Photochemical reactions endoperoxides

Formation of endoperoxides of type 178 can be interpreted as a L,4s + cycloaddition that involves singlet oxygen and therefore constitutes a photochemical reaction. Similarly, formation of hydroperoxides from nonconjugated olefins with an allylic hydrogen generally appears to be a concerted ene reaction as indicated in Scheme 65. Other mechanisms have... 

In contrast to the commonly observed, highly selective photochemical reactions, thermal rearrangements of endoperoxides often lead to more complex mixturex of products.A naturally occurring compound, the terpene endoperoxide ascaridole (3a) rearranges into isoascaridole (4a) upon irradiation (long-wavelength excitation at 366 nm) while similar cyclohexadiene endoperoxides 3 result in mixtures of bis-epoxides 4 and epoxyketones 5 upon thermolysis (Scheme 5). ... 

Carless, H.A.J., Atkins, R., and Fekarurhobo, G.K., Thermal and photochemical reactions of unsaturated bicyclic endoperoxides. Tetrahedron Lett., 26, 803, 1985. 

There are photocalorimetric enthalpy data for two other ozonides, both coming from sensitized photochemical dioxygenation reactions in non-polar solvents . This study reports the enthalpies of the reactions of 2,5-dimethylfuran and 1,3-diphenylisobenzofuran to their corresponding endoperoxides to be exothermic by —45 20 and —92 25 kJmoU (the latter value is an average of the values given for different solvents). From... 

The photochemical dimerization of unsaturated hydrocarbons such as olefins and aromatics, cycloaddition reactions including the addition of 02 ( A ) to form endoperoxides and photochemical Diels-Alders reaction can be rationalized by the Woodward-Hoffman Rule. The rule is based on the principle that the symmetry of the reactants must be conserved in the products. From the analysis of the orbital and state symmetries of the initial and final state, a state correlation diagram can be set up which immediately helps to make predictions regarding the feasibility of the reaction. If a reaction is not allowed by the rule for the conservation of symmetry, it may not occur even if thermodynamically allowed. 

In water, however, the endoperoxide is exclusively transformed by SN2 reaction with the solvent into the corresponding ring-opened hydroperoxide (Eq. 55), which then undergoes intramolecular fragmentation to the desired product [12]. Limitations in the choice of the solvent may in turn restrict the choice of the sensitizer for reasons of solubility and photochemical (and thermal) stability. As a result, experimental conditions for an optimal chemical yield may call for a sensitizer exhibiting a relatively small d>A. 

Another interest in the use of triplet oxygen lies in the oxidation of dienes with photochemical activation (Type 1, above) with formation of endoperoxides as products. The first example of this reaction was observed in the early 1970 s. Thus, reaction of ergosteryl acetate (107) in the presence of trityl tetrafiuoroborate and Lewis acids in the presence of light yielded the endoperoxide 108 (equation 28), With certain Lewis acids this reaction could be thermally, rather than photochemically, activated. Cation radicals were shown to be the intermediate active species, as was borne out by a comparative oxidation of the isomeric lumisteryl acetate which was inactive under these conditions but reacted easily with singlet oxygen . This reaction was later extended to other substrates. Thus, the intermediacy of cation radicals was also indirectly observed by the fact that the r-butyl substituted 1,3-cyclodiene 109 gave a dimeric product 110 (equation 29) via the cation radical intermediate in addition to the usual endoperoxide llOa ,... 

Hydrogen abstraction from lipids by triplet states of benzophenone derivatives followed directly by the use of laser flash techniques allows the separation of physical quenching processes from chemical reactions of the excited state. 98 xhe production of 2 by the photochemical decomposition of aromatic endoperoxides has also been studied by ps kinetic procedures. 99 mechanistic study has also been reported on the phototransformation of 3-nitrophenol in aqueous solution. 99 There is a strong wavelength dependence of the low quantum yield for the phototransformation in this system. 

Thermolysis of the endoperoxide in C5D5 gave lQ2( Ag) luminescence at 1270 nm which decayed exponentially as the peroxide reacted by a first-order reaction. In agreement with the photochemical work above and that reported earlier " , phthalocyanine quenched IO2 monomol emission. Weak luminescence near 700 nm was strongly enhanced by the addition of phthalocyanine. The 700 nm luminescence decay is also exponential, but the rate constant for the apparent decay is twice that observed at 1270 nm, in agreement with the photochemical work. At the same temperature and endoperoxide concentration, the decay curve of the luminescence at 700 nm is almost exactly the square of that at 1270 nm (Fig. 5). 

Pyrene and 4-methyl- and 4,5-dimethyl-pyrene have been prepared from 4//-cyclopenta[de/]phenanthrenes by a reaction sequence involving carbox-ylation and then conversion, through the ester, into the carbinol, which undergoes Wagner-Meerwein rearrangement. 5,12-Diphenylnaphthacene undergoes photochemical oxidation,giving a mixture of endoperoxides (397) and (398) the major product, (397), was subsequently converted into the diol (399). 

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