Photooxygenation reactions yields

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. 

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. 

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... 

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... 

Dimethylfuran has been chosen as a substrate for the photooxygenation reactions. This substrate yields the ozonide of 1,2-dimethyl cyclobutadiene which undergoes a facile cleavage by the alcoholic solvent to the corresponding 2-alkoxy-5-hydroperoxy-2,5-dimethylfuran derivative.104,105... 

This explanation was shown to be incorrect by investigation 95) of reactions of a number of a-diketones (benzil, biacetyl, 1-phenyl-1,2-propanedione) in the presence of olefins in oxygen saturated solutions. Slow consumption of diketone was observed with relatively rapid consumption of olefin and concomitant formation of epoxides, often in high yield. Many of the olefins which underwent this reaction do not form epoxides at all with singlet oxygen. For example, tetraphenylporphin-sensitized photooxygenation of tetramethylethylene afforded hydroperoxide 111 quantitatively while a biacetyl-sensitized reaction yielded the epoxide 112. Further, it was shown that the... 

Wittig reaction of 10 with pentyl phosphonium salt in the presence of NaH gave Z-olefin 11 selectively. Dye-sensitized photooxygenation of 11 proceeded exclusively to give the endoperoxides 12a and 12b in almost quantitative yield as a roughly 1 1 mixture of two diastereomers. 

This problem was solved by Adam and coworkers in 1994-1998. They presented a high-yielding and diastereoselective method for the preparation of epoxydiols starting from enantiomerically pure allyhc alcohols 39 (Scheme 69). Photooxygenation of the latter produces unsaturated a-hydroxyhydroperoxides 146 via Schenck ene reaction. In this reaction the (Z)-allylic alcohols afford the (5, 5 )-hydroperoxy alcohols 146 as the main diastereomer in a high threo selectivity (dr >92 8) as racemic mixmre. The ( )-allylic alcohols react totally unselectively threolerythro 1/1). Subsequent enzymatic kinetic resolution of rac-146 threolerythro mixture) with horseradish peroxidase (HRP) led to optically active hydroperoxy alcohols S,S) and (//,5 )-146 ee >99%) and the... 

Similarly, oxygenation of 1,2-digermetrene gave the digermin 91b in 80% yield. Comparable results were also obtained when methylene blue (MB) was employed as the sensitizer On the other hand, the reaction does not involve singlet oxygen, since 90a is stable under photooxygenation in the presence of the photosensitizer TPP. 

The methylene-blue sensitized photooxygenation of 164 yields the isolable dioxetane 165 (70%), the first of its kind to be isolated from an imidazole, presumably via the sequence of Schemes 10 and 26. Some of its reactions are illustrated (Scheme 44). ... 

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... 

Diaryl- and tetraaryl-substituted furans such as tetraphenylfuran (384) for example, yield generally cw-diaroylethylenes such as 380, probably via intermediate ozonide formation.257 Secondary reactions seem to depend very much on the nature of the solvents. Thus, cis-dibenzoylstilbene (380) has been observed by direct photooxygenation of 384 in CS2 as well as by methylene blue-sensitized photooxygenation of 384 in methanol.257,258 However, when the latter reaction was carried out in acetone, epoxide 386 and the enolbenzoate 387 were obtained.258... 

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