Singlet state oxygen reactions

Reaction with ground state or excited singlet state oxygen, and... 

The formation of endoperoxides served as a useful criterion to reveal the singlet state oxygen molecules produced in a given reaction 93, 95). Moreover, it was recently shown that the triplets of ketones, such as. benzyl and benzophenone, may be used to sensitize the formation of oxygen singlets (0(5). In these reactions, 1.3-cyclohexadiore is oxidized to endoperoxide [Eq. (37)], and no dimers of cyclohexadine form, even though these are the predominant products in the org gen-free reaction 97). In the absence of ketone sensitizer, no endoperoxide forms. 

The lack of any difference in the rate of isomerization between fluoro-sulfonic acid solutions of 34 which had been thoroughly degassed, and those which were saturated with oxygen, suggests that the reaction does not proceed via a triplet mechanism. In fluorosulfonic acid no unproton-ated acid is detected, ruling out the possibility of n,7r excitation. Thus, there is little doubt in this case that it is the Tr,Tr singlet state which is the reactive species. Experiments carried out with a variety of methyl-substituted protonated cydohexadienones have likewise ruled out the... 

Following the discovery of the ene reaction of singlet molecular oxygen ( Ap (Scheme 15) in 1953 by Schenck [88], this fascinating reaction continues to receive considerable mechanistic attention today. The importance of a path via the perepoxide intermediate or a perepoxide-Iike transition state [13] or the perepoxide quasi-intermediate [70] was proposed for the ene reactions of singlet oxygen with alkenes affording allylic hydroperoxides. 

On the other hand. Type II process competes efficiently with the electron-transfer pathway in aerobic environments where the concentration of ground triplet state molecular oxygen is relatively high ( 0.27 mM), and singlet molecular oxygen (1O2) is the most abimdant ROS generated under these conditions, with a quantum yield 0.48 (Valle et al., 2011), eqn. 8. It is also possible an electron-transfer reaction from 3RF to 02 to form anion superoxide, but this reaction occurs with very low efficiency <0.1% (Lu et al., 2000). 

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