Radical stability nitrones

The study of the stabilizing effect of the nitrone group in cumyl radical (205) (Scheme 2.72) (362) and cumyl cation (206) (Scheme 2.73) (363) has shown that the nitrone group appears to be a super radical stabilizer and, at the same time, a weak cation stabilizer. ... 

Selected values of krei (80 °C) are shown in Table I. The much smaller rate enhancement of proximal azoxy (krei = 2.00) compared to distal azoxy (kj.gi = 9.45) is in excellent accord with ethylene loss from 65 but not 68. Furthermore, theoretical calculations showed that the spin density on the substituent is much greater and that on the benzylic position is much smaller for distal azoxybenzyl radical 89 than for the proximal analog 90. Distal azoxy is among the most effective radical stabilizing groups but is not as strong as nitrone (kj-gi = 13.5). Obviously, both of... 

The presence of /3-hydrogen in the nitroxide radical may lead to disproportionation reactions. In spin-trapping experiments, N-t-butyl-a-phenyl nitrone yields rather unstable spin adducts. This type of radical can be stabilized by coordination to Nin. The Ni11 complex with N-oxy-A-r-butyl-(2-pyridyl)phenylmethanamine (923) reveals a distorted octahedral geometry with antiferromagnetic interactions between the unpaired electrons of the metal ion and the radical spins.00... 

In contrast, one-electron polarographic redaction of a,p-dinitrostilbene yields an anion-radical, wMch is stabilized in a nitronic form with a carboradical center. These radicals possess an enhanced electron affinity and are prone to the capture of the second electron at the first wave potential, with the formation of a stable dinitronic dianion. In the case of a,p-dinitrostilbene, the cathodic reduction cannot be stopped at the one-electron step (Todres 1991 see Scheme 2.12). 

Cyclization of nitro-stabilized radicals provides another method for the generation of cyclic nitronates (221). Oxidation of the aci-foim of nitroalkanes with ceric ammonium nitrate generates the ot-carbon centered radical, which in the presence of an alkene, leads to the homologation of the a-radical. In the case of a tethered alkene of appropriate length, radical addition leads to a cyclic nitronate (Scheme 2.20). 

Di(octadecyl)hydroxylamine (18) (Seltzer et al., 1989 PospiSil and Nespurek, 1997) was recently introduced commercially for the stabilization of PO, PP in particular. 18 is a hydrolysis-resistant processing stabilizer used in combination with aromatic phosphites, and a long-term heat stabilizer used in combination with suitable HAS. Di-alkylhydroxylamine is considered as scavenger of radicals POO" and P. The latter are trapped by nitroxide or nitrone, arising from the parent hydroxylamine (Eq. 3-7). 

Spin traps come in basically two types nitroso compounds and nitrone compounds. Reactive free radicals react with the carbon of the nitrone functional group to form a radical adduct that always has a nitroxide group, which is an unusually stable type of free radical. Nitrones are the most useful spin traps for the in vivo detection of free radical metabolites because of the stability of the resulting radical adduct. However, identification of the parent radicals can be difficult because adducts derived from different radicals often have very similar EPR spectra. A comprehensive review of this area through 1992 has recently been published [48]. 

Flash photolysis of misonidazole, metronidazole, and nitrobenzothiazoles has been carried out in [1369-1371], Laser flash-photolysis (355 nm) allows to determine relatively stable anion-radicals of misonidazole and metronidazole in aqueous solutions [1370], Solvated electrons have been formed at harder irradiation, the result of which interaction with nitroimidazole molecules is generation of their radical anions [1372], The authors [1372] have also found that fluorescence intensity of metronidazole is about 20 times more than that of misonidazole in same conditions. Photochromic properties of benzothiazole derivatives containing nitro and methyl groups in the ortho positions with respect to each other were studied by flash photolysis [1371], The application of the thermodynamic approach to predict the kinetic stability of formed nitronic acids is limited owing to specific intramolecular interactions. The lifetime of photoinduced nitronic acid anions tends to increase with rise in the chemical shift of the methyl protons. The rate constants photoinduced nitronic acids and their anions increase as the CH3C-CN02 bond becomes longer [1371],... 

The ways of getting around this problem involve increasing the lifetime of the radicals by some physical or chemical means. One such approach involves stabilizing the radicals by immobilization, for example, by freeze-quenching a reaction mixture [80]. The disadvantage of this method is that an immobilized radical is generally much harder to characterize and identify than one in fluid solution. Other approaches make use of the chemical reactivity of radicals, for example, their ability to add to the double bonds in nitrones and nitroso compounds. This has led to the development of the spin-trapping procedure [81,82], in which a transient radical is reacted with the... 

