A radical anion

M.p. 296 C. Accepts an electron from suitable donors forming a radical anion. Used for colorimetric determination of free radical precursors, replacement of Mn02 in aluminium solid electrolytic capacitors, construction of heat-sensitive resistors and ion-specific electrodes and for inducing radical polymerizations. The charge transfer complexes it forms with certain donors behave electrically like metals with anisotropic conductivity. Like tetracyanoethylene it belongs to a class of compounds called rr-acids. tetracyclines An important group of antibiotics isolated from Streptomyces spp., having structures based on a naphthacene skeleton. Tetracycline, the parent compound, has the structure ... 

The unique chemical behavior of KO2 is a result of its dual character as a radical anion and a strong oxidizing agent (68). The reactivity and solubiHty of KO2 is gready enhanced by a crown ether (69). Its usefiilness in furnishing oxygen anions is demonstrated by its appHcations in SN2-type reactions to displace methanesulfonate and bromine groups (70,71), the oxidation of benzyHc methylene compounds to ketones (72), and the syntheses of a-hydroxyketones from ketones (73). 

Subsequently Birch and Krapcho and Bothner-By independently postulated the mechanism shown in Eq. (2) and the latter authors presented kinetic data in support of it. Reversible electron addition to the aromatic ring affords a radical-anion (36), the formation of which in other solvents has... 

Flavin coenzymes can exist in any of three different redox states. Fully oxidized flavin is converted to a semiqulnone by a one-electron transfer, as shown in Figure 18.22. At physiological pH, the semiqulnone is a neutral radical, blue in color, with a A ax of 570 nm. The semiqulnone possesses a pAl of about 8.4. When it loses a proton at higher pH values, it becomes a radical anion, displaying a red color with a A ax of 490 nm. The semiqulnone radical is particularly stable, owing to extensive delocalization of the unpaired electron across the 77-electron system of the isoalloxazine. A second one-electron transfer converts the semiqulnone to the completely reduced dihydroflavin as shown in Figure 18.22. 

Several intermediates are involved in the latter reaction. The first is a radical anion resulting from electron transfer from sodium to the alkyne. This then deprotonates ammonia leading to a vinyl radical. The process repeats (electron transfer and deprotonation), and involves a vinyl anion intermediate. 

Irradiation of lomefloxacin 271 in dilute neutral aqueous solution (in which it exists as a zwitter ion) in Pyrex-filtered 500 W medium pressure mercury (Helios Italquartz) at 17°C gave pyrrolo[3,2,l-(/ ]quinoline 272 (99JOC5388). Under this condition, reductive defluorination via a radical anion took place. This study is important because of the phototoxicity of the fluorinated compounds which could be used as antibacterials (Scheme 49). 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

FIGURE 3. Qualitative energy model of a radical-anion pair in sulfoxides where A = CH3, C2H5 B = SOCH3, "SOC2H5 u is the potential energy R(AB) is the distance between A and B. Reproduced by permission of the authors from Reference 16. 

Electrochemical reductions of sulphones have been reviewed212, and have been discussed at intervals213-215. There is evidence that the cathodic reduction reaction proceeds via a radical anion, followed by a cleavage reaction, as outlined in equation (91)212,213. 

A. Radical Anions of Sulfoxide- and Sulfone-containing Aromatic... 

The cleavage of alkyl aryl sulfones by sodium amalgam and alcohols65 probably proceeds also through the intermediacy of a radical anion, followed by splitting to the arylsulfinate anion and an alkyl radical. Both the sulfinate anion and the disproportionation products of the radical have been observed. 

Equation (28) shows that changes in the structure of the interfacial region can lead to catalysis through purely physical factors, namely the distribution of potential (Frurakin, 1961). Thus, if the reactant is uncharged and a radical anion is generated, then a positive shift in 2 would lead to an increase in the rate of reaction. Marked effects of this... 

In certain reactions where nucleophilic substitutions would seem obviously indicated, there is evidence that radicals and/or radical ions are actually involved. The first step in such a process is transfer of an electron from the nucleophile to the substrate to form a radical anion ... 

Some of the reactions in this chapter operate by still other mechanisms, among them an addition-elimination mechanism (see 13-15). A new mechanism has been reported in aromatic chemistry, a reductively activated polar nucleophilic aromatic substitution. The reaction of phenoxide with p-dinitrobenzene in DMF shows radical features that cannot be attributed to a radical anion, and it is not Srn2. The new designation was proposed to account for these results. 

An important synthetic application of this reaction is in dehalogenation of dichloro- and dibromocyclopropanes. The dihalocyclopropanes are accessible via carbene addition reactions (see Section 10.2.3). Reductive dehalogenation can also be used to introduce deuterium at a specific site. The mechanism of the reaction involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

For some halides, it is advantageous to use finely powdered lithium and a catalytic amount of an aromatic hydrocarbon, usually naphthalene or 4,4 -di- -bu(ylbiphcnyl (DTBB).28 These reaction conditions involve either radical anions or dianions generated by reduction of the aromatic ring (see Section 5.6.1.2), which then convert the halide to a radical anion. Several useful functionalized lithium reagents have been prepared by this method. In the third example below, the reagent is trapped in situ by reaction with benzaldehyde. 

The other mechanism, if in fact there is one, is perhaps via a radical anion as per the SET mechanism suggested by Heitz (14,15) for other polymerizations. The ability of the long silicon chain to delocalize electrons would assist in stabilizing either radical anions or straight anionic species. A simple radical is preferred by Zeigler (9) for the formation of the highest MW fraction in the polymerization of PMDS, a product for whose route no evidence has accrued In this work. 

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