Anion-radicals acidity

It resembles tetracyanoethylene in that it adds reagents such as hydrogen (31), sulfurous acid (31), and tetrahydrofuran (32) to the ends of the conjugated system of carbon atoms suffers displacement of one or two cyano groups by nucleophilic reagents such as amines (33) or sodiomalononittile (34) forms TT-complexes with aromatic compounds (35) and takes an electron from iodide ion, copper, or tertiary amines to form an anion radical (35,36). The anion radical has been isolated as salts of the formula (TCNQ) where is a metal or ammonium cation, and n = 1, 1.5, or 2. Some of these salts have... 

However, in all cases it appears important to check that the increase of current for the mediator is determined by the SET (catalytic current) and not by the protonation of the mediator anion radical e.g., in cases of acidic sulphones acting as proton donors. 

At this point, special mention37 should be made of the behaviour of highly conjugated ethylenic sulphones in weakly acidic media. For example, in the case when R1 =Ph (Z isomer), a fairly stable anion radical was obtained in dry DMF. However, either in aprotic (consecutive two one-electron transfer) or in protic media (ECE process, occurrence of the protonation step on anion radical), C—S bond cleavage is observed. The formation of the corresponding olefins by C—S bond cleavage may occur in high yield, and is nearly quantitative when R1 = H and R2 = Ph for an electrolysis conducted in... 

Amino acids, sulphoxide, radiolysis of 909 a-Amino acids, reactions of 776, 777 a-Aminosulphones, synthesis of 176 Aminosulphonyl radicals 1093 Aminosulphoxides rearrangement of 740 synthesis of 336 Andersen synthesis 60 / -Anilinosulphoxides, synthesis of 334, 335 Anion radicals 1048-1050 ESR spectra of 1050-1054 formation of during electrolysis 963 during radiolysis 892-897, 899, 903 Annulation 778, 781, 801, 802 Antibiotics, synthesis of 310 Arenesulphenamides 740 Arenesulphenates 623 reactions of 282 rearrangement of 719 Arenesulphinates 824, 959 chiral 618... 

The ratio ARH/ARj (monoalkylation/dialkylation) should depend principally on the electrophilic capability of RX. Thus it has been shown that in the case of t-butyl halides (due to the chemical and electrochemical stability of t-butyl free radical) the yield of mono alkylation is often good. Naturally, aryl sulphones may also be employed in the role of RX-type compounds. Indeed, the t-butylation of pyrene can be performed when reduced cathodically in the presence of CgHjSOjBu-t. Other alkylation reactions are also possible with sulphones possessing an ArS02 moiety bound to a tertiary carbon. In contrast, coupling reactions via redox catalysis do not occur in a good yield with primary and secondary sulphones. This is probably due to the disappearance of the mediator anion radical due to proton transfer from the acidic sulphone. 

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). 

The results of a recent investigation of the dependence of the cleavage rate constant upon the solvent of two similar anion radicals, those of 3-nitrobenzyl chloride and 3-chloroacetophenone,85 may likewise be interpreted as the outcome of a competition between the Lewis acid solvation of the developing halide ion and of the negatively charged oxygen atoms in the initial state. 

In dichloromethane, the acidic ESE cation radical undergoes a rapid proton transfer (k = 1.9 x 109 s ) to the CA anion radical within the contact ion pair (CIP) to generate the uncharged radical pair (siloxycyclohexenyl radical and hydrochloranil radical) in Scheme 6. Based on the quantum yields of hydro-chloranil radical (HCA ), we conclude that the oxidative elimination occurs by geminate combination of the radical pair within the cage as well as by diffusive separation and combination of the freely diffusing radicals to yield enone and hydrochloranil trimethylsilyl ether, as summarized in Scheme 6. 

In a related example, the [D, A] complex of hexamethylbenzene and maleic anhydride reaches a photostationary state with no productive reaction (Scheme 17). However, if the photoirradiation is carried out in the presence of an acid, the anion radical in the resulting contact ion pair14 is readily protonated, and the redox equilibrium is driven toward the coupling (in competition with the back electron transfer) to yield the photoadduct.81... 

The attachment of an electron to an organic acceptor generates an umpolung anion radical that undergoes a variety of rapid unimolecular decompositions such as fragmentation, cyclization, rearrangement, etc., as well as bimolecular reactions with acids, electrophiles, electron acceptors, radicals, etc., as demonstrated by the following examples.135"137... 

Compared with the anodic oxidation of a 1,3-diene, the cathodic reduction of a 1,3-diene may be less interesting since the resulting simple transformation to monoolefin and alkane is more conveniently achieved by a chemical method than by the electrochemical method. So far, only few reactions which are synthetically interesting have been studied15. The typical pattern of the reaction is the formation of an anion radical from 1,3-diene followed by its reaction with two molecules of electrophile as exemplified by the formation of the dicarboxylic acid from butadiene (equation 22)16. 

