Diaryl nitroxides

Stable diaryl nitroxides such as 4,4 -dimethoxydiphenyl nitroxide (III) are somewhat better antioxidants than the dialkyl nitroxides. The III inhibited rate depends on both the oxygen pressure and the substrate... 

Secondary arylamines also form hydroxylamines initially, but these invariably oxidise further to the radicals Ar2N and ArgN (O) The diphenylimino radical (At2N) also results from oxidation by non-peroxidic reagents and it exists in equilibrium with its dimer, tetraphenylhydrazine (equation 80) Diaryl nitroxide radicals... 

The stability of nitroxides will be well known to readers acquainted with the spin-labelling technique (Berliner, 1976), but it must be recognized that nitroxides employed as spin labels or spin probes are almost invariably di-t-alkyl nitroxides. Diaryl and many aryl t-alkyl nitroxides are also sufficiently persistent to be isolated, and it has recently been shown that several acyl t-alkyl nitroxides can also be obtained pure (Perkins and Ward, 1973 Alewood et al., 1978). However, other nitroxides are less persistent. Monosubstituted nitroxides, RN(H)Q-, rapidly disproportionate to nitroso-... 

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,... 

By extrapolating data of Lattimer and coworkers (24, 40), it may be anticipated that the ozonation of DPPD proceeds most probably via nitroxide and aminyl formation pathways. Dinitrone and N-N or N-C and C-C coupling products should therefore be considered as ozonation products formed via free radical intermediates. The reactivity of DPPD with aldehydes will be very limited. All these mechanistic features together with the low ozonation rate of DPPD in comparison with N,N -disec.alkyl PD (26) may be responsible for the generally reported poor antiozonant efficiency of N,N -diaryl PD in comparison with N-sec.alkyl-N -aryl and N,N -disec.alkyl PD. 

Aryl, diaryl and divinyl nitroxides 6.9.1 Monoaryl nitroxides... 

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