Generation of singlet oxygen

SECTION 10.2. ADDITION OF OXYGEN AT CARBON-CARBON DOUBLE BONDS 

Many alkenes present several different allylic hydrogens and in this type of situation it is important to be able to predict the degree of selectivity. A useful generalization of this aspect is that there is a preference for abstraction of hydrogen from the more congested side of the double bond, Exactly how this selectivity relates to the details of the reaction mechanism remains to be clarified. 

The allyl hydroperoxides generated by singlet oxygen oxidation are normally reduced to the corresponding allyl alcohol. The net synthetic transformation is then 

Litt and A. Nickon, Oxidation of Organic Compounds—III, Advances in Chemistry Series, No. 77, American Chemical Society, Washington, D.C. (1968), pp. 118-132. 

The excited oxygen molecule will decay to the ground-state triplet if it does not encounter an olefin suitable for reaction. The rate of this decay process has been shown to depend strongly on the identity of the solvent. Measured lifetimes range 

The reactivity order of olefins is that expected for attack by a relatively electrophilic reagent. Reactivity increases with the number of alkyl substituents on 

Certain olefins react with singlet oxygen in a different manner, giving the cyclic dioxetane adduct  

This reaction is not usually a major factor with olefins bearing only alkyl substituents, but becomes important with vinyl ethers, for example. 

Singlet oxygen undergoes 4 + 2 cycloaddition reactions with dienes, generating peroxides  

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Delmelle, M. (1978). Generation of singlet oxygen by the retina. Photochem. Pathobiol. 27, 731-734. 

Schmidt R (2006) Photosensitized generation of singlet oxygen. Photochem Photobiol 82 1161-1177... 

Danilov OB, Belousova IM, Mak AA, Belousov VP, Grenishin AS, Kiselev VM, Kris ko AV, Ponomarev AN, Sosnov EN (2003) Generation of singlet oxygen with the use of optically excited fullerenes and fullerene-like nanoparticles. Optics and Spectroscopy 95 833-842. 

Evidence for adiabatic photolytic cycloreversions at room temperature has been obtained more frequently in recent years [121,122], The adiabatic generation of singlet oxygen by photochemical cycloreversion of the anthracene and 9,10-dimethylanthracene endoperoxides 105 and 106 proceeds with wavelength-dependent quantum yields of 0.22 and 0.35, respectively, and involves the second excited singlet state of the endoperoxides [123]. Photodissociation of the 1,4-endoperoxide from l,4-dimethyl-9,10-diphenylanthracene was found to yield both fragments, i.e., molecular oxygen and l,4-dimethyl-9,10-diphenylanthracene, in their electronically excited state [124]. 

As illustrated in Scheme 3.26, the role of the Pd(II) porphyrin complex is the photochemically sensitized generation of singlet oxygen Oj), which forms the allylic hydroperoxide 14. Subsequent Fe3+ porphyrin-catalyzed hydrogen abstraction from 14 yields the peroxy radical 16, while the catalytic Fe3+ species is recycled by homolytic 0—0 bond cleavage of 14 to give the alkoxy radical 17. Both radicals 16 and 17 are involved in the dark reaction where the remaining cyclohexene is consumed. 

Harriman, A. et al. (1989) Metallotexaphyrins a new family of photosensitisers for efficient generation of singlet oxygen, Chem. Commun. 314-316. 

Scheme 58 An early example of the photochemical generation of singlet oxygen within a glass microreactor.

However, the chemiluminescent reaction in the autoxidation of (31) is not inhibited by radical scavengers 208 it may proceed via a 1,2-dioxete (62). The proposed 211) thermal generation of singlet oxygen in the presence of (31) could not be confirmed in various experiments 208). 

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