Photooxidations with singlet oxygen

Some substituted alkenes react with singlet oxygen to form a dioxetane in a -f- cycloaddition reaction. Most dioxetanes readily decompose to carbonyl compounds in an exothermic reaction that is accompanied by a bluish luminescence. The chemiluminescence will be dealt with in more detail in Section 7.6.4. 

The ordering of the reactant orbitals is obtained from ionization potentials of molecular oxygen and butadiene. The reactant configuration with singly occupied MOs jfi and COj) correlates with a highly excited product configuration, while the configuration Ttf - Ttf with doubly occupied MOs 

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Among oxi tions producing allylic alcohols or their derivatives the modem variants of selenium dioxide oxidations are by far the most popular. Systems based on metal acetates, particularly palladium tri-fluoroacetate, can be very useful and are receiving increasing attention but the Kharasch-Sosnovsky reaction, once very common for allylic oxidation, is now rarely used. Sensitized photooxidation with singlet oxygen, a very well-known procedure, is still somewhat unpredictable and has periu K received less consideration than it deserves. 

When benzimidazole is photooxidized with singlet oxygen in methanol mainly dimeric products are isolated <85CZ231>. 2-Aminobenzimidazole behaves similarly, but 2-chlorobenzimidazole was... 

Oxidation photooxidation t,/2 = 0.32-3.2 h estimated, based on estimated rate constant for the reaction with hydroxyl radicals in air (Atkinson 1987 quoted, Howard et al. 1991) photooxidation t,/2(aq.) = 1,57 - 157 yr estimated, based on measured rate constant for the reaction with singlet oxygen in benzene (Stevens et al. 1974 quoted, Howard et al. 1991). 

Figure 4. Methanolic photooxidation of methoprene (reaction with singlet oxygen (40)...

This process continually generates lipid free radicals. The formation of nonradical products resulting from the combination of two radical species can terminate this chain reaction or propagation. Alternatively, unsaturated lipids can form hydroperoxides by reacting with singlet oxygen produced by sensitized photooxidation, which is a non-free-radical process. 

Tetradehydrodianthracene undergoes 4 + 2-cycloaddition with electron-deficient dienes such as 1,2,4,5-tetrazines.263 The Diels-Alder reactions of [3.3 ortho-anthracenophanes witli A-(p-nitro, chloro, or methoxy-substituted phenyl)malehnides yield approximately equal quantities of inside and outside adducts.264 The photooxidation of bulky water-soluble 1,4-disubstituted naphthalenes with singlet oxygen yields both the expected 1,4- and the unexpected 5,8-endoperoxides.265... 

Photooxygenation of vinylsilane in the presence of acetic anhydride and pyridine furnishes a convenient procedure for the synthesis of a-trimethylsilylenones 185 in moderate to good yields (equation 156)278. Dye-sensitized photooxidations of allylsilane with singlet oxygen produces a mixture of (E)- and (Z)-3-trimethyl-silylallyl hydroperoxides 186 (equation 157)279. 

MPT reacts somewhat more slowly with singlet oxygen than PTH. The combined rate constants of physical quenching (e.g. Equation 32) and chemical reaction (e.g. Equation 27) have been calculated from photooxidation experiments in bromobenzene/methanol (2/1) to be 1.2x10 M s-1 for MPT and 4.2x10 for PTH (26). Hovey has calculated... 

The photooxidation of colchicine with singlet oxygen has been reported by Seitz and co-workers, Fig. (5) [22]. This [4+2] cycloaddition produced the endoperoxide 42 stereoselectively, which could be isolated in 85% yield. Endoperoxide ring opening with triphenylphosphine... 

Pyrene can sensitize the photooxidation of 1,3-diphenylisobenzofuran (72) in DTAC micellar solutions 61>. The reaction involves sensitization of singlet oxygen by pyrene which diffuses into another micelle and reacts with (72). Indole and tryptophan, which also react with singlet oxygen, quench the above reaction in ethanol solutions. However, in micellar systems they enhance the rate of reaction. Because of the high local concentrations of the quencher, the pyrene excited state is quenched by indole and tryptophan which leads to the photooxidation of (72) by a Type I process. 

Sensitized photooxidation of silyl enolates with singlet oxygen followed by reduction of the hydroperoxides with PPhs in methanol has been used for the preparation of chiral and achiral ketones such as (/f)-6-methyl-2-cyclohexen-l-one starting from R)-2-methy Icy clohexanone. ... 

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