Photosensitized oxygenation reactions

Apparent first-order rate constant phototransformation al X > 285 nm, k = (1.38 0.12) x 102 h 1 in purified water, and k = (1.68 0.12) x 10 2 h 1 in Capot river water with t,/2 40 h (Zamy et al. 2004) Oxidation half-life ranged from t,/2 5 h of midday sunlight during summer to t,/2 = 12 h during winter estimated from kinetic data for oxygenation reactions photosensitized by humic substances in water exposed to sunlight (Zepp et al. 1981) ... 

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... 

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... 

A3-/ -Menthene (68) readily undergoes the photosensitized oxygenation reaction, giving rise to only four hydroperoxides, the reduction products of which are the alcohols 69 through 72 although carefully looked for, no pulegols (73,74) could be found in the reduction mixture.188 A slow photosensitized oxygenation takes place with A8-/ -menthene (75) and with A1(7),8-p-menthadiene (77) from which after... 

Of the two (-)-A2-carenes only the cis isomer 175 undergoes a photosensitized oxygenation reaction forming as the sole product the tram-alcohol 176 (after reduction), while the tram isomer 177 is completely inert,202 thus supporting an earlier determination of the structures of the A2-carenes.203 The cis isomer probably exists only in the conformation 175b, in which the allyl hydrogen at C4 has a favorable... 

No reactions have been observed when methylenecyclopentane (182), methylenecyclohexane (183), or A1(7)-p-menthene (184) were submitted to photosensitized oxygenation.187 On the other hand, methylene-cycloheptane (185)1B7, jS-pinene (187)197, A4<10)-carene (190),202 and sabinene (193)205 undergo slow photosensitized oxygenation reactions giving rise to the formation of primary alcohols 186,189,191,192, and 194, respectively, after reduction of the primarily formed... 

Relative Free Energies of Activation of Olefins in Type II Photosensitized Oxygenation Reactions... 

The method of photosensitized oxygenation was successfully applied in the preparation of alcohols 265-270 from sylvestrene (264),207 and seems to be the most simple and successful method for the preparation of optically active rose oxides (272,273) from (+)- or (—)-citronellol C271).177 It may also be used for the preparation of certain organo-metallic hydroperoxides. Thus, the triphenyl-tin derivative of tri-methylethylene (274) undergoes a photosensitized oxygenation reaction with a rate similar to that of tetramethylethylene, giving rise to the hydroperoxides 275 and 276 219... 

Dehydration products have been observed from photosensitized oxygenation reactions with cycloheptatriene (304),220 AMialin (307) 222 3/1- and 3a-hydroxy-A4-cholestenes (312a and 312b, respectively),226,227 and 2-hydroxy-10-methyl-1,9-octalin (316).226... 

Thiosemicarbazones, such as 331, and thiourea (333) give rise to sulfinic acids when subjected to photosensitized oxygenation reaction conditions.62,233 It was suggested that by analogy to olefin oxygenation the primary products may be of hydroperoxidic nature... 

Cyclopentadienes, 1,3-cyclohexadienes, 1,3-cycloheptadienes, as well as furan and aklyl-substituted furans, have been investigated as substrates of photosensitized oxygenation reactions, while aromatic compounds such as anthracenes and tetracenes as well as aryl-substituted carbo-and heterocyclic pentadienes were studied in direct and indirect (photosensitized) photooxygenation reactions. 

In order to establish the general validity of the mechanism of photosensitized oxygenation reactions, substrates other than 2,5-dimethyl-furan have been investigated.81 The quantum yield of product formation is given by the general equation... 

Ketones Aliphatic ketones formed by autoxidation of lipids also contribute to the flavor of oils and food products. For example, Guth and Grosch (13) identified l-octen-3-one as one of the odor-active compounds in reverted soybean oil. This compound was described as metallic and mushroom-like. The reaction pathway for the formation of l-octen-3-one from the linoleate-10-hydroperoxide via the p-scission route is illustrated in Figure 2. 10-Hydroperoxide of linoleate is not the usual hydroperoxide formed by autoxidation of linoleate however, it is one of the major hydroperoxides formed by the photosensitized oxidation (singlet oxygen reaction) of linoleate (14). 

Whilst 1-methylcyclopropene is inert towards singlet oxygen, the photosensitized oxidation of cyclopropenes 288 rapidly produces numerous products which include enones. This latter free radical oxidation is thought to provide 289 and 290 which afford products by secondary rearrangement singlet oxygen is not involved in these reactions. 

---