In this classical Haber-Weiss cycle iron is being reduced by superoxide anion radical (02T), ascorbic acid or glutathione and subsequently decomposes hydrogen peroxide - formed by spontaneous dismutation of 02T - in the Fenton reaction to produce 0H. This iron-driven 0H formation has a stringent requirement for an available iron coordination site, a sine qua non met not only by hexaaquoiron(III) but by most iron chelates (28). Thus, Fe-EDTA, -EGTA, and -ATP retain a reactive coordination site and catalyze the Haber-Weiss cycle. Phytic acid, however, occupies all available iron coordination sites and consequently fails to support 0H generation (Figure 6). 

One way of stabihzing the initial radical or anion radical is therefore the addition of an acid. Expulsion of a base should produce a similar effect. This is indeed the case (Scheme 2.21), and the secondary radical thus formed is similarly easier to reduce than the starting molecule in most cases. RX is a molecule containing a low-lying orbital able to accommodate the incoming electron, thus leading to the primary radical, RX -, before the nucleophile X- is expelled. We consider here the case of a stepwise process in which the reaction pathway involves the intermediacy of the primary radical rather than a... 

The direct electrochemical reduction of carbon dioxide requires very negative potentials, more negative than —2V vs. SCE. Redox catalysis, which implies the intermediacy of C02 (E° = —2.2 V vs. SCE), is accordingly rather inefficient.3 With aromatic anion radicals, catalysis is hampered in most cases by a two-electron carboxylation of the aromatic ring. Spectacular chemical catalysis is obtained with electrochemically generated iron(0) porphyrins, but the help of a synergistic effect of Bronsted and Lewis acids is required.4... 

The product is exclusively carbon monoxide, and good turnover numbers are found in preparative-scale electrolysis. Analysis of the reaction orders in CO2 and AH suggests the mechanism depicted in Scheme 4.6. After generation of the iron(O) complex, the first step in the catalytic reaction is the formation of an adduct with one molecule of CO2. Only one form of the resulting complex is shown in the scheme. Other forms may result from the attack of CO2 on the porphyrin, since all the electronic density is not necessarily concentrated on the iron atom [an iron(I) anion radical and an iron(II) di-anion mesomeric forms may mix to some extent with the form shown in the scheme, in which all the electronic density is located on iron]. Addition of a weak Bronsted acid stabilizes the iron(II) carbene-like structure of the adduct, which then produces the carbon monoxide complex after elimination of a water molecule. The formation of carbon monoxide, which is the only electrolysis product, also appears in the cyclic voltammogram. The anodic peak 2a, corresponding to the reoxidation of iron(II) into iron(III) is indeed shifted toward a more negative value, 2a, as it is when CO is added to the solution. 

Radiolysis of isobutyric acid at 195 K results in the formation of only one radical intermediate, the hydrogen abstraction radical III. The decarboxylation radical and the anion radical are both unstable at this temperature and react forming the abstraction radical and other products. The hydrogen which is abstracted is generally that which is attached to the carbon atom a to the carboxyl group. 

A variety of radical products is observed following gamma radiolysis of the N-acetyl amino acids at 77 K (6), depending on the nature of the side chain of the parent amino acid. In the case of N-acetyl alanine, for example, the intermediates are (i) the anion radical IV (ii) the decarboxylation radical V (iii) the deamination radical VI and (iv) the alpha carbon radical VII. 

One-electron reversible reduction of triarylphosphine sulfides in DMF/Et4NI leads to an anion radical [198, 203, 204]. The second step of the reduction, occurring at slightly more negative potentials, results in different products of cleavage of P—S or P—C bonds - triphenylphosphine, benzene, and diphenylthiophosphinic acid (Scheme 42). 

Q , 8-Ethylenic sulfones exhibit a behavior that could be considered as specific and totally different from that of Q , 8-ethylenic ketones or nitriles. Thus, with the present series, there is practically no case of dimerization or double-bond saturation. This seems to be because of the fact that reduction of compounds of this series cannot be completed neither in acidic nor in aqueous solutions because a fast cleavage occurs at the level of the anion radical. 

The superoxide anion radical (O2 ) is produced when oxygen accepts one electron. This radical has a short lifetime in aqueous solutions, where it mainly undergoes spontaneous dismutation to hydrogen peroxide and oxygen (Reaction 1). The superoxide radical is in equilibrium with its conjugated acid, the hydroperoxyl radical (HO2), which is a stronger oxidant and generally more reactive than O2... 

---