Radical-trapping studies

Radical trapping studies, 14 277 Radicidation, 8 655 Radioactive decay, 21 287—288 particles associated with, 21 291 Radioactive decay properties of uranium isotopes, 25 393 Radioactive emission, interaction with tracer molecules, 21 276 Radioactive iodine, protection from,... 

Udassin, R. Ariel, 1. Haskel, Y. Kitrossky, N. Chevion, M. Salicylate as an in vivo free radical trap studies on ischemic insult to the rat intestine. Free Radic. Biol. Med. 10 1-6 1991. 

The BDE theory does not explain all observed experimental results. Addition reactions are not adequately handled at all, mostly owing to steric and electronic effects in the transition state. Thus it is important to consider both the reactivities of the radical and the intended coreactant or environment in any attempt to predict the course of a radical reaction (31). Application of frontier molecular orbital theory may be more appropriate to explain certain reactions (32,33). Radical reactivities have been studied by esr spectroscopy (34-36) and modeling based on general reactivity and radical polarity (37). Recent radical trapping studies have provided considerable insight into the course of free-radical reactions, particularly addition polymerizations, using radical traps such as 2,4-diphenyl-4-methyl-l-pentene (a-methylstyrene dimer, MSD) (38-44) and 1,1,3,3-tetramethyl-2,3-dihydro-liT-isoindol-2-yloxyl (45-49). 

Radical traps used for the study of radical monomer reactions should meet a number of criteria ... 

Tabic 3,11 Radical Trapping Agents for Studying Initiation... 

Chalfont ei a m were the first to apply the spin trapping technique in the study of radical polymerization. They studied radicals produced during S polymerization initiated by r-butoxy radicals with 108 as the radical trap. Since... 

Alkyl mercuric hydrides are generated in situ by reduction of an alkyl mercuric salt with sodium borohydridc (Scheme 3.91). Their use as radical traps was first reported by Hill and Whitesides491 and developed for the study of radical-olefin reactions by Giese,489490 Tirrell492 and coworkers. Careful choice of reagents and conditions provides excellent yields of adducts of nucleophilic radicals (e.g. -hexyl, cyclohexyl, /-butyl, alkoxyalkyl) to electron-deficient monomers (e.g. acrylics). 

Continuing his studies on the metallation of tetrahydro-2-benzazepine formamidines, Meyers has now shown that the previously unsuccessful deprotonation of 1-alkyl derivatives can be achieved with sec-butyllithium at -40 °C <96H(42)475>. In this way 1,1-dialkylated derivatives are now accessible. The preparation of 3//-benzazepines by chemical oxidation of 2,5- and 2,3-dihydro-l/f-l-benzazepines has been reported <96T4423>. 3Af-Diazepines are also formed by rearrangement of the 5//-tautomers which had been previously reported to be the products of electrochemical oxidation of 2,5-dihydro-lAf-l-benzazepine <95T9611>. The synthesis and radical trapping activities of a number of benzazepine derived nitrones have been reported <96T6519, 96JBC3097>. 

The chemical reactivity of the organoruthenium and -osmium porphyrin complexes varies considerably, with some complexes (M(Por)R2, M(Por)R and Os(OEP)(NO)R) at least moderately air stable, while most are light sensitive and Stability is improved by handling them in the dark. Chemical transformations directly involving the methyl group have been observed for Ru(TTP) NO)Me, which inserts SO2 to form Ru(TTP)(N0) 0S(0)Me and Ru(OEP)Me which undergoes H- atom abstraction reactions with the radical trap TEMPO in benzene solution to yield Ru(OEP)(CO)(TEMPO). Isotope labeling studies indicate that the carbonyl carbon atom is derived from the methyl carbon atom. "" Reaction of... 

Pharmacologic neuroprotection, which might be expected to prevent tissue necrosis or apoptosis until tissue reperfusion can be achieved with rt-PA, is a theoretically attractive adjunct to rt-PA treatment. Despite positive studies in animals, all evaluations of neuroprotective agents in humans have failed. Most recently, the promising initial results for intravenous NXY-059, a ffee-radical-trapping agent, were not replicated in a confirmatory phase III trial (unpublished data). 

Lai, C.S. and Piette, L.H. (1978) Spin-trapping studies of hydroxyl radical production involved in lipid peroxidation. Arch. Biochem. Biophys. 190, 27-38. 

