Singlet-oxygen solvent effect

Recently the solvent effect on the [4+2] cycloaddition of singlet oxygen to cyclic dienes has been subjected to a multiparameter analysis. A pre-equilibrium with charge-transfer character is involved, which is affected by the solvent through dipolarity-polarisability (n ) and solvophobic interactions ( Sjf and Another multiparameter analysis has been published by Gajewski, demonstrating the... 

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. 

Previous studies have shown that the rate of the O2 ene reaction with alkenes shows neghgible dependence on solvent polarity . A small variation in the distribution of the ene products by changing solvent was reported earlier . However, no mechanistic explanation was offered to account for the observed solvent effects. It is rather difficult to rationahze these results based on any of the currently proposed mechanisms of singlet oxygen ene reactions. Nevertheless, product distribution depends substantially on solvent polarity and reaction temperature only in substrates where both ene and dioxetane products are produced ° . 

The observed solvent effect on the stereoselectivity of singlet oxygen with ester 79 was rationalized by examination of the possible transition states of this reaction (Scheme 22). 

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. 

The addition of singlet oxygen to alkenes also gives dioxetanes. A number of mechanisms have been proposed and the literature abounds with theoretical and experimental results supporting one or more possible intermediates (a) 1,4-diradicals, (b) 1,4-dipolar, (c) perepoxides, or (d) concerted (Scheme 95). Both ab initio and semi-empirical calculations have been done and to date the controversy is still not resolved. These mechanisms have been reviewed extensively (77AHC(21)437, 80JA439, 81MI51500 and references therein) and will not be discussed here, except to point out that any one mechanism does not satisfactorily account for the stereospecificity, solvent effects, isotope effects and trapped intermediates observed. The reaction is undoubtedly substrate-dependent and what holds for one system does not always hold for another. 

Substituent effects in the dye-sensitized (Rose Bengal or methylene blue) singlet oxygen photooxidation of a wide variety of furans have been investigated.287 Solvents effects on the value of p, which became less negative as polarity of the medium was increased was indicative of... 

Table 4 presents comprehensive photophysical data of 2T-7T (di-n-butyl), including singlet oxygen quantum yields (4>a) in benzene. Considering the absorption and emission spectra, there is a relatively small effect of solvent on these maxima. In the fluorescence excitation spectra, there is a considerable red shift, 1600 cm-1, in the maxima with an excellent linear correlation between 1/n and the maximum of the first transition, as well as the 0-0 energy particularly for 4T-7T. 

Recent developments in the stereoselective singlet oxygen allylic photo-oxygenations of alkenes have been reviewed. A number of factors, such as solvent, electronic effects, and non-bonded interactions that dictate the ene product selectivity, and also the various mechanisms of this reaction, have been highlighted.239... 

Foote, C.S. and R.W. Denny. 1971. Chemistry of singlet oxygen. XIII. Solvent effects on the reaction with olefins. ]. Am. Chem. Soc. 93 5168-5172. 

Zwitterions [32] and biradicals [33] have been also proposed as intermediates. These mechanistic possibilities were insufficient for rationalizing the lack of correlation between the reaction rates and solvent polarity [14,31], An early theoretical study by Goddard [33] favored a biradical intermediate. However, isotope effect studies [34], the lack of Markovnikov-type directing effects [14,31], and the fact that radical scavengers have no effect on the reaction eliminated a biradical mechanism for the singlet-oxygen-alkene ene reaction. 

Singlet-Oxygen Reactions Solvent and Compartmentalization Effects... 

Table 2 Solvent Effect on Singlet-Oxygen Quenching Rate Constants...

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