Photooxidative pathways

A similar photooxidation pathway was found for Mg(ll)TPP. It reacted readily with molecular oxygen to give the corresponding 15,16-dihydrobiliverdin, similar to the one shown for Mg(II)OEP in Scheme 1. Further studies have proposed that the photooxygenation of metallo-me o-tetrasubstituted porphyrins proceeds via a one-molecule mechanism involving only one oxygen molecule. Most likely, the first intermediates formed upon photooxygenation are short-lived peroxides. Such compounds are very unstable and a possible dioxetane structure is shown in formula 31. 

Not all photooxidation pathways can be attributed to radical cations as the only intermediate. For example, in the benzophenone-sensitized reaction, some and P—and a—>p isomerization (in the case of a-dGuo) is observed (Vialas et al. 1999). while no such isomerization occurs upon the Mn-TMPyP-mediated oxidation by KHS05 (Vialas et al. 1999). Here, Iz is the only detected product [90% yield Vialas et al. 1998] which points to different intermediates in these two systems. Ascorbate or cysteine, may intercept G% and in their presence the oxazolone yield is strongly reduced (Douki et al. 1999). 

Figure 18. Photooxidation pathways for histidine (1), tyrosine (2), tryptophan (3), ana methionine (4) (16). Wavy lines through structures indicate ring scissions. Cysteine is oxidized to cystine, in some cases, without sensitizing dye cystine may...

We conclude that the photooxidative degradation of SAN is initiated by the styrene units. The mechanism of SAN presents three ways of degradation a classical photooxidation pathway of PS, an oxidation of the acrylonitrile units initiated by the oxidation of the adjacent styrene units and a degradation of the acrylonitrile units due to the formation of acids among the photoproducts. 

Fig. 7. (A) Photooxidation pathways of the alternate electron donors Cyt b559 and Chiz in PS II. (B) effect of illumination temperature on the photooxidation of competing electron donors in PS II as monitored by EPR signal intensities. See text for discussion. Plot (B) reproduced from Thompson and Brudvig (1988) Cytochrome b559 may function to protect photosystem II from photoinhibition. Biochemistry 27 6656.

Krieg, M. and Whitten, D.G. (1984a) Self-sensitized photo-oxidation of protoporphyrin IX and related porphyrins in erythrocyte ghosts and microemulsions a novel photooxidation pathway involving singlet oxygen, J. Photochem., 25 235-252. 

Fig. 6. The initial degradation pathway for thermooxidation and photooxidation. The free radical X is generated by the effect of heat or light on impurities,...

Photooxidation is also observed to reduce the photoluminescence of Alq3 [174]. However, the details of the chemical pathway is not so thoroughly explored for this material. 

Chen, J., Eberlein, L., and Langford, C.H. (2002) Pathways of phenol and benzene photooxidation using Ti02 supported on a zeolite. Journal of Photochemistry and Photobiology A Chemistry, 148 (1-3), 183-189. 

The principal pathway leading to degradation of acrylonitrile in air is believed to be photooxidation, mainly by reaction with hydroxyl radicals (OH). The rate constant for acrylonitrile reaction with OH has been measured as 4.1 x 10" cm /molecule/second (Harris et al. 1981). This would correspond to an atmospheric half-life of about 5 to 50 hours. This is consistent with a value of 9 to 10 hours measured in a smog chamber (Suta 1979). 

The fluorescence lifetime was determined to be 1124ps for 35a, 785 ps for 35b, and 831 ps for 43 in dichloromethane, whereas in the corresponding amorphous films a nonexponential decay with shorter time constants was observed [118, 119]. These lifetimes are similar to the parent oligophenyls but different from fluorene (10 ns) [120, 121]. When applying oligophenyls as luminescent films, however, we must consider that photooxidation may occur if molecular oxygen is present [122, 123], The proposed pathway for the decomposition is... 

The present discussion is only concerned with the structure/redox capacity of the site responsible for the oxidation of water. The starting point is the evidence that the photosynthetic pathway is triggered by photooxidation of the chlorophylls in photosystem II. The need for chlorophylls to recover the electrons lost in photooxidation (in order to regenerate their ability to absorb light) induces water to undergo oxidation, according to ... 

The regioselectivity for the ene pathway, in the photooxidation of several para substituted /3,/3-dimethylstyrenes, was recently found to depend on the electronic nature of the aryl substituents . Electron-withdrawing substituents, such as p-CFs or p-F,... 

Here pn is 1,2-diaminopropane and bn is 2,3-diaminobutane. Decomposition of the amine cation radicals obtained by photooxidation of the ligands en, bn, and pn have been discussed by Moeller. The products of Co(en)33+ photolysis can be satisfactorily explained by postulating that carbon-carbon bondbreaking is the principal step in decomposition of the cation radical H2NCH2CH2NH2t.58 Presuming a similar mechanism to obtain in photoreduction of Co(pn)33+, there are then two possible reaction pathways leading to different products. 

In 1963, E. J. Bowen published his classic review The Photochemistry of Aromatic Hydrocarbon Solutions, in which he described two major reaction pathways for PAHs irradiated in organic solvents photodimerization and photooxidation mediated by the addition of singlet molecular oxygen, 02 ) (or simply 102), to a PAH (e.g., anthracene). For details on the spectroscopy and photochemistry of this lowest electronically excited singlet state of molecular oxygen, see Chapter 4.A, the monograph by Wayne (1988), and his review article (1994). For compilations of quantum yields of formation and of rate constants for the decay and reactions of 02( A), see Wilkinson et al., 1993 and 1995, respectively. 

In Chapter 15 we address the consequences of the direct interaction of organic compounds with sunlight. This also forces us to evaluate the light regime in natural systems, in particular, in surface waters. Chapter 16 then deals with reactions of organic chemicals with photochemically produced reactive species (photooxidants) in surface waters and in the atmosphere. Note that in Chapters 15 and 16, the focus is on quantification of these processes rather than on a discussion of reaction pathways. 

In this study, we suggest that CO was not the intermediate of C02 in the photooxidation of benzene. It was also shown that the selectivities to C02 and CO were 93% and 7%, respectively, and almost independent of the concentration of 02, and H20 in the range of 0.8% to 2.2%. The invariability of the values may imply that the formation of C02 and CO may proceed in different pathways in the photoreaction and the contribution of each pathway may not be much changed by varying these conditions. 

An additional oxidation pathway is observed for 13 with a methyl group at the C-atom of the silene moiety. Here, the primary adduct of 13 and 302 under photo lytic conditions, i.e. triplet diradical 17, can either ring-close to give the corresponding dioxetane (the dioxetane is not stable under the photolytic conditions required to induce the photooxidation, and therefore cannot be observed) leading to products derived from it, or produce dimethylvinylsilyl hydroperoxide (18) via H-abstraction from a methyl group (equation 6). 

By H NMR monitoring of the oxidation of benzene oxide-oxepine with dimethyldioxirane (DMDO), a significant by-product, oxepine 4,5-dioxide, was identified <1997CRT1314>. This fact supports the hypothesis that the route from oxepine to muconaldehyde proceeds via oxepine 2,3-oxide with a minor pathway leading to symmetrical oxepine 4,5-oxide. The DMDO oxidations provide model systems for the cytochrome P450-dependent metabolism of benzene and atmospheric photooxidation of benzenoid hydrocarbons. 

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