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Nitroarenes radical anions

The negative charge in nitroarene radical anions is localized on the nitro group ... [Pg.856]

Diethoxyphosphinyl-2-buten-4-olide reacts with lithiated dithianes followed by an intermolecular Wittig-Homer reaction to produce fused 7-lactones (equation 37). The reaction of 2-lithio-l,3-di-thianes with nitroarenes gives 2- or 4-[(l,3-dithian)-2 -yl]cyclohexa-3,S(or 2,S-)-diene-l-nitronate compounds (conjugate addition products), free nitroarene radical anions (redox products), 1,3-dithianes and 2,2 -bis( 1,3-dithianes). ... [Pg.569]

Nitroarenes, on the other hand, are strong electron acceptors and easily undergo one-electron reduction (12, 13). Thus, nitrobenzene, to cite one example, has been customarily used as an effective quencher in chain reactions involving radical anion intermediates, such as in SRN1 reactions (3). Under different conditions, nitroarene radical anions are reactive species. In particular, Zinin (14) reported that treatment of nitroarenes with hot alkaline alcoholic solutions results in products of reduction, mainly the azoxy derivative (equation 2). These complex multistep processes involve nitroarene radical anion intermediates and are quite effectively inhibited by oxygen (10, 15, 16). In 1964, Russell et al. (17) wrote that apparently much of the chemistry of aromatic nitro, nitroso and azo compounds in basic solution involves electron-transfer processes . [Pg.330]

An analogous explanation can be invoked to account for the counterion effects observed in the base-promoted reduction of nitroarenes described earlier. Indications in this sense have been obtained by means of in situ electron paramagnetic resonance (EPR) analysis of reacting solutions (32). In Figure 4, for example, plots are shown of the concentration of 4-ClC6H4N02 and of the intensity of the EPR signal due to 4-ClC6H4N02, as a function of time. Experiments of this sort provide evidence that these alkoxy-promoted reductions involve nitroarene radical anion intermediates. [Pg.338]

Carbanions of many different types, on the other hand, are known to undergo one-electron oxidation at the expense of nitroarenes (17). Examples range from enolates to the conjugate base of (CH3)2SO (11) and include the interesting studies on fluorene derivatives of Guthrie (37). Because of the ascertained ability of carbanions to produce nitroarene radical anions, mech-... [Pg.341]

An alternative proposal involves H transfer from the a-C of the alkoxide to one of the oxygens of the nitro group followed by ionization of the neutral radical so obtained to its conjugate base, the nitroarene radical anion (16). [Pg.342]

Strong indications exist that the overall rate of the reduction process depends on the rate of decay of the intermediate nitroarene radical anion ArN02 to the dianion species ArN022-. It is not known whether this step occurs via second-order dismutation (equation 11) or electron transfer from alcohol-derived reducing radicals formed in the process. [Pg.346]

An electron transfer alone to the nitro compound is not sufficient to promote oxygen transfer, since the nitroarene radical anion generated independently by electrochemical techniques in the presence of phosphine does not afford phosphine oxide. The ion pair is a reasonable representation of the rate-limiting transition state, and neither the N-0 bond breaking nor the P-0 bond making are determining factors in the kinetics of the oxygen-transfer process. [Pg.110]

The reaction with nitrite proceeds smoothly and with relatively high yields of the corresponding nitroarene (see Sec. 10.6). Obviously a major part of the driving force of this reaction is the formation of a stable, i. e., an energetically favorable, radical, nitrogen dioxide. With the hydroxide ion — a much stronger nucleophile than the nitrite ion — the reaction is expected to produce very unstable radicals, the hydroxy radical OH and the oxygen radical anion O, from the diazohydroxide (Ar - N2 — OH) and the diazoate (Ar-N20 ) respectively. Consequently, dediazoniation in alkaline aqueous solution does not follow the simple Scheme 8-41 with Yn = OH, but instead involves diazoanhydrides (Ar — N2 —O —N2 —Ar) as intermediates (see Sec. 8.8). [Pg.195]

In aprotic solvents, such as DMF [106] and ammonia [107], nitrobenzene is reduced reversibly to the radical anion and at a more negative potential to the dianion in the absence of electrophiles the dianion is stable, but if impurities are present, the second wave becomes irreversible. In some cases the radical anion of a nitroarene may dimerize 9-nitroanthracene radical anion dimerizes thus to the dianion of 9,9 -dinitro-9,9, 10,10 -tetrahydro-9,9 -biantryl [108]. Radical anions of nitroarenes may be used as electron transfer reagents (Chapter 29). [Pg.391]

There are several parallels in the reduction chemistry of nitroarenes and aromatic N-oxides, such as similar kinetics of electron transfer reactions of the radical-anions and the effects of prototropic equilibria on radical lifetimes in aqueous solution [16]. The benzotriazine di-N-oxide, tirapazamine (Figure 1,16) is currently in Phase III clinical trial as a hypoxic cell cytotoxin in conjunction with cisplatin [132]. The mechanism of its action appears to involve the one-electron reduction product [133] cleaving DNA [134], probably also sensitizing the damage by a radical-addition step [135-138]. [Pg.640]

