Radicals chloramines

Protonated /V-chloroalkyl amines under the influence of heat or uv light rearrange to piperidines or pyrroHdines (Hofmann-Lriffler reaction) (88). The free-radical addition of alkyl and dialkyl-/V-chloramines to olefins and acetylenes yields P-chloroalkji-, P-chloroalkenyl-, and 8-chloroalkenylamines (89). Various N-hiomo- and N-chloropolyfluoroaLkylarnines have been synthesized whose addition products to olefinic double bonds can be photolyzed to fluoroazaalkenes (90). 

Witko-Sarsat, V., Nguyen Anh Thu, Knight, J., and Descamps-Latscha, B. (1992). Pholasin a new chemiluminescent probe for the detection of chloramines derived from human phagocytes. Free Radic. Biol. Med. 13 83-88. 

A more practical, atom-economic and environmentally benign aziridination protocol is the use of chloramine-T or bromamine-T as nitrene source, which leads to NaCl or NaBr as the sole reaction by-product. In 2001, Gross reported an iron corrole catalyzed aziridination of styrenes with chloramine-T [83]. With iron corrole as catalyst, the aziridination can be performed rmder air atmosphere conditions, affording aziridines in moderate product yields (48-60%). In 2004, Zhang described an aziridination with bromamine-T as nitrene source and [Fe(TTP)Cl] as catalyst [84]. This catalytic system is effective for a variety of alkenes, including aromatic, aliphatic, cyclic, and acyclic alkenes, as well as cx,p-unsaturated esters (Scheme 28). Moderate to low stereoselectivities for 1,2-disubstituted alkenes were observed indicating the involvement of radical intermediate. 

As mentioned earlier, MPO-hydrogen peroxide-chloride system of phagocytes induces the formation of lipid peroxidation products in LDL but their amount is small [167-169], It was proposed that HOCL can decompose the lipid hydroperoxides formed to yield alkoxyl radicals [170]. It was also suggested that chloramines formed in this process decompose to free radicals, which can initiate lipid peroxidation [171]. 

HOCl-mediated protein oxidation accelerates under pathophysiological conditions. Thus, proteins from extracellular matrix obtained from advanced human atherosclerotic lesions contained the enhanced levels of oxidized amino acids (DOPA and dityrosine) compared to healthy arterial tissue [44], It was also found that superoxide enhanced the prooxidant effect of hypochlorite in protein oxidation supposedly by the decomposition of chloramines and chlor-amides forming nitrogen-centered free radicals and increasing protein fragmentation [45], In addition to chlorination, hypochlorite is able to oxidize proteins. The most readily oxidized amino acid residue of protein is methionine. Methionine is reversibly oxidized by many oxidants including hypochlorite to methionine sulfide and irreversibly to methionine sulfone [46] ... 

For example, the chloramine LV failed to rearrange when heated with base, a condition not unfavorable for the non-chain radical mechanism.812 Yet the corresponding hydroxylamine rearranged with phosphorus pentachloride in ether.813... 

The concentration of copper(II) has a pronounced effect on the course of the reaction. In the presence of very low copper(II) concentrations, oxidation of allyl radical 69 is slow and major amounts of allyl radical dimer are formed. In the presence of very high concentrations of copper(II), radical 68 is oxidized rapidly before addition to diene can take place. An optimum yield of product 71 can therefore only be achieved at certain copper(II) concentrations. The metal-ion-promoted addition of chloramines to butadiene appears to follow the same mechanism93. 

Chemical/Physical. Under atmospheric conditions, the gas-phase reaction of o-xylene with OH radicals and nitrogen oxides resulted in the formation of o-tolualdehyde, o-methylbenzyl nitrate, nitro-o-xylenes, 2,3-and 3,4-dimethylphenol (Atkinson, 1990). Kanno et al. (1982) studied the aqueous reaction of o-xylene and other aromatic hydrocarbons (benzene, toluene, w and p-xylene, and naphthalene) with hypochlorous acid in the presence of ammonium ion. They reported that the aromatic ring was not chlorinated as expected but was cleaved by chloramine forming cyanogen chloride. The amount of cyanogen chloride formed increased at lower pHs (Kanno et al., 1982). In the gas phase, o-xylene reacted with nitrate radicals in purified air forming the following products 5-nitro-2-methyltoluene and 6-nitro-2-methyltoluene, o-methylbenzaldehyde, and an aryl nitrate (Chiodini et ah, 1993). 

