1.3- Dienes triplet oxygen

The reactions outlined in Schemes 8-11 indicate that 1,4-diradicals can trap triplet oxygen to give 6-membered cyclic peroxides in variable yields. For example, even under 10 atm oxygen pressure, Ph2CO-sensitized photolyses of the azoalkanes 42 afford the dienes 43 and bicydo[2.1.1]hexanes 44 as the major products while the bicyclic peroxides 45 are formed in very poor yields 1.5%) (Scheme Failure in oxygen... 

ThiyI and selenyl radical-mediated domino reactions of dienes and of vinylcyclopropanes with triplet oxygen... 

The formal addition of hydrogen atom to the terminal double bond of vinylcyclo-propanes and of 1,5-diene systems under triplet oxygen provides another sequential... 

However, it has been recently suggested that oxirane intermediates also play a part in this reaction, producing some of the minor products (Scheme 59)152. Dienes do not appear to be good substrates for this reaction, at least not with triplet oxygen, as the cation-radical Diels-Alder dimerization is much faster unless the alkene is sterically hindered153. 

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 ,... 

Titanium tetrachloride has been found to catalyze a unique reaction between triplet oxygen and ergosteryl acetate [512a] to first form the 5,8-peroxide which reacts in a facile manner with unreacted olefin to give 6a-chloro-5-hydroxy-5a-ergosta-7,22-dien-3j3-yl acetate, equations (313) and (314). 

Alkenes and 1,3-dienes also react with triplet oxygen in the presence of ArjN, Lewis Acids or PhjC with ultraviolet irradiation (Barton el al., 1972, 1974, 1975 Hayes et al., 1978) to form cyclic peroxides such as [19]... 

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