Nitrogen-centred Radicals

Figure 5-36. Oxidation of 5.20 yields an iron(ii)-nitrenium complex 5.21 by the loss of the electron from the radical nitrogen centre. The nitrenium centre has only six electrons on the nitrogen and is electron deficient.

The third class of compounds to be discussed in this chapter are those in which an RE group (E = S, Se, Te) is attached to a nitrogen centre. This category includes amines of the type (REfsN and the related radicals [(RE)2N] , as well as organochalcogen(ir) azides, REN3, and nitrenes REN (E = S, Se). Covalent azides of the type RTe(N3)3 and R2Te(N3)2, in which the chalcogen is in the +4 oxidation state, have also been characterized. 

Free-radical cyclization on to unsaturated CN bonds and also the cyclization of a range of nitrogen-centred radicals have continued to attract interest and have been reviewed. Aryl radicals, generated from BusSnH- or TTMSS-mediated homolytic cleavage of aryl-bromide bonds, have been shown to cyclize on to the nitrogen atom of imidate esters in the 5-exo mode (Scheme 9). Loss of an ethyl radical leads to the observed A-acylindolines. No cyclization in the 6-endo mode was detected. 

The reaction of amino acids with HOCl was smdied using EPR spin trapping and UV-vis spectroscopy. Some nitrogen-centred radicals, which then undergo a variety of abstraction, rearrangement, and fragmentation reactions, were detected (Scheme 27). 

A paper has reported that nitrogen-centred radicals (29) have been generated by one-electron reduction (at carbon or metal electrodes) of stable nitrenium ions of the general structure (28). The reactivity of the radicals is influenced by the substituents attached to the two nitrogen atoms that are directly linked to the ion nitrenium centre." ... 

Nitrogen-centred free radicals or radical cations... 

For amines having an a-hydrogen atom, electrochemical oxidation leads to the imine as the first detectable intemrediate. In the absence of another nucleophile, this is not usually a useful reaction since the imine is hydrolysed by water present in the solvent leading to a mixture of products [80, 81], Oxidation of ten-butylamine, which has no a-hydrogen atom, leads to loss of a proton flrom the nitrogen atom and the dimerization of nitrogen centred radicals. The product isolated in moderate yields is azo-teri-butane 17 [82], The reaction can be carried out in an... 

Hydrazine derivatives are more easily oxidised than the corresponding amines. Tetraalkylhydrazines afford the short-lived radical-cation and these redox couples have standard redox potentials in the range 0.0 to +0.1 V vs. see [136]. In the oxidised form, the two nitrogen lone pair p-orbitals interact to cause delocalization of the radical-cation centre. Substrates such as 31 where the a-carbon-hydrogen... 

Oxidation of ketone phenylhydrazones generates a radical-cation centre on the nitrogen atom adjacent to the benzene ring. The radical-cation is delocalised by both the hydrazone group and the phenyl ring. Reactions of 1,3,5-triphenyl-A -pyazolines illustrate the properties of these radical-cations. Two one-electron waves are seen at a rotating disc electrode in acetonitrile and for 1,3.5-triphenyl-pyrazoline, Ey. = 0.82 and 1.68 V vs. see [33]. The delocalised radical-cation is... 

Acrylonitrile. - Used widely in the polymer industry, acrylonitrile is considered to be a possible human carcinogen. DNA damage is believed to involve metabolism to an epoxide, followed by adduct formation with DNA. There is also evidence for radical-mediated damage to DNA. For example, Ohnishi et al. have trapped a nitrogen-centred radical derived from acrylonitrile, the formation of which was associated damage to DNA.22... 

Metallic silver was found to react with NBS and provided a convenient high yield route to the silver salt of succinimide (equation 19).280 Spin trapping experiments were performed to show that nitrogen-centred radicals were intermediates in the reaction. 

The oxidation of 5.20 (by air, nitric acid, iron(m) etc.) results in electron loss from the nitrogen-centred radical and the formation of an iron(n) nitrenium species 5.21 (Fig. 5-36), which then undergoes deprotonation to yield an iron(n) complex of a tetraene 5.22 (Fig. 5-37). 

The key feature of these reactions is the interplay between metal-centred and ligand-centred radical species, as discussed in Chapter 5. To summarise, the process involves generation of a nitrogen-centred radical, which is stabilised by charge transfer from the... 

As oximes and hydrazones are not good electron acceptors, the mechanism for these reactions most likely involves an intermediate ketyl radical anion, which adds to the C=N double bond. A three-electron-two-orbital interaction involving the nitrogen-centred radical with the lone pair of the adjacent... 

Hll. Hawkins, C. L., and Davies, M. J., Hypochlorite-induced damage to proteins Formation of nitrogen-centred radicals from lysine residues and their role in protein fragmentation. Biochem. J. 332, 617-625 (1998). 

Williams and coworkers performed several other experiments to show that the observed radical is really a carbon-centred radical, and did not rely only on the typical splitting of a-CH2 group. They also radiolysed A-dideuterioallylamine in freon matrices, obtaining the same spectrum as in the all-hydrogen molecule. This indicates that the deuterium atoms are not bound to the radical centre, ruling out a nitrogen-centred radical. 

The antioxidant role of NO comes Ifom its reaction with oxygen, carbon and nitrogen centred radicals and can be seen to have a scavenger role under a range of conditions [112]. This is because of the unpaired electrons of NO which react rapidly with alkoxyl and alkyl hydroperoxyl radicals at near diffusion reaction rates (2x10 M s [113]). It is these reactions that have been suggested to have a modulatory role in enzyme- or metal-catalysed... 

The spectrum from cis-polyisoprene tensile tested in (nominally) oxygen free nitrogen, shown in Fig. 32, is a six line spectrum with approximately 12 G hyper-fine splitting. Polymer radicals are centred around the free spin -value (g-= 2) and structure in the spectrum is invariably due to nuclear hyperfine structure. The peroxy... 

Figure 3.8. Radical formation due to mechanochemical degradation measured by using Tinuvin 770 as a spin trap and detecting the alkyl radicals via the change in nitrogen-centred radical concentration by ESR. Adapted from Sohma (1989a).

In contrast to the diphenylamine derivatives, transformation of the nitrogen radical (XII) to a nitroxyl radical, N-0-, has not been observed. Due to the longer lifetime of the nitrogen-centred radical (XII) by resonance stabilisation, dimerisation and oligomerisation take place whilst maintaining the -NH- function. Indications have been found that the products of Reaction (4.37) may result from direct reaction of phenyl-a-naphthylamine with oxygen [27, 35]. 

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