Photosensitized oxygenation

Thiazole is relatively resistant to oxidation, and very few reactions of this type have been recorded. Under photosensitized oxygenation, triphenylthiazole affords various products of ring cleavage depending on... 

Fig. 21. Kinetic curves of photosensitized oxygen absorption by thermally treated PAN samples during irradiation with white light. ( ) PAN (450 °C,...

Zepp RG, GL Baugham, PA Scholtzhauer (1981b) Comparison of photochemical behaviour of various humic substances in water II Photosensitized oxygenations. Chemosphere 10 119-126. 

Orito et al. (206-208) developed an attractive conversion of indenobenzazepines to protopines through photosensitized oxygenation of the... 

Fiedor, J., L. Fiedor, J. Winkler et al. 2001. Photodynamics of the bacteriochlorophyll-carotenoid system. 1. Bacteriochlorophyll-photosensitized oxygenation of (3-carotene in acetone. Photochem. Photobiol. 74 64-71. 

Oxidation t,/2 = 10 h for photosensitized oxygenation with singlet oxygen at near-surface natural water, 40°N, midday, midsummer (Zepp Schlotzhauer 1979)... 

Surface water computed t,/2 = 0.13 h at near-surface of a water body, for direct photochemical transformation, and t,/2 = 0.79 d for direct photolysis in a 5-m deep inland water body with no sediment-water partitioning, t,/2 = 1.2 d with sediment-water partitioning to top cm bottom sediment and t,/2 = 10 h for photosensitized oxygenation with singlet oxygen at near-surface natural water, 40°N, midday, midsummer (Zepp Schlotzhauer 1979) ... 

Air direct photolysis t,/2 = 1.17 h predicted by QSPR method in atmospheric aerosol (Chen et al. 2001). Surface water photolysis t,/2 = 0.35 h near surface water, 40°N midday, midsummer and photosensitized oxygenation t,/2 = 1.5 h at near surface water, 40°N, midday, midsummer (Zepp Schlotzhauer 1979). photolysis t,/2 = 0.18 h in aqueous solution when irradiated with a 500 W medium pressure mercury lamp (Chen et al. 1996). 

Hydroxy epoxidation of dienes.2 Photosensitized oxygenation of dienes when catalyzed by titanium(IV) isopropoxide results in an epoxy alcohol, formed by an oxygen transfer from an allylic hydroperoxide. 

Quantitative kinetic data showed that photosensitized oxygenations of various pollutants (e.g., 2,5-dimethylfuran and the insecticide Disulfoton) in air-saturated natural water samples containing aquatic HS and in distilled water containing soil-extracted or commercial HA/FA were at least one order of magnitude faster than those in distilled water. 

Enamines, reaction with quinones, 20, 3 Enantioselective aldol reactions, 67, 1 allylation and crotylation, 73, 1 Ene reaction, in photosensitized oxygenation, 20, 2 Enolates ... 

SCEIEME 26. Synthesis of the allene moiety of carotenoids via biomimetic photosensitized oxygenation... 

Mechanistic studies were started by Gaffron15-17 in 1926. At this time Moureu, Dufraisse, and Dean18 discovered the direct photooxygenation of rubrene and Windaus and Brunken19 discovered the photosensitized oxygenation of ergosterol. The structures of both the products formed were elucidated some years later and shown to be those of transannular peroxides (endoperoxides).20-23... 

Schenck, the main contributor to the field of photosensitized oxygenation reactions, strongly advocated the view that the interaction between an electronically excited sensitizer and oxygen should result in a (chemical) addition reaction rather than in a (physical) energy-transfer reaction.7,26,60-62... 

Activation Energies of the Termination Step (13) in Photosensitized Oxygenation Reactions (after ref. 124)... 

As has been pointed out earlier, the kinetic results (ft = k6/k9 independent of the nature of the sensitizer) as well as the observed product distributions obtained in photosensitized oxygenation reactions and in (chemically prepared) singlet oxygen oxygenation reactions (see p. 45, for example) leave but little doubt that 102(1Ag or 1S9+) is the common oxygenating species. ... 

B. Olefins as Substrates of Photosensitized Oxygenation Reactions— The Schenck Reaction... 

In 1943 Schenck25,26 discovered the photosensitized oxygenation of olefins which leads to the formation of a,/J-unsaturated hydroperoxides... 

Hydrogen abstraction from the two optically active compounds 16 and 19 gives rise to the formation of symmetrical radicals from which only the racemic cis- and frans-alcohols in a ratio of 1 1 were formed. On the other hand, photosensitized oxygenation of 16 and 19 resulted... 

According to the reaction scheme of the Schenck reaction one should expect four more products in the case of (+)-carvomenthene (19) and at least four more products in the case of (+)-limonene (16). The fact that with limonene no products were obtained which originate from an oxygen attack on the A8 double bond, is in agreement with the general rule that type II photosensitized oxygenation reactions occur much faster with tri- and tetrasubstituted ethylenes than with di- or monosubstituted ones (see p. 71). 

The photosensitized oxygenation of (+)-carvomenthene, 19, was also carried out by Kenney and Fisher,182 who did not find alcohol 20 but, in addition to the other alcohols, a carvomenthene epoxide. Probably because of these findings, the authors concluded that this photosensitized oxygenation reaction occurs by a nonconcerted, nonstereoselective reaction mechanism. According to all our experiences, epoxidation is not a primary reaction in photosensitized oxygenation reactions of olefins. It may occur as an after-reaction of the primarily formed hydroperoxides its occurrence depends on the hydroperoxide and olefin concentration as well as on the reaction temperature.184... 

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