Radical anion formation

The authors proposed the following picture of the silylene anion-radical formation. Treatment of the starting material by the naphthalene anion-radical salt with lithium or sodium (the metals are denoted here as M) results in two-electron reduction of >Si=Si< bond with the formation of >SiM—MSi< intermediate. The existence of this intermediate was experimentally proven. The crown ether removes the alkali cation, leaving behind the >Si - Si< counterpart. This sharply increases electrostatic repulsion within the silicon-silicon bond and generates the driving force for its dissociation. In a control experiment, with the alkali cation inserted into the crown ether, >Si — Si< species does dissociate into two [>Si ] particles. 

Both theory and experiment point to an almost perpendicular orientation of the two butadiene H2C=C(t-Bu) moieties (see Scheme 3.53). On passing from the neutral molecule to its anion-radical, this orthogonal orientation should flatten because the LUMO of 1,3-butadiene is bonding between C-2 and C-3. Therefore, C2-C3 bond should be considerably strengthened after the anion-radical formation. The anion-radical will acquire the cisoidal conformation. This conformation places two bulky tert-butyl substituents on one side of the molecule, so that the alkali metal counterion (M+) can approach the anion-radical from the other side. In this case, the cation will detain spin density in the localized part of the molecular skeleton. A direct transfer of the spin population from the SOMO of the anion-radical into the alkali cation has been proven (Gerson et al. 1998). 

As for solvents, liquid ammonia or dimethylsulfoxide are most often used. There are some cases when tert-butanol is used as a solvent. In principle, ion-radical reactions need aprotic solvents of expressed polarity. This facilitates the formation of such polar forms as ion-radicals are. Meanwhile, the polarity of the solvent assists ion-pair dissociation. This enhances reactivity of organic ions and sometimes enhances it to an unnecessary degree. Certainly, a decrease in the permissible limit of the solvent s polarity widens the possibilities for ion-radical synthesis. Interphase catalysis is a useful method to circumvent the solvent restriction. Thus, 18-crown-6-ether assists anion-radical formation in the reaction between benzoquinone and potassium triethylgermyl in benzene (Bravo-Zhivotovskii et al. 1980). In the presence of tri(dodecyl)methylammonium chloride, fluorenylpi-nacoline forms the anion-radical on the action of calcium hydroxide octahydrate in benzene. The cation of the onium salts stabilizes the anion-radical (Cazianis and Screttas 1983). Surprisingly, the fluorenylpinacoline anion-radicals are stable even in the presence of water. 

Stepwise chemical reduction of humic acid caused a variation in spin content as shown in Figure 7. The initial rise in radical content is attributed to anion radical formation caused by sodium the following decrease in spin content with further addition of sodium is probably caused by the reduction of these anion radicals. The subsequent increase in radical content could be caused by the one-step reduction of the remaining quinone moieties. 

There are several cases of hydroxylation according to the hidden radical mechanism, within a solvent cage. As assumed (Fomin Skuratova 1978), hydroxylation of the anthraquinone sulfonic acids (AQ-SO3H) proceeds by such a pathway, and OH radicals attack the substrate anion radicals in the solvent cage. Anthraquinone hydroxyl derivatives are the final products of the reaction. In the specific case of dimethylsulfoxide as a solvent, hydroxyl radicals give complexes with the solvent and lose their ability to react with the antraquinone sulfonic acid anion radicals (Bil kis Shein 1975). The reaction is stopped just after anion radical formation, Scheme 1-102 ... 

Anion-radicals were obtained by alkali-metal reduction of phospholes in ether solvents.602 Sodium and potassium gave radicals rapidly whereas lithium failed. The radicals persisted several days at — 80° but decomposed above - 30°. The persistence of the radicals and their relatively large phosphorus hyperfine splitting, e.g., 186, by comparison with anion-radicals from phosphines, were interpreted in terms of aromatic character.602 The results obtained here contrast with results obtained earlier for 187 where phenyl cleavage and small phosphine-like phosphorus splittings had been observed for the products of attempted anion-radical formation.603 Chemiluminescence on oxidation of the anion-radical of 1,2,5-triphenylphosphole has been reported.604... 

Electrolysis of carbonyl compounds provides pinacols, alcohols or hydrocarbons, depending on the conditions, such as pH, the nature of the electrode, and its potential. Fundamental studies have been carried out on the mechanisms of hydrocarbon formation using acetone as a substrate. Although several electrodes, such as Cd, Pt, Pb or Zn, are recommended, carbonyl compounds, including aryl and alkyl derivatives, require strong aqueous acidic media for reduction to the hydrocarbons. The mechanism of the electrolytic reduction is probably similar to that of Clemmensen reduction, which starts from anion radical formation by one-electron transfer, as indicated in Scheme 3. The difference is that electrolytic reduction takes place in an electric double layer, rather than on the surface of the zinc metal. 

