Hydroperoxides formation with singlet oxygen

The allylic hydroperoxides generated by singlet oxygen oxidation are normally reduced to the corresponding allylic alcohol. The net synthetic transformation is then formation of an allylic alcohol with transposition of the double bond. 

We studied dienic elastomer films (EPDM, poly-octenamer, poly-isobutylene-co -iso-prene) which react with singlet oxygen, 02, according a "ene" reaction described elsewhere1 12. The resulting hydroperoxidized polymers can also be treated by nitric oxide and acetyl chloride but the reactions result now in the formation of nitrate and perester respectively ... 

This process continually generates lipid free radicals. The formation of nonradical products resulting from the combination of two radical species can terminate this chain reaction or propagation. Alternatively, unsaturated lipids can form hydroperoxides by reacting with singlet oxygen produced by sensitized photooxidation, which is a non-free-radical process. 

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... 

Copper deficiency is reported to cause a 2-fold increase in the level of lipid hydroperoxides in rat liver mitochondria and microsomes along with decreases in Cu-Zn SOD [696]. Cytosolic and mitochondrial glutathione peroxidase and catalase activities were also decreased [696]. Increased lipid peroxidation was suggested to be due to the accumulation of hydrogen peroxide rather than superoxide. Alternatively, some lipid peroxidation may be due to the formation of singlet oxygen [444] and its oxidation of glutathione to its disulphide and/or oxidation of Fe(II)catalase to the inactive Fe(III) form. 

Olefins containing at least one allylic hydrogen are suitable substrates and are of special importance and interest with regard to the intrinsic mechanism involved in their reactions with singlet oxygen. Allylic hydroperoxides are formed, but the mechanism of their formation is clearly distinct from that by which allylic hydroperoxides are produced in thermal or photochemically initiated (see example above for a Type I process) autoxidation reactions. This has unequivocally been shown with optically active limonene as a substrate, which gives rise to different products in free radical and Type II photo-oxygenation reactions (22, 57, 61). 

Epoxldation. Dye-sensitized photooxidation of aliphatic olefins results in formation of altylic hydroperoxides from reaction with singlet oxygen. Photo-IMiidation with -diketone sensitizers follows a different course epoxides are the foain products. Of several a-diketones examined, epoxides are obtained in highest yields with biacetyl, which is oxidized at the same time mainly to acetic acid. In some cases yields of epoxides in biacetyl photooxidation are 90% or more. Both cis- and trans-alkenes yield mainly tran.r-epoxides. Both epoxidatioh and ketone destruction are much slower with benzil. The nature of this epoxidation reaction is discussed, although many points of ambiguity remain. This reaction is Of practical importance. ... 

Polydienes (polybutadiene, polyisoprene and their copolymers and analogues) react very effectively with singlet oxygen CO2) generated in the gaseous phase by microwave discharge or in photosensitized (dye) reactions. Reaction of 02 with polyisoprene leads to the formation of three polymeric hydroperoxides [800, 1569, 1571, 1572, 1787, 2095, 2096] ... 

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