Radical traps, nitroxides

Adding a radical trap like BulNO to the reaction mixture this reacts with radicals (R ) forming nitroxide radicals Bul(R )NO that can be detected by ESR. 

Nitroxides have the property of quenching fluorescence. Thus radical trapping with nitroxides containing fluorophores (e.g. 114) can be monitored by observing the appearance of fluorescence.511015 The method is highly sensitive and has been applied to quantitatively determine radical yields in PLP experiments (Section... 

The reaction of radicals with nitroxides is reversible. 09 This means that the highest temperature that the technique can reasonably be employed at is ca 80 °C for tertiary propagating species and ca 120 °C for secondary propagating species.22 These maximum temperatures are only guidelines. The stability of alkoxyamines is also dependent on solvent (polar solvents favor decomposition) and the structure of the trapped species. This chemistry has led to certain alkoxyamines being useful as initiators of living polymerization (Section 9.3.6). At elevated temperatures nitroxides are observed to add to monomer albeit slowly. 3IS 5" 523... 

If di(tcrt-butyl)nitroxide (a radical trap) is present, the reaction with phenylacetonitrile-potassium does not proceed entirely. Acetonitrile-potassium (which is in equilibrium with potassium amide) forms only aminopyridine in the presence of the trap (Moon et al. 1983). Consequently, amination is a classical nucleophile reaction, and the formation of pyridyl acetonitrile is a reaction of the typs- These two reactions are quite different. A stronger CH acid leads to a well-defined synthesis. 

The Sjjf.fl character of the reaction was ascertained by the effect of light irradiation and addition of a radical trap. Namely, under light irradiation, the half-reaction time was considerably shortened (3 instead of 41 min). Addition of di(tert-butyl)nitroxide completely quenched the reaction—neither C- nor 0-substitution was observed after 4 h. The radical trap may only react with the R radicals that escaped the solvent cage where R, Nu, and X have been formed. This means that, in the... 

In chloroprene containing 0.05M azobisisobutyronitrile and 0.02M 2,2,6,6-tetramethyl-4-piperidone-l-oxyl an induction period of 22 minutes was observed, followed by retarded oxidation. In the absence of the initiator 110 p.p.m. of N,N-dimethyl-4-nitrosoaniline inhibited oxidation for 1 hour. Nitroxide radicals and their nitroso precursors (17) do not function as peroxy radical traps since they cause no inhibition and little retardation of the initiated oxidation of cumene at 60°C. 

The graft copolymerization of acrylonitrile onto polystyrene was attempted using benzoyl peroxide, di-/-butylperoxide, and 2,5-dimethyl-2,5-di-(/-butylperoxy)hexane as initiators. In all cases no increase in mass of the polystyrene was observed. Attempts were also made to test whether the polystyryl radical was ever formed by combining the initiator and the polymer or the initiator, polymer and a nitroxide radical trap. In the first case the formation of a radical must lead to cross-linking of the polymer and in the second case the polystyryl radical will be trapped by the nitroxide. ... 

An investigation into the initiation mechanism of copolymerization of ethyl vinyl ether and acrylonitrile by /-butoxyl radicals lias shown that the reaction between the two monomers competes successfully with radical trapping by the nitroxide radical trap (5).37 The /-butoxyl radicals react 3-6 times faster with ethyl vinyl ether than acrylonitrile the authors proposed that this is due to selective interaction of one monomer with the radical species rather than a solvent polarity effect. 

Quantitative nitroxide-trapping experiments should be carried out under thoroughly de-oxygenated conditions since oxygen will act as a competitive radical trap for carbon-centred radicals. Consequently, several freeze-thaw degassing cycles using pressures <10 3 mm Hg are usually required. 

Nitroxide mediated polymerization (NMP) is another type of controlled radical polymerization technique used to synthesize polymer hybrids. It relies on the reversible trapping of growing macro-radicals by nitroxide to form dormant species in which the C-ON covalent bond is thermally cleavaged (Fig. 19). At a polymerization temperature, the equilibrium between dormant and active species is strongly shifted to the dormant side, which Emits the irreversible chain termination reaction. 

Inhibition by radical traps, such as TEMPO 17, was used to explain the involvement of radicals in the course of transition metal-catalyzed reactions (Fig. 7). Typical cross-coupling reactions, such as Heck or Suzuki-Miyaura reactions, proceeded even with nitroxyls as substrates, although the yields were sometimes low. Thus, nitroxyls do not necessarily interfere very much with the course of two-electron catalytic processes [79-81]. However, it must be critically mentioned that 17 and related nitroxides are both oxidants and reductants for metal species. 

The reaction of oxyhaemoglobin with phenylhydrazine was studied in whole blood samples (Maples et al., 1988). Hydrazine-based drugs induce destruction of red blood cells with resulting haemolytic anaemia. Using DMPO as a trap, nitroxide radicals were detected, but these had solid-state or immobilized spectra with broad parallel and perpendicular features. Whilst no firm identification is possibly based on ESR spectra, except that the trapped radical must be a high polymer, various lines of evidence lead to the conclusion that the adduct was formed from a sulphydryl radical on oxyhaemoglobin. Chloroform extracts gave the phenyl radical adduct in accord with in vitro studies. 

V.W. Bowry and K.U. Ingold, Kinetics of nitroxide radical trapping. 2. Structural effects, J. Am. Chem. Soc. 1992, 114, 4992M996. 

Open-ring nitroso- [224] 144 and nitrocompounds [126] 145 were reported as the ultimate products of ROO reaction with HAS derived nitroxides. A-Oxide 146 was mentioned among the ultimate products of HAS as well [225]. Some alkyl radical trapping and 102 quenching activities should be preserved in 146. 

Nitroxides as Carbon Radical Traps in Non-Chain Synthetic Sequences... 

Radical trapping by a nitroxide was used to determine the ratio between (3-fragmentation and hydrogen abstraction by alkoxy radicals generated by thermal decomposition of peroxy pivalate esters.The ratio of fragmentation hydrogen abstraction increased sharply with radical substitution and stability. 

Separation of the trapped radicals showed that the vapor phase of smoke from each cigarette type produced a unique set of free radicals (four to ten distinct peaks). Vapor mixtures used to model tobacco smoke consisted of NO, air, isoprene, and methanol. The model systems produced a set of free radicals that consisted of four major and several minor peaks, two of which matched peaks in tobacco smoke chromatograms. Quantities of free radicals trapped from cigarettes tested varied from 54 + 2 nmol to 66 + 9 mnol. The cigar tested produced 185 + 9 mnol of free radicals. In their experiments, oxygen competed with the nitroxide trap for MSS vapor-phase radicals. A kinetic analysis of the O2 competition shows that actual radical concentrations in the smoke were approximately 100-fold higher than measured. 

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