Singlet oxygen/oxidation

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. 

Stratton, S.P., Schaefer, W.H., and Liebler, D.C., Isolation and identification of singlet oxygen oxidation products of beta-carotene, Ghent. Res. Toxicol, 6, 542, 1993. 

The allylic hydroperoxides generated by singlet oxygen oxidation are normally reduced to the corresponding allylic alcohol. The net synthetic transformation is then formation of an allylic alcohol with transposition of the double bond. 

The reactivity order of alkenes is that expected for attack by an electrophilic reagent. Reactivity increases with the number of alkyl substituents.163 Terminal alkenes are relatively inert. The reaction has a low AHl and relative reactivity is dominated by entropic factors.164 Steric effects govern the direction of approach of the oxygen, so the hydroperoxy group is usually introduced on the less hindered face of the double bond. A key mechanistic issue in singlet oxygen oxidations is whether it is a concerted process or involves an intermediate formulated as a pcrcpoxide. Most of the available evidence points to the perepoxide mechanism.165... 

In the literature, there are also several examples where the specific oxidation reaction is dependent on the presence of a diene. Often the substrates are conjugated dienes where there is simultaneous oxidation of both double bonds. An important prototype of this reaction is the well-known singlet oxygen oxidation of dienes to endoperoxides, for example the oxidation of a-terpinene (4) to ascaridole (5) (equation 1). 

Stille coupling (12, 56).1 The key step in a synthesis of (E)-neomanoalide (4) involves palladium-catalyzed coupling of an allyl halide with an a-stannylfuran. Thus 1 and 2 couple in the presence of Pd(dba)2 and P(C6H5), to form 3 as a 1 1 mixture of (E)- and (Z)-isomers in 66% yield. Conversion of 3 to 4 involves reduction (DIBAH) and selective singlet oxygen oxidation of a 2-silylfuran to a butenolide. 

Alkyl-4-hydroxybutenolides. This group is present in some marine sponges and is believed to arise from 3-alkylfurans. This transformation can be realized in vitro by singlet oxygen oxidation in the presence of a hindered base such as ethyl-diisopropylamine. ... 

Dioxaphosphirane, a novel three membered ring of P-O-O species, has been suggested as an intermediate for singlet oxygen oxidation of trivalent phosphorus and for the decomposition of bis(phosphinic) peroxides . Akiba and coworkers reported the first isolation and structure of the dioxaphosphirane species, the phosphate 75 bearing Martin ligands and a novel three-membered ring . ... 

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