Singlet oxygen photooxidation

Figure 4.15 Dye sensitised singlet oxygen photooxidation reactions.

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... 

Singlet oxygen photooxidation also yields allylic hydrope-roxide shifted double bonds, according to the "ene pro-... 

The singlet oxygen photooxidation of polymers with pendant double bonds depends on the presence of methylene groups iy close vicinity to the double bond. For example 1.2-polybutadiene <1 08 not react with singlet oxygen whereas 1.2-poly(l,4-hexadiene) is reactive ... 

Photooxidafions are also iudustriaHy significant. A widely used treatment for removal of thiols from petroleum distillates is air iu the presence of sulfonated phthalocyanines (cobalt or vanadium complexes). Studies of this photoreaction (53) with the analogous ziuc phthalocyanine show a facile photooxidation of thiols, and the rate is enhanced further by cationic surfactants. For the perfume iudustry, rose oxide is produced iu low toimage quantifies by singlet oxygen oxidation of citroneUol (54). Rose bengal is the photosensitizer. 

Similarly, photooxidation of dihydrocoralyne (108) in hot methanol at pH 8, subsequent addition of sodium methoxide and additional irradiation yielded 6,7-dimethoxyisoquinolone and 3-methyl-3,5,6-trimetho-xyphthalide via the betainic intermediate 109 (77H45) (Scheme 39). It was demonstrated earlier that dihydrocoralyne is oxidized to this betaine in quantitative yields under physiological conditions (76H153). The autoox-idative degradation of the mesomeric betaine was rationalized by the addition of singlet oxygen. 

Okada and Mukai [48] showed a preference for a contrasteric approach of singlet oxygen to anti face of 7-isopropylidene double bond in photooxidation of 7-isopropylidenenorbomene followed by reduction with dimethyl sulfide (Scheme 32). They explained the stereoselectivity by applying the orbital mixing rules (Scheme 33). The r orbital of the exocyclic double bond enlarges its extention in the anti face. 

The photooxidation of cyclopentadiene by singlet oxygen is one step of an industrial process to make 2-cyclopentene-l, 4-diol [40]. Hence the driver is a commercial one, namely to develop a continuous synthesis of this molecule. 

Photooxidation of Dienes Investigated in Micro Reactors Cas/liquid reaction 24 [CL 24] Oxidation of cyclopentadiene by singlet oxygen to 2-cyclopentene-1,4-diol... 

On the other hand, carbonyl quenching by polyenyl radicals in PVC has been invoked in order to account for autoinhibition during photooxidative degradation (52). Some workers have mentioned the possibility of singlet oxygen formation in PVC via the quenching of excited polyenes (25,29) or cyclohexadienes (53). 

The oxidation of 2,5-disubstuted furans by singlet oxygen was exploited for the synthesis of [5,5,5] and [6,5,6] bis-spiroketals <06OL1945>. An unusual regioselective photooxidation of 3-bromofuran to 2- and 3-bromo- hydroxybutenolides, as depicted below, was reported. The mechanism for the observed base-dependent regioselective deprotonation of the endoperoxide intermediate was not determined <06OL4831>. 

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). 

Pace, A. and Clennan, E.L. (2002). A new experimental protocol for intrazeolite photooxidations. The first product-based estimate of an upper limit for the intrazeolite singlet oxygen lifetime. J. Am. Chem. Soc. 124, 11236-11237... 

Clennan, E.L. and Sram, J.P. (1999). Photooxidation in zeolites. Part 2 a new mechanistic model for reaction selectivity in singlet oxygen ene reactions in zeolitic medsia. 

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