Photo-oxygenation reactions, solvent effects

Santamaria, J. and Jroundi, R., Electron transfer activation — a selective photooxidation method for the preparation of aromatic aldehydes and ketones. Tetrahedron Lett., 32, 4291, 1991. Bokobza, L. and Santamaria, J., Exciplex and radical ion intermediates in electron transfer reactions. Solvent effect on the photo-oxygenation of 1,4-dimethylnaphthalene sensitized by 9,10-dicyanoan-thracene,/. Chem. Soc., Perkin Trans. 2, 269, 1985. 

Bokobza, L. and Santamaria, J., Exciplex and radical ion intermediates in electron transfer reactions solvent effect on the photo-oxygenation of 1,4-dimethylnaphthalene sensitized by 9,10-dicyanoan-... 

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... 

Many mechanisms had been proposed in the past to rationalize this selectivity (trioxanes, perepoxide, exciplex, dipolar, or biradical intermediates) however, it is now generally accepted [100] that the mechanism proceeds through an intermediate exciplex which has the structural requirements of a perepoxide. This assumption is supported (a) by the lack of stereoselectivity in the reactions with chiral oxazolines [101] and tiglic acid esters [102], (b) by the comparison of the diastereoselectivity of dialkyl-substituted acrylic esters [103] with structurally similar nonfunctionalized alkenes, (c) by intermolecular isotope effects [104] in the photo-oxygenation of methyl tiglate, and (d) by solvent effects on regioselec-tivity [105],... 

The hydroxyl group at the allylic position has a significant effect on the syn/anti methyl stereoselectivity [67,68] and the diastereoselectivity [63,64] of the photo-oxygenation ene reaction (see Sec. II.B). To assess the effect of the hydroxyl at the more remote homoallylic position, the reaction of O with the geminal dimethyl trisubstituted homoallylic alcohols (85, 86, 89) and the cis dis-ubstituted 90 was examined in nonpolar solvents [116], The regioselectivity trend was compared with that of the structurally similar trisubstituted alkenes (87, 88, 91) [105], The results are summarized in Table 12. 

In the photo sensitized oxygenation reactions of alkenes, not only the influence of the solvent on the reaction rate but also the effect of solvent on product distribution i.e. from competing hydroperoxide, 1,2-dioxetane, and entfo-peroxide formation) is rather small [550, 551],... 

The photo conversion of m-nitroanisole to m-nitrophenolate ion by hydroxide ion, Eq. (14), is sensitized by benzophenone and quenched by oxygen when the solvent is acetonitrile-water. A heavy atom effect is observed, the reaction being prevent-... 

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