Autoxidation photosensitized

Secondary alcohols, such as isopropyl alcohol, j -butyl alcohol, 2-pentanol, 3-pentanol, cyclopentanol, and cyclohexanol, have been autoxidized to hydroxyaLkyl hydroperoxides (1, X = OH R = H) (10,44). These autoxidations usually are carried out at ca 20°C with uv radiation in the presence of a photosensitizer, eg, benzophenone. a-Oxygen-substituted dialkyl peroxides (2, X = Y = OH and X = Y = OOH), also are formed and sometimes they are the exclusive products (10). 

Complex reactions occur on the autoxidation of pyrroles (see Section 3.05.1.4) predictably, susceptibility to autoxidation increases with increasing alkyl substitution, llie photosensitized reaction of pyrrole and oxygen yields 5-hydroxy-A -pyrrolin-2-one, probably by way of an intermediate cyclic peroxide (Scheme 28) (76JA802). 

The type I mechanism is a radical process, and involves the excited state of the photosensitizer in electron-transfer processes, as indicated in Scheme 1. The reactions there are essentially photochemically stimulated autoxidation processes. 

Butenes were subjected to photosensitized reaction with molecular oxygen in methanol. 1-Butene proved unreactive. A single hydroperoxide, l-butene-3-hydroperoxide, was produced from 2-butene and isolated by preparative gas chromatography, Thermal and catalyzed decomposition of pure hydroperoxide in benzene and other solvents did not result in formation of any acetaldehyde or propionaldehyde. The absence of these aldehydes suggests that they arise by an addition mechanism in the autoxidation of butenes where they are important products. l-Butene-3-hydroperoxide in the absence of catalyst is converted predominantly to methyl vinyl ketone and a smaller quantity of methyl vinyl carbinol —volatile products usually not detected in important quantities in the autoxidation of butene. 

Like the porphyrins, the phthalocyanines can undergo photooxidation and act as photosensitizers for the production of singlet oxygen" One of the few chemical synthetic applications was the acceleration of the autoxidation of cumene and photooxidation of pinenes". ... 

Singlet oxygen. A probable intermediate in photosensitized autoxidations. Journal of the American Chemical Society, 86, 3880-3881. 

Following the autoxidation of cyclohexa-1,3-diene and cyclopenta-diene at 15 to 20°, Hock and Depke147 isolated small yields of the cyclic peroxides 147 and 148, respectively. 147 can be obtained in better yields by photosensitized oxidation of cyclohexa-1,3-diene (see Section IV, B) and by the action of hydrogen peroxide and sodium hypochlorite on cyclohexa-1,3-diene154. In the last reaction, oxygen... 

In true photosensitized autoxidation, the addition of a sensitizer, such as chlorophyll, eosin, or rose bengal, is essential. This method was pioneered by Windaus and Brunken178 in the steroid series (ergosterol peroxide) and by Schenck and Ziegler179 in the terpene series (ascari-dole). Schenck et al.10 180,181 generalized the applicability of photosensitized autoxidation. 

Schenck and Ziegler170 obtained the naturally occurring endoper-oxide ascaridole (166) by irradiation of an alcoholic solution of a-terpinene in the presence of eosin. Endoperoxides similar to 166 are formed in the photosensitized autoxidation of cyclopentadiene,182 cyclohexa-1,3-diene,179,182 cyclohepta-1-3-diene,183 and their phenyl-... 

The photosensitized autoxidation of terpinolene (170) leads initially to an allyl shift of the double bond with formation of the hydroperoxide intermediate (171), which reacts further to give the cyclic... 

In type I photosensitized oxidation, the triplet state sensitizer abstracts a hydrogen or electron from the unsaturated oil, producing radicals that initiate chain propagation as in autoxidation. However, chain-breaking antioxidants do not stop this reaction as new radicals are produced photochemically. In type II photooxidation, the energy of the triplet sensitizer is transferred to molecular oxygen, converting it... 

Hydroperoxide positional distributions in unsaturated fatty acids undergoing autoxidation and photosensitized oxidation are presented in Table 4. 

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). 

When alkenes are treated with oxygen that has been photosensitized (p. 341), they are substituted by OOH in the allylic position in a synthetically useful reac-tion. Although superficially similar to autoxidation, this reaction is clearly different because 100% allylic rearrangement always takes place. The reagent here is not... 

Indeed, the systematic studies by Wasserman et aL360-362 on the photosensitized autoxidation of oxazoles have shown that this system may be directed to undergo facile destruction of the aromatic ring selectively... 

The highly reactive intermediate isoimides of the type 209 have actually been obtained as reaction products when fused-ring oxazoles are employed as substrates. For example, the photosensitized autoxidation of the phenyl-substituted oxazole 211 in methanol yields the iV-benzoylimino anhydride (212 ).126-362 Further studies on the reaction of oxazoles with singlet oxygen... 

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