Phthalimide and substituted phthalimides are relatively easily reduced in DMF 1.4V (versus SCE)]. In alkaline or aprotic solution, two, one-electron reduction steps are observed in voltammetric experiments [134-137] anion radical formation at the first wave has been confirmed by esr spectroscopy [55,135,136]. Considerable attention has been given to the detailed mechanism of reduction of phthalimide derivatives [134— 137] ... 

The potentials for porphyrin a anion radical formation of M (0EP) complexes are linearly related to the electronegativity of the central metal ion and the most... 

Fig.4. Nitrobenzene anion radical formation at —0.4 V under potentio-static control at the silver amalgam electrode. The points at the curves indicate the start and the end resp. of the potential step. 

Fewer theoretical studies have been performed on the base radical ions [6, 16, 33, 34]. They appear to undergo a strong flattening of the amino groups which become nearly planar in the radical cations [33]. In our laboratory, we have performed 3-21G and 6-3IG full geometry optimizations of the four DNA base radical ions [16, 34] and observed that geometrical relaxation and destabilization of the bases upon cation radical formation is more significant than upon anion radical formation. Such destabilization of the cation radical appears to influence the rate of interpair electron transfer and the adiabatic ionization potential [35]. 

Treatment with the cells with the redox cycler paraquat, as expected, increased superoxide radical anion formation significantly. Incubation with CML or CasCML resulted in a major decrease of MitoSOX-fluorescent cells in HEK-FL and Caco-2, with CML causing the more explicit effects. Since mitochondrial superoxide anion radical formation in HEK-DC... 

Most enones are reduced to anion radicals by organo cuprates. It is likely, that this reaction is connected with the alkylation. Both the formation of anion radicals and of conjugate adducts are not observed, when the redox potential of the enone becomes too negative (H.O. House, 1976). 

Tire reduction of TAF 100 by metallic potassium resulted in the formation at room temperature of the stable anion radical 109, which yielded a simple nine-line ESR pattern caused by the two sets of two equivalent nitrogens with Ani = 3.40 and An2 = 0.81 G (79JOC3211).Tlie nonequivalency of the nitrogens was explained by the association of the potassium cation with one of the two diazacylopentadienyl moieties (Scheme 44). 

C. Formation of MAIs By Anionic Chain Polymerization-Anion Radical Transfer... 

The above discussion indicates that nucleophilic organic radicals also induce the formation of arenediazenyl radicals. This was shown by Bespalov (1980) for the lithium salt of the tetracyanoquinodimethane anion radical (8.54). 

A large number of other sensitizers has been investigated for use in photolytic de-diazoniation. The excited states of these compounds (S ) react either by direct electron transfer (Scheme 10-97), as for pyrene, or by reaction with an electron donor with formation of a sensitizer anion radical which then attacks the diazonium ion (Scheme 10-98). An example of the second mechanism is the sensitization of arenedi-azonium ions by semiquinone, formed photolytically from 1,4-benzoquinone (Jir-kovsky et al., 1981). 

Shislov and coworkers13 studied the photochemical transformation of the paramagnetic particles of irradiated polycrystalline DMSO-d6 in order to evaluate the energy of the electrons involved in the formation of the anion-radical pair. 

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... 

This behaviour is shown in the voltammetry of 68 which leads under suitable conditions to the formation of benzylidene acetone at the cathodic interface. The latter structure exhibits a reversible step, i.e., formation at — 1.25 V of a fairly stable anion radical. 

The two main conditions (besides the stability of the a radical towards the solvent to observe such an electron catalysis are a sufficient high rate of addition of the nucleophile and the thermodynamic inequality E°ll >E°12 implying a fast displacement of the latter equilibrium to the direction of the formation of the anion radical of 71. 

This review is concerned with the formation of cation radicals and anion radicals from sulfoxides and sulfones. First the clear-cut evidence for this formation is summarized (ESR spectroscopy, pulse radiolysis in particular) followed by a discussion of the mechanisms of reactions with chemical oxidants and reductants in which such intermediates are proposed. In this section, the reactions of a-sulfonyl and oc-sulfinyl carbanions in which the electron transfer process has been proposed are also dealt with. The last section describes photochemical reactions involving anion and cation radicals of sulfoxides and sulfones. The electrochemistry of this class of compounds is covered in the chapter written by Simonet1 and is not discussed here some electrochemical data will however be used during the discussion of mechanisms (some reduction potential values are given in Table 1). 

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... 

More recently it has become apparent that proton equilibria and hence pH can be equally important in aprotic and other non-aqueous solvents. For example, the addition of a proton donor, such as phenol or water, to dimethylformamide has a marked effect on the i-E curve for the reduction of a polynuclear aromatic hydrocarbon (Peover, 1967). In the absence of a proton donor the curve shows two one-electron reduction waves. The first electron addition is reversible and leads to the formation of the anion radical while the second wave is irreversible owing to rapid abstraction of protons from the solvent by the dicarbanion. 

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