Products in the Presence of Oxygen

In the presence of 02, most radicals are converted into the corresponding per-oxyl radicals with the notable exception of heteroatom-centered radicals which do not react with 02 at an appreciable rate (Chap. 8.2). However, even though peroxyl radical reactions may dominate in the reactions induced by the autoxi-dation of Fe(II)EDTA or Fe(II)NTA (Chap. 2.5), in the case of 2 -deoxynucleo-sides the subsequent reactions seem to be considerably modified by the presence of the transition metal ion, i.e. product ratios are found in these reactions which are different from those observed by ionizing radiation in the absence of Fe(II)/ Fe(III) (Murata-Kamiya et al. 1998). A basis for understanding these differences may be the various redox reactions that the peroxyl radicals will undergo with Fe(II)/Fe(III) (cf. Yurkova et al. 1999 Theruvathu et al. 2001 see also Chaps 2.5 and 8.3). 

In acid solutions, but also in neutral solutions at a high steady-state radical concentration, CV -elimination becomes too slow to be of importance as compared to the bimolecular decay of the peroxyl radicals. This leads to a very different product distribution (Table 10.15). 

Typical decay pathways, with tetroxides as short-lived intermediates (Chapter 8.8), lead to Ug, dialuric acid and O2 [reaction (125)], as well as to two mol dialuric acid and H2O2 [reaction (126)]. 

A detailed kinetic study is still missing for Cyt and its derivatives, but for dCyd product data are available (Wagner et al. 1999). They are compiled in Table 10.16 together with the products observed upon menadione photosensitization. 

In the y-radiolysis, similar reactions are expected to occur as discussed above for the Ura system. Here, however, about equal amounts of OH and 02 -are formed initially and thus the latter will play an even larger role on the way to the products, and thus it is even more difficult to come up with a well-substantiated reaction scheme. An interesting product is 5, 6-cyclo-5-hydroxy-5,6-dihydro-2 -deoxyuridine. Its formation is discussed below in the context of the reactions of alkyl radicals. 

The OH-adduct radicals of the pyrimidines react with oxygen at close to diffusion-controlled rates, yielding the corresponding peroxyl radicals. In basic solutions, but also in neutral solutions provided that these peroxyl radicals have a sufficiently long lifetime, the C(5)-OH,C(6)-peroxyl radicals can undergo superoxide elimination after deprotonation at N(l) [reactions (26) and (27)] 

In acid solutions, but also in neutral solutions at high steady-state radical concentrations, the superoxide elimination becomes too slow compared with the bimolecular decay of these peroxyl radicals [reactions (28)-(31)]. This leads to a very different product distribution, as seen in Table 5. There is evidence that in their bimolecular decay peroxyl radicals can give rise to the formation of oxyl radicals which may undergo fragmentation (see, e.g., [37, 38]) [e.g., reaction (30)], leading to products with the pyrimidine cycle destroyed (e.g., l-N-formyl-5-hydroxyhydantoin. Other pyrimidine-ring cleavage reactions are conceivable but at present not supported by product data). 

However, if non-tertiary, oxyl radicals in aqueous solution are capable of undergoing a rapid 1,2-H-shift [44-46] [reaction (34)] in competition with fragmentation. Then after addition of oxygen, superoxide is eliminated [reaction (36)]. Thus under these conditions also, superoxide radicals are likely intermediates which are expected to react with peroxyl radiceds, reducing them to the corresponding hydroperoxides. These are abundant, though somewhat unstable, products in the radiolysis of air-saturated pyrimidine solutions. They are considered to be important precursors of the pyrimidine glycols observed under various conditions [1]. 

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Photolysis of 1,4-diphenylphthalazine 2-oxide in various solvents gives 1,3-diphenyl-isobenzofuran (52) as the primary product. In the presence of oxygen, deoxygenation to 1,4-diphenylphthalazine and oxidation of the initially formed 1,3-diphenylisobenzofuran (52) to 1,2-dibenzoylbenzene also take place (Scheme 17). 

The photopolymerization of this monomer with a mercury arc89,9°) produces small yields of low molecular-weight products. In the presence of oxygen an induction period is noted and the polymers contain an appreciable amount of peroxide units in the chains9 ). The photolysis of 2-vinylfuran was briefly described by Hiraoka92 cyclopentadiene and CO were reported as products. It is not certain if free radicals are involved in this photodecomposition, but presumably they are. 

It is evident from the nature of the products, especially those formed with toluene present, that the photoreaction in weakly acidic medium involves incursion of a radical species. The complete suppression of reactions leading to the above products, in the presence of oxygen, strongly suggests that it is an excited triplet trityl ion which undergoes reaction. It is postulated that the primary photochemical process is the abstraction of a hydrogen atom by the triplet trityl ion to form the radical cation 90, which was proposed as an intermediate in the dimerization reactions carried out in strong acid (Cole, 1970). 

C-C Cleavage in Epoxides. Radical cations generated by photoinduced electron transfer from epoxides (130) or aziridlnes (131) also ring open, giving oxidative products in the presence of oxygen. For example, dicyanoanthracene sensitizes the conversion of aryl epoxides to ozonides, eq. 48,... 

The reaction of canthaxanthin (/3,/3-carotene I,4 -dione) with zt z-chloroperbenzoic acid furnishes dihydrooxepins <1997TL7853>. Two isomeric 1,2,4-trioxolanes 166 have been obtained more recently as products of the same reaction when it was carried out with potassium 6-G-18 crown ether in CH2C12 the 13,14-CM-isomer is the main product in the presence of oxygen, whereas the 13.14-/ram-isomcr predominates when the solvent was degassed <1999T2307>. 

These radicals initiate chain reactions to produce the final low-weight products. In the presence of oxygen, additional reactions generating the superoxide radical are possible ... 

As almost all foreign compounds are distributed via the bloodstream, the components of the blood are exposed at least initially to significant concentrations of toxic compounds. Damage to and destruction of the blood cells results in a variety of sequelae such as a reduced ability to carry oxygen to the tissues if red blood cells are destroyed. Aromatic amines such as aniline and the drug dapsone (4,4-diaminodiphenyl sulphone) are metabolized to hydroxylamines, and in the latter case the metabolite is concentrated in red blood cells. Also, nitro compounds such as nitrobenzene, which can be reduced to hydroxylamines, are similarly toxic to red blood cells. These hydroxylamines are often unstable and can be further oxidized to reactive products, in the presence of oxygen in the... 

Photolysis of 2-phenylpropionic acid in solution (acetonitrile/methanol/benzene) under a nitrogen atmosphere produces ethylbenzene 2, the meso- (3) and dl- (4) forms of 2,3-diphenylbutane, l-(2-ethylphe-nyl)-l-phenylethane 6, l-(4-ethylphenyl)-l-phenylethane 7, and acetophenone 5, involving homolytic cleavage of the CC bond a to the carboxyl group affording 1-phenylethyl radical (PER) as the key intermediate. In solution, the radical is not stereoselective and can be converted to a mixture of meso-and d/-diphenylbutane. Acetophenone is the major product in the presence of oxygen (Scheme 5, photolysis of 2-PPA in solution). ... 

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