In the HPPD ozonation, the free radical chemistry seems to be more important than in DOPPD, due to the stabilizing effect provided by the diarylamine moiety (40). The nitroxide pathway is therefore of relevant importance in HPPH ozonation to amine oxide and side chain oxidation pathways. Because of the influence of N-substituent effects the ozonation of HPPH occurs only on the aliphatic side of the molecule and a nitrone is the most abundant ozonation product. 

The absolute rate of dissociation of the /j-nitrobenzyl chloride radical anion has been measured as 4 x 10 s The m-nitro isomer does not undergo a corresponding reaction, owing to the fact that the m-nitro group does not provide any resonance stabilization of the benzylic radical. The kinetics of the overall reaction supports a concerted electron transfer involving dissociation at the stage of electron transfer from the nitronate to the benzylic halide. ... 

Although polycyclic azo N,N -dioxides of type were patented (224) as excited-state quenchers and radical Inhibitors for example, for stabilizing pigments and polymers—It Is not yet known If those compounds attained any practical utilization. Besides the l-02-quenchlng ability, nitrones such as can effectively trap R radicals (225), and cyclic azo dioxides similar to can photochemlcally expel NO (226), thus affording the cyclic nltroxldes In both cases. 

Wider applications include nitrone spin traps, which are light insensitive, nontoxic, and lead to reasonably robust adducts. The adduct spectra do not distinguish between different primary radicals as these get inserted at the //-position further away from the nitroxide center. Phenyl-/er/-butylnitrone (PBN) and 5,5-dimethylpyrroline (DMPO) are most widely used for superoxide detection in hydro-phobic and hydrophilic compartments of biological samples, respectively. DMPO is the standard of choice for application in cell cultures as it readily crosses cell membranes. The spectrum of its superoxide adduct, DMPO-OOH, is shown in Figure 13. The interpretation of the trapping experiments is complicated by the poor stability of DMPO-OOH,... 

Nitrones formed by reaction (2.15) represent spin traps and can accept ARs. In some cases, such adducts can be more stable than radical adducts of the initial generation, so only adducts of the second and third generations will be observed in ESR experiments. ARs of high stability are formed when the nitrogen is connected with the tertiary carbon atom and the disproportionation is excluded [3]. Such radicals are, for example, di-t-butylaminoxyl (1), derivatives of 4,4-dimethyloxazolidineoxyl (2), 2,2,5,5,-tetramethylpyrrolinoxyl (3) and derivatives of piperidine-1-oxyl (4). 

In spin trapping experiments, relatively stable ESR-active compounds, the spin adducts, are formed by reaction of radicals with ESR-silent compounds, the spin traps, added to the smnpie. The most commonly used spin traps are nitroxides and nitrones, which form stabilized radicals by reaction with other radicals (23). Based on the characteristics of the spin adduct (e.g. hyperfine pattern, coupling constants, and g-value), an assignment of the radical in question is often possible. However, due to lack of specificity of the often-used nitroxides, like N-r-butyl-a-phenylnitro-ne (PBN), a valid verification of the radicals trapped depends on identification by tecimiques such as HPLC-MS. Despite the lack of spectral resolution, spin tr q>ping seems to be a promising technique for prediction of the oxidative susceptibility of dairy products (see later sections). 

This reaction also can be made to proceed with tertiary benzyl nitro compounds lacking the p-nitro substituent. The nitro substituent at the benzyl position provides sufficient stabilization to permit the electron transfer to proceed, generating the radical anion. This species decomposes to a tertiary benzyl radical by loss of nitrite ion. Substituted nitrocumyl systems can alkylate nitronate anions in HMPA sol-... 

Diels-Alder reactions, 133, 135 epoxidation, 69-72, 516 grafting on polyethylene, 462 hydroformylation, 44 hydrogenation, 41, 42 isomerization catalysts, 133, 484 isomerization during polymerizations, 484 isomerization kinetics, 484 isopropyl alcohol radical reaction, 207 MA copolymerization, 532, 534, 541 Michael reactions, 63-66 nitrone adducts, 224, 225 olefin copolymerization, 288 olefin ene reactions, 162 phenanthrene adducts, 181 plasticizers use, 14 production—synthesis, 14, 78-81 radical copolymerization, 270, 275-277, 307, 315, 317, 333, 345, 365, 379 radical polymerization, 239, 264, 287 reaction with allyl alcohol, 46 reaction with sodium bisulfite, 53 styrene copolymerization, 365, 483 tetraalkyl methylenediphosphonate adduct, 66 transesterification, 46 /7-xylylene copolymerization, 359 dialkyl stannyl, PVC stabilizer, 275 diaryl, synthesis from MA, 80 pyridinium, betaine intermediate, 216... 

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