Albano, E., Lott, K.A.K., Slater, T.F., Stier, A., Symons, M.C.R.and Tomasi, A. (1982). Spin trapping studies on the free radical products formed by metabolic activation of carbon tetrachloride in rat liver microsomal fractions, isolated hepato-cytes and in vivo in the rat. Biochem. J. 204, 593-603. 

Connor, H.D., Gao, W., Nukina, S., Lemasters, J.J., Mason, R.P. and Thurman, KG. (1992). Evidence that free radicals are involved in graft failure following orthotopic liver transplantation in the rat an electron paramagnetic resonance spin trapping study. Transplantation 54, 199-204. 

Secondary Photochemical Processes. While the nature of the primary photochemical step may be described as still uncertain, the nature of the subsequent secondary steps is best characterized as obscure. A previous trapping study during exhaustive irradiation (30) demonstrated that silylenes are formed somewhere along the line and implicated silyl radicals as well since the formation of Si-H bonds was observed, presumably by hydrogen atom abstraction. 

The substituted hydroxylamine C NOPP from reaction 7) can take part in various dark reactions, even at ambient temperature. From a study of the low molecular weight model I in the liquid phase, two decomposition pathways are possible (reaction 8) (12). The products from the disproportionation reaction 8a were only observed in the absence of a radical trap such as O2. In a given solvent ks kaa-Uo (solvent air saturated and degassed respectively). Both k8a and ke were found to increase by an order of magnitude on going from a non-polar solvent (iso-octane) to a polar solvent (methanol or tert.-butyl hydro peroxide, BuOOH). 

We consider, primarily, events in solids since most e.s.r. studies have been carried out on radicals trapped in solids. Only relatively persistent organometallic radicals have been studied by liquid-phase e.s.r. with in situ radiolysis, because of the technical difficulties involved. In most solid systems at low temperature radical centres are physically trapped in the rigid matrix and hence can be studied by e.s.r. without difficulty. However, although radicals as such may be immobile, this does not necessarily apply to electron-gain or -loss centres, particularly if these are charged, since electron-transfer may be facile. 

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

Organometallic radicals are important intermediates in biological and catalytic reactions. The structure and formation mechanism of radicals trapped in y-irradiated molecular sieves exposed to methanol and ethylene have been studied by EPR spectroscopy. It was found that Ag CH2OH+ radical with one-electron bond between Ag and C is formed by the attack of -CH2OH hydroxymethyl radical on Ag+ cation. 

Activation rate constants (k) in ATRP/ATRA are typically determined from model studies in which copper complex is reacted with alkyl halide in the presence of radical trapping agents such as TEMPO [127,128,129], Rates are determined by monitoring the rate of disappearance of alkyl halide in the presence of large excess of the activator (Cu X/L) and TEMPO. Under such pseudo-first order conditions, the activation rate constant can be calculated ln([RX]0/[RX]() vs.t plots (slope =-k) Cu C/... 

Hydroxyurea reacts with oxy, deoxy and metHb in vitro to form iron nitrosyl hemoglobin (HbNO) and transfers NO to 2-6% of the iron heme groups [115]. Trapping studies using cyanide and carbon monoxide indicate that hydroxyurea oxidizes both oxy and deoxyHb to metHb and reduces metHb to deoxyHb specifically identifying the reaction of hydroxyurea and metHb as the critical reaction in the formation of HbNO from hydroxyurea [115]. Scheme 7.16 depicts the proposed mechanisms of N O and HbNO formation during the reaction of deoxy and metHb with hydroxyurea. Oxidation of hydroxyurea by metHb produces deoxyHb and the nitroxide radical (25,... 

Numerous synthetic and mechanistic studies were done to investigate this reaction further, and a variety of enediynes have been thermalized in the presence of radical traps such as 1,4-cyclohexadiene. Even though large excesses of radical traps were employed, the yields of the substituted benzenes were often moderate at best. Most important of all, Tour et al.50 demonstrated that 1,4-naphthalene diradicals generated in solution couple to eventually form a polymer [Eq. (9)]. 

Motton el al. made a spin-trapping study of the radicals induced by a xenon arc lamp from promazine and chlorpromazine. The results suggested that the radical from photodechlorination of chlorpromazine was sufficiently more reactive than the promazine radical to amount for the order of magnitude difference in the potencies of their phototoxicities [233],... 

---