Molecular Oxygen Cation-Radical

In any event, 02SbFg and 02AsFg act as very effective one-electron oxidants with regard to aromatic amines, nitrogen- and sulfur-containing heterocyclic compounds (Dinnocenzo and Banach 1986, 1988,1989) as well as to perfluorobenzene, benzotrifiuoride, and perfluoronaphthalene (Richardson et al. 1986). The oxidation proceeds in Freon (CHCIF2) at temperatures from -115 to -145°C, and the formation of the organic cation-radicals has been firmly established. 

Carbon dioxide, CO2, is a typical component of the gaseous environment for reactions in air or in the presence of air traces. Therefore, both interactions between CO2 and organic ion-radicals as well as reactions of 62 with uncharged molecules of organic compounds should be considered. Interaction of CO2 with organic anion-radicals leads, as a rule, to carboxylic acids COj anion-radicals are not formed. Even such a one-electron reductant as the superoxide ion (in aprotic medium) simply adds to carbon dioxide CO2 + O2 - 00-C02. The additional product accepts an electron  

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Fig. 8.9 Possible mechanisms of the bioluminescence reaction of dinoflagellate luciferin, based on the results of the model study (Stojanovic and Kishi, 1994b Stojanovic, 1995). The luciferin might react with molecular oxygen to form the luciferin radical cation and superoxide radical anion (A), and the latter deproto-nates the radical cation at C.132 to form (B). The collapse of the radical pair might yield the excited state of the peroxide (C). Alternatively, luciferin might be directly oxygenated to give C, and C rearranges to give the excited state of the hydrate (D) by the CIEEL mechanism. Both C and D can be the light emitter.

Studies on carotenoid autoxidation have been performed with metals. Gao and Kispert proposed a mechanism by which P-carotene is transformed into 5,8-per-oxide-P Carotene, identified by LC-MS and H NMR, when it is in presence of ferric iron (0.2 eq) and air in methylene chloride. The P-carotene disappeared after 10 min of reaction and the mechanism implies oxidation of the carotenoid with ferric iron to produce the carotenoid radical cation and ferrous iron followed by the reaction of molecular oxygen on the carotenoid radical cation. Radical-initiated autoxidations of carotenoids have also been studied using either radical generators like or NBS.35... 

Addition of 1,5-dithiacyclooctane to zeolite CaY in the presence of molecular oxygen results in spontaneous oxidation to mono- and bis-sulfoxides through formation of the corresponding radical cation characterized by ESR and diffuse reflectance of UV-Vis spectroscopy.51... 

The second example depicted in Scheme 3.64 is the trioxotriphenylamine cation-radical. Kuratsu et al. (2005) compared structures of the cation-radical and its neutral counterpart. The neutral compound has a shallow bowl structure, whereas the cation-radical has a planar structure. In the latter, spin delocalization embraces a whole molecular contour, involving the oxygen atoms. This contribntes to the cation-radical stability. (The solid species is easily formed after oxidation of the neutral parent compound with tris(p-bromophenyl)aminiumyl hexafluorophosphate in methylene... 

Paraquat (1,1 dimethyl, 4,4 bipyridyl) is a nonselective contact herbicide. It is used almost exclusively as a dichloride salt and usually is formulated to contain surfactants. Both its herbicidal and toxicological properties are dependent on the ability of the parent cation to undergo a single electron addition, to form a free radical that reacts with molecular oxygen to reform the cation and concomitantly produce a superoxide anion. This oxygen radical may directly or indirectly cause cell death. Diquat, l,T-ethylene-2,2 -dipyridylium, is a charged quaternary ammonium compound often found as the dibromide salt. The structure of diquat dibromide and that of the closely related herbicide paraquat can be seen in Fig. 4.5. 

Type 1 intrazeolite photooxygenation of alkenes has been also reported to give mainly allylic hydroperoxides (Scheme 42). In this process, the charge transfer band of the alkene—O2 complex within Na-Y was irradiated to form the alkene radical cation and superoxide ion. The radical ion pair in turn gives the allylic hydroperoxides via an allylic radical intermediate. On the other hand, for the Type II pathway, singlet molecular oxygen ( O2) is produced by energy transfer from the triplet excited state of a photosensitizer to 02. 

Electron spin resonance (ESR) methods have been used to observe the formation of the radical cations and dications of benzoll,2- 4,5- ]bis[l,2,3]trithiole 13 and benzo[l,2-4 4,5- ]bis[l,2,3]dithiazole 17, and the experimental results confirm the ab initio calculations performed <2003EJ04902, 1997JA12136>. ESR has also been used to confirm the formation of superoxides upon photolysis of aryl benzobisthiazoles and aryl benzobisoxazoles in the presence of molecular oxygen <2003MM4699>. 

When carried out in the presence of molecular oxygen, PET reactions between some donor-acceptor pairs yield oxygenated products. The radical anions of acceptors with appropriate reduction potentials reduce O2 to superoxide ion, O2, which then couples to the cations. In the PET reaction between trani-stUbene and... 

In some cases, the structures of oxygenation products have been crucial for assigning the structures of unusual radical cations. Eor example, the endo-peroxides (83 and 85) support the structures assigned to radical cations (24 and 84 ) derived from l,l-diaryl-2-methylenecyclopropane (23) and 2,5-diaryl-l,5-hexadiene, respectively.Time-resolved spectroscopic data suggest that 83 is generated by coupling of triplet biradical (24 ) with (triplet) molecular oxygen. 

In neutral aqueous solutions, the ultimate product of one-electron abstraction from guanine is the guanine neutral radical. In DNA, this radical is formed via the deprotonation of the guanine radical cation arising, e.g., from hole localization or directly via proton-coupled electron transfer from guanine to an appropriate electron acceptor. The G(-H) radicals do not exhibit observable reactivities with molecular oxygen (fc<10 s ) [93]. 

Most mechanistic studies have focused on elucidation of the role of alkali promoters. The addition of Li+ to MgO has been shown to decrease the surface area and to increase both methane conversion and selective C2 production.338,339 As was mentioned, however, besides this surface-catalyzed process, a homogeneous route also exists to the formation of methyl radicals.340-342 The surface active species on lithium-doped catalysts is assumed to be the lithium cation stabilized by an anion vacancy. The methyl radicals are considered to be produced by the interaction of methane with O- of the [Li+0-] center330,343 [Eq. (3.32)]. This is supported by the direct correlations between the concentration of [Li+0 ] and the concentration of CH3 and the methane conversion, respectively. The active sites then are regenerated by dehydration [Eq. (3.33)] and subsequent oxidation with molecular oxygen [Eq. (3.34)] ... 

Tetrahydropterins are highly reactive towards oxidation (e.g. 542 — 544) even molecular oxygen can cause hydroxylation. The autoxidation is due to the electron donating groups such as amino and hydroxy, whereas removal of such substituents enhances the stability of the reduced pteridine nucleus tremendously (96CHEC-li(7)70l). The reaction appears to proceed via single electron transfer. The radical cation (543) can be observed by cyclic voltammetry. 

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