epoxide formation is more difficult to rationalize. However, we do point out that photochemical formation of oxygen atoms has previously been observed in other systems.141 All the other products were formed either thermally or photochemically from these two primary photoproducts (Fig. 34). The thermal (acid catalyzed) formation of 1,1-diphenylacetaldehyde from the epoxide during photooxygenation of 30 (Fig. 34) was independently verified by addition of an authentic sample of the epoxide to NaY. The formation of diphenylmethane in the reaction of 30 but not 31 is also consistent with the well-established facile (at 254 nm but not 366 or 420 nm) Norrish Type I... 

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. 

Two main types of intermediates may be involved in these reactions free radicals and molecules in electronically excited states. For our present purpose it is convenient to divide photooxygenation reactions into type I processes, in which free radicals and electronically excited molecules are involved, and type II processes, in which only electronically excited molecules occur as intermediates.1... 

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. 

There are several reviews on various aspects of photooxygenation reactions. Bowen2 has reviewed type II direct photooxygenation reactions of polycyclic aromatic compounds in solution. 

Livingston3 has discussed type I and type II direct and indirect photooxygenation reactions at some length, stressing the mechanistic aspects of these reactions. Therefore, especially in the case of type II indirect photooxygenation reactions, very little is reported with respect to substrates used and products formed. 

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

Already in his early investigations on type II direct and indirect photooxygenation reactions, Gaffron18,32 33 showed that the quantum yields of these processes are independent of the intensity as well as of the exciting wavelength of light. He also showed that in the direct photooxygenation of rubrene, two rubrene molecules have to take part in the reaction sequence which bads to the stable endoperoxide. 

Both these mechanisms involve free radicals which should cause the initiation of chain reactions.56-59 Livingston3 has discussed type I direct and indirect photooxygenation processes in detail. In the latter case benzoyl peroxide or azoisobutyronitrile are photolyzed into free radicals. These induce chain reactions, the quantum yields of which are generally greater than unity and inversely proportional to the square root of the light intensity.3... 

Type II photooxygenation reactions, on the other hand, have been found to occur with quantum yields which do not exceed unity and which are independent of the absorbed light intensity.3,16,32,33 In addition, free-radical intermediates can be excluded on the basis of product analysis, since olefins, for example, afford different products in type II than in type I photooxygenation processes (see Sect. IV). Therefore, the two mechanisms discussed above can be omitted as possible mechanisms of photooxygenation reactions with which we are concerned in this article. 

It was the complex formation mechanism which governed the discussion of type II photooxygenation reactions since it was proposed for the first time.38,38... 

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. 

Recently some quantitative analyses of the kinetics of type II direct and indirect photooxygenation reactions have been made. These dealt with the photooxygenation of 2,5-dimethylfuran7 61 and allylthiourea86 sensitized by xanthene dyes and chlorophyll, respectively, and with the... 

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