Azole radicals anionic

Oxidation of azole anions can give neutral azole radicals which could, in principle, be tt (139) or a- (140) in nature. ESR spectra indicate structure (141 hyperfine splittings in G) for imidazolyl radicals, but both tt- and cr-character have been observed for pyrazolyl radicals. Tetrazolyl radicals (142 4 143) are also well known (79AHC(25)205). Oxidation of 2,4,5-triarylimidazole anions with bromine gives l,l -diimidazolyls (144) which are in equilibrium with the dissociated free radical (145) (70AHQ 12)103). 

Nitro derivatives of azoles owing to electron deficiency are capable of being reduced with the formation of radical ions. 3-Nitropyrazole in the conditions of pulse radiolysis, depending on pH medium, forms radical anions (RA) or radical dianions (RDA), which were registered by ESR method [849] (Scheme 3.16). A hyperfine structure (HFS) constant (a, mT) is a main parameter in the ESR spectrum indicating the interaction of the unpaired electron with all magnetic active nuclei of radical. 

Ab initio (B3LYP/6-31G or UHF/6-31G ) quantum-chemical calculations of spin density distribution in nitroazole radical anions (Scheme 3.29) are in a good agreement with experimental data (Scheme 3.28, Figs. 3.2 and 3.3) and show that the largest positive spin density is located at the carbon atom of azole ring, where the vicarious C-amination is realized [277]. 

Reaction of 1-chloro-l,2,4-triazoles with chloride ion in acetonitrile results in complete dechlorination even at 1 100 [Cl ]/[./V-chloroazole] molar ratio. It is proposed that the reaction proceeds according to the scheme including a one-electron transfer from chloride ion to IV-chloroazole to form IV-chloroazolyl radical anion, and its fast decay to chloride ion and triazolyl radical, which removes a hydrogen atom from the solvent to give NH-azole. Chloride ion reacts with a new molecule of starting N-chloroazole to continue the reaction chain (90IZV2814) (Scheme 43). 

The emphasis here is on the properties of glutathione (or cysteine if data for cysteine, but not GSH are available). Dithiols which form cyclic or stabilized disulphide radical anions, such as dithiothreitol (//ireo-l,4-dimercapto-2,3-butanediol) [29-33], or the reduced form of lipoic acid (6,8-dithiooctanoic acid) [15, 34-37], in effect have uncharacteristically high values of to be good models for glutathione. In addition, biological effects may be complicated by thiol/disulphide exchange since these thiols will reduce many disulphides [38-40]. Arenethiyl radicals (e.g. phenylthiyl or derived from 2-mercaptoimidi-azole) are closer to phenoxyl radicals in nature than aliphatic thiyl radicals [41-44]. 

Conditions under which anion-radicals are commonly made, e.g., reaction with an alkali metal in an ethereal solvent, are strongly basic. Azoles without a nitrogen substituent in general, therefore, lose the proton from the heteroatom before the electron is added and, when the latter is added, the product is a dianion radical. Conversely, on oxidation of an azole to give a cation-radical, if the heteroatom is unsubstituted, the proton may be lost, yielding a neutral azolyl radical. Thus, from azoles various radicals may be formed dianion, anion, neutral, or cation-radicals according to the nature of the substituent at N. 

Tsveniashvili and co-workers acquired polarographic data for series of 2,4-disubstituted, 2,5-disubstituted, and 2,4,5-trisubstituted oxazoles and investigated the effects of substituents on the electrochemical reduction potential. They explained their results by two one-electron reversible steps to produce a dianion, which was then protonated. Rogers and co-workers described anion-pi radicals formed during electrochemical reduction of fused oxazoles, e.g., 2-phenylbenzox-azole. 

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