Dienes photochemical oxygenation

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. 

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 ,... 

More detailed investigations by Rabek and Pajak [508] show that DPPH (concentration 10-3—10-4 mole l-1) added to a solution of cis-l,4-polyisoprene in benzene or n-heptane accelerates the photodegradation in the presence as well as in the absence of oxygen. It was discovered that the products of photochemical decomposition of DPPH have stronger sensitizing properties with respect to the photodegradation of diene polymers than has the undecomposed DPPH. 

The ozone concentration in the troposphere during the daytime is typically about 1 pphm (parts per hundred million parts of air by volume) [20], Values up to 100 pphm were measured in some photochemical smog areas. The molecular mechanism of the ozone aging of diene based elastomers was studied in detail and is well understood [19,21], Products or intermediates different from those arising in autoxidation or photo-oxidation of polymers were identified ozonides (3), zwitterions (4), diperoxides (5), polyperoxides (6), polymeric ozonides (7) and terminal aldehydes (8). Reactivity of aminic antiozonants (AOZ) with these species accounts for the protection of rubbers against atmospheric 03. AOZ must also possess antioxidant properties, because the free radical processes are concerted with ozonation due to the permanent presence of oxygen. 

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). ... 

The photochemical isomerization of polyfluorocyclohexa-1,3-dienes (Scheme 3) offers a unique route to the corresponding bicyclo [2,2,0 ]hex-2-enes. A further interesting photoch nical isomerization is that of perfluoro-cyclohexene to -(methylenecyclopentane) and -(1-methylcycIopentene) (Scheme 4) which occurs in the presence of oxygen. Also formed were higherboiling compounds, several of which were identified by cobalt trifluoride fluorinationof themselves or of appropriate substrates (see p. 4). Perfluoro-(1-methylcyclopentene) (30%) is also the product of pyrolysis of perfluoro-cyclohexene over nickel shavings at 480 °C (see pp. 58 and 359) at 550 °C,... 

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