Singlet oxygen reactivity solvent

Katsumura, Kitaura and their coworkers [74] found and discussed the high reactivity of vinylic vs allylic hydrogen in the photosensitized reactions of twisted 1,3-dienes in terms of the interaction in the perepoxide structure. Yoshioka and coworkers [75] investigated the effects of solvent polarity on the product distribution in the reaction of singlet oxygen with enolic tautomers of 1,3-diketones and discussed the role of the perepoxide intermediate or the perepoxide-Uke transition state to explain their results. A recent review of the ene reactions of was based on the significant intervention of the perepoxide structure [76], which can be taken as a quasi-intermediate. 

Although this mechanism could explain the inertness of di-t-butyl sulphide towards oxidation due to the absence of a-hydrogen atoms, it was later ruled out by Tezuka and coworkers They found that diphenyl sulphoxide was also formed when diphenyl sulphide was photolyzed in the presence of oxygen in methylene chloride or in benzene as a solvent. This implies that a-hydrogen is not necessary for the formation of the sulphoxide. It was proposed that a possible reactive intermediate arising from the excited complex 64 would be either a singlet oxygen, a pair of superoxide anion radical and the cation radical of sulphide 68 or zwitterionic and/or biradical species such as 69 or 70 (equation 35). 

The change of p with solvents may be due to either a change of the lifetime of singlet oxygen (1 /kB) or a change of the reactivity of the substrate toward singlet oxygen (kg), or both. Systematic studies of solvent effects are, therefore, very much needed. 

Gorman, A. A., Gould, I. R., Hamblett, I., Time resolved Study of the Solvent and Temperature Dependence of Singlet Oxygen ( Ag) Reactivity toward Enol Ethers Reactivity Parameters Typical of Rapid Reversible Exciplex Formation, J. Am. Chem. Soc. 1982, 104, 7098 7104. 

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