Nitroxides spin trapping

DEL 12a] Delaittre G., Dietrich M., Blinco J.P. et al, Photo-Induced Macromolecular Functionalization of Cellulose via Nitroxide Spin Trapping ,... 

Keane, J.F.N., Newer aspects of the synthesis and chemistry of nitroxide spin traps, Chem. Rev., 78, 37, 1978. 

Many nitrones and nitroso-compounds have been exploited as spin traps in elucidating radical reaction mechanisms by EPR spectroscopy (Section 3.5.2.1). The initial adducts are nitroxides which can trap further radicals (Scheme 5.17). 

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... 

When the lifetime of the radicals is very short and direct ESR detection is not an option, spin trapping is used to detect radicals at ambient temperatures. The method is based on the scavenging of radicals, P, by a spin trap, leading to the formation of a spin adduct with higher stability in most cases, this adduct is a nitroxide radical. 

Unfortunately, due to the above shortcomings of hydroxylamine derivatives as spin traps, the uncertainties of the mechanism of their reactions with superoxide are added. Although it is supposed that nitroxide radicals are formed by oxidation with superoxide (Reaction (6)), this reaction cannot be an elemental stage because superoxide cannot abstract a hydrogen atom. 

The factors which determine the answers to these questions are central to the following discussion. Some of them were already well understood when the earliest reviews of the technique first appeared (Perkins, 1970 Lagercrantz, 1971 Janzen, 1971) others are only now becoming clear as quantitative data on the kinetics of reactions of spin traps and of nitroxides are accumulated. 

In the spin-trapping context, diaryl or aryl t-alkyl nitroxides are effectively stable. Both, however, when present in very high concentrations, may decay slowly by a bimolecular process (10), provided that steric constraints do not twist the aryl group out of conjugation with the nitroxide function. In contrast,... 

Spin-adduct spectra often reveal splittings to substituent atoms other than hydrogen. Indeed, since the spin-trapping technique provides a convenient route to many nitroxides containing structural features likely to be of spectroscopic interest, it has frequently been used to this end. Chlorine and bromine splittings, as well as those from fluorine, have been encountered and many nitroxide spectra have been reported in which there is splitting from a second nitrogen, from phosphorus, or even from a metal atom. 

The pre-eminent advantage of C-nitroso-compounds as spin traps is that in the spin adduct the scavenged radical is directly attached to the nitroxide nitrogen. Consequently, the esr spectrum of the spin adduct is likely to reveal splittings from magnetic nuclei in the trapped radical, and these will greatly facilitate its identification. A simple example is presented in Fig. 2, which shows the spectrum of the spin adduct of the methyl radical with 2-methyl-2-nitroso-... 

Relatively simple spectra are obtained from spin adducts of the hindered nitroso-arenes, and these may be further refined by deuteration of the spin trap (Terabe et al., 1973). In spite of being substantially dimerized, even in dilute solution,6 nitrosodurene (ND) has two considerable advantages over MNP. Firstly, it is more reactive towards radical addition (Table 5, p. 33). Secondly, it is not sensitive to visible light, and even on ultraviolet irradiation any photodecomposition is apparently not a major source of nitroxides. 

The tri-t-butylnitrosobenzene, TNB, is monomeric even in the solid state, but the principal advantage of this scavenger, exemplified in the mechanistic studies described in Section 3 (p. 47), is that it functions as an ambident spin trap (Terabe and Konaka, 1973). Thus, primary alkyl radicals add to form nitroxides in the normal way, but with t-alkyl radicals, addition occurs at oxygen, alkoxyaminyl radicals (ArNOR) being formed. Secondary alkyl radicals give mixtures of both species (Fig. 5). The alkoxyaminyl radicals have a lower g-value than the nitroxides (ca. 2.004 vs. 2.006) and their spectra are therefore centred at slightly higher field positions than those of the nitroxides. 

The esr parameters for a large number of alkyl aryl nitroxides have been collected recently, in part as a source of reference for spin trapping studies (Rockenbauer et al., 1978) the conformations of these nitroxides are also discussed. A selection of spectral parameters for spin adducts of the more important nitroso traps which have been discussed in this Section is presented in Table 2. 

The spectra of spin adducts have been thoroughly examined in the preceding Sections, with emphasis on the ease with which structural information may be extracted. It has also been noted that some spin traps which are thermally or photochemically labile may give rise to nitroxides other than those deriving from the radical reaction under investigation. More seriously, there may be other possible sources of nitroxides in these systems which can cause (and almost certainly have caused) false mechanistic conclusions to be drawn. 

Very shortly after the technique of spin trapping was first proposed as a versatile general method for probing free-radical reactions, it was pointed out by Forrester and Hepburn (1971) that both nitroso-compounds and nitrones are susceptible to nucleophilic attack. The resulting hydroxylamines are particularly sensitive to oxidation (14a,b) and this gives nitroxides Under many circumstances, the possibility of such a reaction is remote, but it should... 

It is not uncommon to find the persistence of a spin adduct quantified in terms of half-life . This is a dangerous practice unless the experimental conditions are precisely defined, or it is known that the nitroxide decays by a unimolecular process. Decay may depend on reaction with a reducing agent present in the system, in which case the concentration of this species will influence the half-life. More commonly, decay will be second order (p. 5), in which case the time for disappearance of 50% of the spin adduct will show a profound dependence on its absolute concentration. The possibility of bimolecular association of nitroxides has been recognized for many years, but only very recently has it been suggested that this may be a complication under experimental conditions employed for spin trapping. Whilst the problem, which was encountered with the important [DMPO-HO ] system (Bullock et al., 1980), seems unlikely to be widespread, it is one which should always be borne in mind in quantitative studies. 

Spin trapping by PBN has also been employed to detect radical formation in a photo-Kolbe reaction in which acetic acid is irradiated (A > 360 nm) in the presence of platinized titanium dioxide powder (Kraeutler et al, 1978). The nitroxide observed was considered to be (PBN—Me ), but the published spectrum clearly shows the presence of a second species spectral overlap might therefore be an alternative to solvent polarity as an explanation of the discrepancy between the observed splitting parameters and those previously reported for this species. Where poor resolution obtains, it is important that... 

Experiments designed to utilize spin trapping to monitor free-radical chemistry in the gas phase were first reported by Janzen and Gerlock (1969). In these, radicals generated by photolysis in a stream of carrier gas were passed over solid PBN. The PBN was then dissolved in benzene, and the solution was found to contain spin adducts of radicals present in the gas stream. Photolysis of t-butyl hypochlorite vapour in this way leads to a nitroxide whose spectrum reveals splitting from two chlorine atoms. This proved to be due to butyl nitroxide (Janzen, 1971 Janzen et al., 1970), and recalls the observation of other nitroxides which apparently result from further reaction of the initial spin adducts. 

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