Photochemical reactions diene + oxygen

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. 

The visible light excitation of diene - NO reactant pairs has been also found to cause an oxygen-atom transfer from NO to diene to form the oxirane biradical [77], which is similar to the photochemical reactions observed in the alkene - NO - Ar systems. Two isomers are considered for the nitrite radical of butadiene ... 

The reaction of aromatics later had an important role in defining the mechanism of the photochemical reacticMis with oxygen, but had a limited synthetic significance. From this point of view, a big step forward was the development by Schenck of the wide-scope photosensitized oxygen addition to dienes to form l,2-diox-4-enes and to alkylalkenes to form propenyl-3-hydroperoxides [99—101]. 

Homoannular dienes e.g. i) readily add excited oxygen in a reaction which is the photochemical equivalent of the Diels-Alder diene-addition reaction fyjJ. The resulting epidioxides e.g. 2) are sufficiently stable for isolation/although they are fairly reactive [72]. 

Conjugated dienes (and compounds that behave like conjugated dienes in the Diels-Alder reaction) react with singlet oxygen to form cyclic peroxides as if molecular oxygen acted as a dienophile. The yields of the peroxides, prepared by photochemical oxidation [13, 55] or by chemical oxidations with hydrogen peroxide and sodium hypochlorite, alkaline hydrogen peroxide and bromine, alkaline salts of peroxy acids [14, 26], or the ozonide of triphenyl phosphite [29], are comparable. 

Another interest in the use of triplet oxygen lies in the oxidation of dienes with photochemical activation (Type 1, above) with formation of endoperoxides as products. The first example of this reaction was observed in the early 1970 s. Thus, reaction of ergosteryl acetate (107) in the presence of trityl tetrafiuoroborate and Lewis acids in the presence of light yielded the endoperoxide 108 (equation 28), With certain Lewis acids this reaction could be thermally, rather than photochemically, activated. Cation radicals were shown to be the intermediate active species, as was borne out by a comparative oxidation of the isomeric lumisteryl acetate which was inactive under these conditions but reacted easily with singlet oxygen . This reaction was later extended to other substrates. Thus, the intermediacy of cation radicals was also indirectly observed by the fact that the r-butyl substituted 1,3-cyclodiene 109 gave a dimeric product 110 (equation 29) via the cation radical intermediate in addition to the usual endoperoxide llOa ,... 

The sensitized photo-oxygenation of 5-methylene-4,7,7-trimethylcyclohepta-l,3-diene was found to give a mixture of the peroxide (233 36 %) and two aP-unsaturated ketones that were not identified (ketone yields = 3.5 % and 1 %). Several reactions of the peroxide (233) were studied, including thermal and photochemical conversion into the syn-bis-epoxide and the potassium hydroxide-methanol conversion into the hydroxyketone (234 X = O), which exists in equilibrium with its bicyclic hemi-ketal form. Lithium aluminium hydride reduction of peroxide (233) gave a mixture of the epimeric alcohols (234 X = H or OH). ... 

Other notable micellar catalysed reactions are the attack of oxygen on an enolate anion, elimination reactions that have more carbanion-like transition states compared with non-micellar conditions and the selective hydroxymercuration of non-conjugated dienes. There has been considerable interest in the effect of micelles on photochemical and electron-transfer reactions. Micelles can increase yields in photoinduced electron transfer and lower the ionization threshold in photoionization. ... 

Ethers were the first functionalized dienes to be polymerized successfully by ADMET [74]. It was found, however, that at least three methylene spacers between the oxygen atom and the olefin were required for successful polymerization with Schrock s catalysts. The resultant polymers were cross-linkable by both chemical and photochemical means [75]. Both [Mo]2 and [W]l were capable of producing the polymer, but polymerization with the molybdenum catalyst proceeded at a rate roughly 10 times faster than that with the tungsten catalyst [76]. ADMET polymerization of diallyl ether was attempted with both [Mo]2 and [Ru]l, resulting in a low molecular weight polymer. The major reaction product for both catalysts is 2,5-dihydrofuran, the result of RCM (Figure 13.6). Divinyl ether was not metathesis-active with any of Schrock s catalysts. 

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