Oxygen singlet, reaction 4- olefins

Singlet oxygen reacts with olefins presumably by the "ene" reaction to form allyflc hydroperoxides (45,57), eg, l-methyl-2-propenyl hydroperoxide [20733-08-8] is produced from 2-butene (eq. 19). The regioselectivity of this reaction has been investigated (58). 

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). 

Figure 6. Ene reaction between singlet oxygen and an olefinic bond. The hydroperoxide may be attached to either of the inital double bond carbons.

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... 

Figure 2.9 Chemical quenching of singlet oxygen, (a) The ene reaction — addition of singlet oxygen to an olefin with allylic hydrogen (b) the ene reaction of cholesterol (c) endoperoxide formation by singlet oxygen to imidazole residue as in histidine.

The fact that singlet oxygen ene-reactions with ds-olefins have less negative activation entropies and about 2-8 times larger reaction rates (e.g., for ds-2-butene AS = -32 e.u., AH =1.6 kcal/mol for trans-2-butene AS = -42 e.u., AH = 0.3 kcal/mol) than with trans-olefins implies that the transition states... 

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). 

Classical chemiluminescence from lucigenin (20) is obtained from its reaction with hydrogen peroxide in water at a pH of about 10 Qc is reported to be about 0.5% based on lucigenin, but 1.6% based on the product A/-methylacridone which is formed in low yield (46). Lucigenin dioxetane (17) has been prepared by singlet oxygen addition to an electron-rich olefin (16) at low temperature (47). Thermal decomposition of (17) gives of 1.6% (47). 

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. 

Singlet molecular oxygen ( A is an electron acceptor powerful enough to react with olefins in the pseudoexcitation band. The [2h-2] cycloaddition and ene reactions and the stereoselectivities are reviewed in this subsection. 

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]. 

On a first, very broad, approximation singlet oxygen behaves somehow like ethylene. Three types of reactions of 102 are usually observed and have been utilized in organic synthesis 606 608) a) the Diels-Alder like cycloaddition to dienes (6.1) b) the ene reaction with olefins (6.2) and c) cycloaddition to activated double bonds (6.3). 

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

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