The A -methyl derivative (32) was obtained from 31 via a Leuckart reaction and isolated as its hydrochloride 32 is also formed in the Hoffmann-Lbflfterreaction(photolysisinsulfuricacid)ofthei r-chloramine (33), since after separation of secondary amines and addition of methyl iodide a 10% yield of the methiodide (34) was obtained. The secondary amine (31) was also converted to its A-acyl and JV -nitroso derivatives (35-37) and (38), respectively, by conventional procedures. Free-radical chlorination of 37 gave the ca o-2-chloro derivative (39) and... 

Nucleic Acids. - Recent EPR studies of radical attack and formation upon DNA are reported elsewhere in this volume, by Sevilla and Becker, and therefore will not be described here in any detail. Findings of particular biomedical relevance include the spin trapping of radicals resulting from the decomposition of chloramines formed in the reaction of HOC1 with polynucleosides, RNA and... 

Hazell LJ, Davies MJ, Stocker R (1999) Secondary Radicals Derived from Chloramines of Apolipoprotein B-10 Contribute to HOCl-Induced Lipid Peroxidation of Low-Density Lipoproteins. Biochem J 339 489... 

Hawkins CL, Davies MJ (2001) Hypochlorite-Induced Damage to Nucleosides Formation of Chloramines and Nitrogen-Centered Radicals. Chem Res Toxicol 14 1071... 

Hawkins CL, Pattison DI, Davies MJ (2002) Reaction of Protein Chloramines with DNA and Nucleosides Evidence for the Formation of Radicals, Protein-DNA Cross-Links and DNA Fragmentation. Biochem J 365 605... 

B. Aminium Cation Radical Cyclizations from N-Chloramines and... 

The use of /V-chloramines, in principle, allows the facile generation of aminyl radicals upon UV photolysis in neutral media. A radical chain can be envisioned for the formation of 2-chIoromethylpyrrolidines (Scheme 7). In practice, however, there is a slow step in this sequence, step A and/or B, such that other reaction pathways, disproportionation or H-abstraction from the solvent, compete. Surzur has studied the reaction in Scheme 7 in the alcoholic solvents MeOH and /-PrOH, which serve as hydrogen atom sources, and achieved acceptable ratios of cyclic products 25 and 26 to acyclic amine 27 (70TL3107). Other /V-chloroalkenylamines gave similar results (71TL903 80TL287). /8-chloro-substituted amine products such as 25 were the sole products when the reactions were carried out in acetic acid-water mixtures these reactions involve aminium cation radicals and are discussed further in Section III,B. 

As stated in the introduction, chloramine-T (where T denotes three crystalline water molecules) is a commonly used nitrene precursor, which is commercially available and costs less than do most other nitrene sources. The benefit of a silver salt in nitrene transfer reactions with chloramine-T is surprisingly simple. Because silver chloride is insoluble in most solvents, substoichiometric amounts of silver salts (like silver nitrate) can be used to remove the chloride from chloramine to facilitate the release of a free nitrene radical, which can aziridinate olefins. Since the amount of silver is near stoichiometric, it should not be called silver-based catalysis, although turnover numbers (TONs) higher than 1 have been observed in some cases. 

Using BDD anodes and MIO cathodes, enrichment of ammonia was also observed - in contradistinction to other studies using higher ammonium concentration and an Ir02 anode (Kim et al. 2005). In the combination of BDD anode/BDD cathode, nitrite was oxidised but only relatively slowly. When nitrate was electrolysed, its depletion was lower than 1 ppm for current densities lower than 200 A m-2. An explanation for the relatively low reaction rate between radicals and nitrite is the assumption that ozone or radicals are consumed in faster reactions such as peroxide formation and chlorine oxidation. OH radicals are also able to oxidise chloramines (Huie et al. 2005). 

Recently it was found that the reaction employs intermediary nitrogen-centered free radical formation via thermal homolysis of the N—Cl bond (H11). The stable chloramine T was effectively used for relatively specific oxidation of cysteinyl residues exposed to the surface of the protein molecule (S25). 

The other amino acid residue present in proteins that is susceptible to oxidation is the indole moiety of tryptophan (Fig. 11). The reducing potential of tryptophan is considerably less than that of cysteine and methionine, so oxidation of tryptophanyl residues usually does not occur until all exposed thiol residues are oxidized. Also, the spontaneous oxidation of tryptophanyl residues in proteins is much less probable than that of cysteinyl and methionyl residues. Tryptophan residues are the only chromophoric moieties in proteins which can be photooxi-dized to tryptophanyl radicals by solar UV radiation, even by wavelengths as long as 305 nm (B12). Tryptophanyl residues readily react with all reactive oxygen species, hypochlorite, peroxynitrite, and chloramines. Oxidative modifications of other amino acid residues require use of strong oxidants, which eventually are produced in the cells. Detailed mechanisms of action of these oxidants is described in subsequent sections of this chapter. 

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