Olefins reaction with singlet oxygen

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. ... 

Dioxetanes are obtained from an a-halohydroperoxide by treatment with base (41), or reaction of singlet oxygen with an electron-rich olefin such as tetraethoxyethylene or 10,10 -dimethyl-9,9 -biacridan [23663-77-6] (16) (25,42). 

Characteristic reactions of singlet oxygen lead to 1,2-dioxetane (addition to olefins), hydroperoxides (reaction with aHyhc hydrogen atom), and endoperoxides (Diels-Alder "4 -H 2" cycloaddition). Many specific examples of these spectrally sensitized reactions are found iu reviews (45—48), earlier texts (15), and elsewhere iu the Engchpedia. 

Cycloaddition reactions can occur with retention of configuration in the pseudoexcitation band (Sect 1.1) whereas [2jt H-2jtJ reactions are symmetry-forbidden in the delocalization band. Experimental evidence is available for the stereospecific [2-1-2] cycloaddition reactions between A and olefins with retention of configuration (Scheme 14) [82]. A perepoxide intermediate was reported to be trapped in the epoxide form [83] in the reaction of adamantylideneadamantane with singlet oxygen affording dioxetane derivatives [84]. 

Surface water t,/2 = 320 h and 9 x 104 d for reaction with OH and R02 radicals for olefins in aquatic system, and t,/2 = 7.3 d, based on oxidation reaction rate constant of 3 x 103 M-1 s-1 with singlet oxygen for unsubstituted olefins in aquatic system (Mill Mabey 1985). 

Frimer, A. A. (1979). The reactions of singlet oxygen with olefins the question of mechanism. Chem. Rev. 79, 359-387... 

Trioxacycloalkanes. 1.2,4-Trioxanes (1), X = OR4 R4 and R4 = alkylene) are generally prepared by the interaction of aldehydes with zwitterionic intermediates made from reaction of singlet oxygen with olefins. They can also be prepared by catalyzed reaction of ketones or... 

The total synthesis proceeds in >10 steps on solid phase and includes various transformations, including an asymmetric Diels-Alder reaction, oxidation with singlet oxygen, and olefin metathesis. This synthesis sequence is among the most advanced and demanding solid-phase syntheses developed so far for chemical genomics experiments. It demonstrates that the total synthesis of complex natural products in multi-step sequences on solid phase is feasible. 

The cyanoaromatic-sensitized electron-transfer photooxygenation (Foote-type) is a useful preparative reaction with a very limited class of unsaturated olefins, namely those substituted by aromatics and, at the same time, totally inert towards singlet oxygen. On the other hand, in the previous sections, it has been many times underlined that electron-transfer reactions may compete with singlet oxygen formation and, above all, that the reactions of 02 may be the only observable outcome in the presence of singlet-oxygen acceptors. 

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... 

Photo sensitized oxygenation of alkenes with singlet oxygen can, in principle, proceed via three competitive reaction pathways [4 + 2]cycloaddition to e do-peroxides, ene reaction of allylic hydroperoxides, and [2 + 2]cycloaddition to 1,2-dioxetanes (see reference [681] for a review). With suitable olefinic substrates, the chemical outcome of such photo-oxygenation reactions can be strongly influenced by the solvent. This is shown in the somewhat simplified Eq. (5-145). 

Reactions of singlet oxygen with isolated olefins having allylic hydrogen atoms are extremely rapid and in this sense isolated double bonds at any point in the polymer are a source of photochemical instability... 

A second characteristic reaction of singlet oxygen is the ene reaction with olefins with an allylic hydrogen to give allylic peroxides. Thus 2,3-dimethyl-hutene-2 (3) is converted in 53% yield into the allylic peroxide 2,3-dimethyl-3-liydroperoxybutcnc-l (4) ... 

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