The formation of o-complex seems to be a crucial step of the reaction between a nucleophile and an electron-deficient nitroarene. At the same time, there might be another strategy. By changing the electron character of substrate through the formation of radical-anions we generate electrrMi rich species, which are able to react with neutral nucleophiles (electron poor). Thus, the choice of appropriate reactants for electrochemical reactions appears to be a crucial point. Indeed, 1,3,5-trinitrobenzene and N-methylformamide (nucleophile/solvent) proved to be suitable reactants [29]. [Pg.264]

Nitrosoarenes are readily formed by the oxidation of primary N-hydroxy arylamines and several mechanisms appear to be involved. These include 1) the metal-catalyzed oxidation/reduction to nitrosoarenes, azoxyarenes and arylamines (144) 2) the 02-dependent, metal-catalyzed oxidation to nitrosoarenes (145) 3) the 02-dependent, hemoglobin-mediated co-oxidation to nitrosoarenes and methe-moglobin (146) and 4) the 0 2-dependent conversion of N-hydroxy arylamines to nitrosoarenes, nitrosophenols and nitroarenes (147,148). Each of these processes can involve intermediate nitroxide radicals, superoxide anion radicals, hydrogen peroxide and hydroxyl radicals, all of which have been observed in model systems (149,151). Although these radicals are electrophilic and have been suggested to result in DNA damage (151,152), a causal relationship has not yet been established. Nitrosoarenes, on the other hand, are readily formed in in vitro metabolic incubations (2,153) and have been shown to react covalently with lipids (154), proteins (28,155) and GSH (17,156-159). Nitrosoarenes are also readily reduced to N-hydroxy arylamines by ascorbic acid (17,160) and by reduced pyridine nucleotides (9,161). [Pg.360]

In our previous paper (ref. 2) we demonstrated the particular role played by one-electron donor centres on magnesia surface in catalytic transfer hydrogenation. Moreover, nitroarenes exhibit high tendency to convert themselves into corresponding anion radicals during adsorption on MgO. Thus, it was expected that esr spectroscopy would reveal new data concerning the reactants activation. [Pg.174]

Strong centres, forming anion radical even from nitrobenzene molecule are poisoned irreversibly, however, their presence is not necessity for the preservation of catalytic activity. Taking into consideration that regenerated MgO which is not able to ionize nitrobenzene molecule is still active in its reduction by hydrogen transfer and that only a few from reduced nitro compounds form ion radicals on catalyst surface one can ascertain that ion radicals formation is not necessary step in nitroarenes (or nitroparaffins) activation. Probably, one-electron donor sites take part only in activation of alcohol what was demonstrated by us earlier. [Pg.176]

Some halogenated alkyl groups that can undergo reductive dehalogenation (Fig. 3, reaction 7) Nitroarenes that can be reduced to nitro anion-radicals, nitrosoarenes, nitroxides, and hydroxylamines (Fig. 7, reaction 4)... [Pg.491]

When analyzing plausible mechanisms of the VNS reactions of nitroarenes with a-chlorocarbanions, one should clarify a few key questions how to proceed the addition and subsequent conversion of adducts and how other substituents may affect both of these steps - rate and orientation of the addition, rate of the elimination, etc. It is well known that nitroarenes are active electron acceptors, whereas carbanions are good electron donors thus, these reactants can enter a single-electron transfer (SET) to form anion radicals of nitroarenes and radicals from carbanions [21, 22]. Further coupling of these electrophilic radicals with nucleophilic anion-radical species could give adducts. This SET pathway, alternative to the direct addition, is often favored by authors and the concept is sometimes abused, see [23] and rebuttal [24]. Nevertheless, numerous observations contradict participation of the SET mechanism in the VNS reactions ... [Pg.54]

Formation of the o -adducts of nucleophiles to nitroarenes is a common initial step for aU Sj ArH reactions ONSH, VNS, cine- and fe/e-suhstitution, etc. The key differences between these reactions are pathways of conversion of the o -adducts into final products. Nitroarenes can act not only as electrophilic partners hut also as active electron acceptors. Since carbanions are good electron donors, the reaction between nitroarenes and carbanions can proceed via a simple direct addition or via an SET to produce the corresponding anion radicals of nitroarenes and radicals from the carbanions. Combination of these paramagnetic species can also produce o -adducts. Indeed, in such systems, ESR spectra of the anion radicals of nitroarenes are often observed. On this basis, SET is often postulated as the way of formation of the a-adducts (Scheme 11.20) [36]. [Pg.280]


See other pages where Nitroarenes radical anions is mentioned: [Pg.269]    [Pg.343]    [Pg.634]    [Pg.269]    [Pg.269]    [Pg.343]    [Pg.634]    [Pg.269]    [Pg.851]    [Pg.428]    [Pg.453]    [Pg.176]    [Pg.185]    [Pg.270]    [Pg.321]    [Pg.122]    [Pg.626]    [Pg.631]    [Pg.640]    [Pg.643]    [Pg.135]    [Pg.155]    [Pg.315]    [Pg.286]    [Pg.159]    [Pg.270]    [Pg.280]    [Pg.294]    [Pg.311]   
See also in sourсe #XX -- [ Pg.95 , Pg.160 ]




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