Anionic electron transfer, potassium

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

A particular case of a [3C+2S] cycloaddition is that described by Sierra et al. related to the tail-to-tail dimerisation of alkynylcarbenes by reaction of these complexes with C8K (potassium graphite) at low temperature and further acid hydrolysis [69] (Scheme 24). In fact, this process should be considered as a [3C+2C] cycloaddition as two molecules of the carbene complex are involved in the reaction. Remarkable features of this reaction are (i) the formation of radical anion complexes by one-electron transfer from the potassium to the carbene complex, (ii) the tail-to-tail dimerisation to form a biscarbene anion intermediate and finally (iii) the protonation with a strong acid to produce the... 

The electrochemical response of analytes at the CNT-modified electrodes is influenced by the surfactants which are used as dispersants. CNT-modified electrodes using cationic surfactant CTAB as a dispersant showed an improved catalytic effect for negatively charged small molecular analytes, such as potassium ferricyanide and ascorbic acid, whereas anionic surfactants such as SDS showed a better catalytic activity for a positively charged analyte such as dopamine. This effect, which is ascribed mainly to the electrostatic interactions, is also observed for the electrochemical response of a negatively charged macromolecule such as DNA on the CNT (surfactant)-modified electrodes (see Fig. 15.12). An oxidation peak current near +1.0 V was observed only at the CNT/CTAB-modified electrode in the DNA solution (curve (ii) in Fig. 15.12a). The differential pulse voltammetry of DNA at the CNT/CTAB-modified electrode also showed a sharp peak current, which is due to the oxidation of the adenine residue in DNA (curve (ii) in Fig. 15.12b). The different effects of surfactants for CNTs to promote the electron transfer of DNA are in agreement with the electrostatic interactions... 

The redox chemistry of [4]radialenes shows similarities as well as differences with respect to [3]radialenes (see elsewhere1 for a more detailed comparison). The simplest [4]radialene for which a redox chemistry in solution is known appears to be octa-methyl[4]radialene (94). It has been converted into the radical anion 94 (with potassium, [2.2.2]cryptand, THF, 200 K) and into the radical cation 94 + (with AICI3/CH2CI2, 180 K)82. Both species are kinetically unstable, but the radical cation is less stable than the radical anion and disappears even at 180 K within 2 hours, probably by polymerization. For the success of the oxidation of 94 with the one-electron transfer system... 

In case of benzene, the potassium salt of its anion-radical can be separated as a precipitate after benzene reduction by potassium in the presence of low concentrations of 18-crown-6-ether. For benzene, the heavy-form content is greatest in the solution, not in the precipitate. It is in the solution where most of the nonreduced neutral molecules remain. Since the neutral molecules are inert toward protons, the anion-radicals combine with the protons to give dihydro derivatives (products of the Birch reaction). Therefore, it is possible to conduct the separation chemically. The easiest way is to protonate a mixture after the electron transfer, than to separate the aromatic compounds from the respective dihydroaromatics (cyclohexadiene, dihydronaphthalene, etc.) (Chang and Coombe 1971, Stevenson and Alegria 1976 Stevenson et al. 1986a, 1986c, 1988). 

A single-electron transfer from cyclooctatetraene dipotassium (C8H8K2) to 2- and 4-nitrostilbenes in THF leads to the formation of paramagnetic potassium salts of the anion-radicals. In this solvent, the salts exist as coordination complexes (Scheme 3.43). 

A major drawback of the Kaplan-Shechter reaction is the use of expensive silver nitrate as one of the reagents, which prevents scale up to an industrial capacity. Urbanski and co-workers modified the process by showing that the silver nitrate component can be replaced with an inorganic one-electron transfer agent like ferricyanide anion. In a standard procedure the nitroalkane or the corresponding nitronate salt is treated in alkaline media with potassium... 

Electron-transfer initiation from other radical-anions, such as those formed by reaction of sodium with nonenolizable ketones, azomthines, nitriles, azo and azoxy compounds, has also been studied. In addition to radical-anions, initiation by electron transfer has been observed when one uses certain alkali metals in liquid ammonia. Polymerizations initiated by alkali metals in liquid ammonia proceed by two different mechanisms. In some systems, such as the polymerizations of styrene and methacrylonitrile by potassium, the initiation is due to amide ion formed in the system [Overberger et al., I960]. Such polymerizations are analogous to those initiated by alkali amides. Polymerization in other systems cannot be due to amide ion. Thus, polymerization of methacrylonitrile by lithium in liquid ammonia proceeds at a much faster rate than that initiated by lithium amide in liquid ammonia [Overberger et al., 1959]. The mechanism of polymerization is considered to involve the formation of a solvated electron ... 

The first results of anionic polymerization (the polymerization of 1,3-butadiene and isoprene induced by sodium and potassium) appeared in the literature in the early twentieth century.168,169 It was not until the pioneering work of Ziegler170 and Szwarc,171 however, that the real nature of the reaction was understood. Styrene derivatives and conjugated dienes are the most suitable unsaturated hydrocarbons for anionic polymerization. They are sufficiently electrophilic toward carbanionic centers and able to form stable carbanions on initiation. Simple alkenes (ethylene, propylene) do not undergo anionic polymerization and form only oligomers. Initiation is achieved by nucleophilic addition of organometallic compounds or via electron transfer reactions. Hydrocarbons (cylohexane, benzene) and ethers (diethyl ether, THF) are usually applied as the solvent in anionic polymerizations. 

Copolymerizations initiated by lithium metal should give the same product as produced from lithium alkyls. Usually the radical ends produced by electron transfer initiation have so short a lifetime they can have no influence on the copolymerization. This is true for instance in the copolymerization of isoprene and styrene (50). The product is identical if initiated by lithium metal or by butyllithium. With the styrene-methylmethacrylate system, however, differences are observed (79,80,82). Whereas the butyllithium initiated copolymer contains no styrene at low conversions, the one initiated by lithium metal has a high styrene content if the reaction is carried out in bulk and a moderate one even in tetrahydrofuran. These facts led O Driscoll and Tobolsky (80) to suggest that initiation with lithium occurs by electron exchange and that in this case the radical ends are sufficiently long-lived to produce simultaneous radical and anionic reactions at opposite ends of the chain. Only in certain rather exceptional circumstances would the free radical reaction be of importance. Some of the conditions required have been discussed by Tobolsky and Hartley (111). The anionic reaction should be slow. This is normally true for lithium based catalysts in hydrocarbon solvents. No evidence of appreciable radical participation is observed for initiation by sodium and potassium. The monomers should show a fast radical reaction. If styrene is replaced by isoprene, no isoprene is found in the copolymer for isoprene polymerizes slowly by free radical initiation. Most important of all, initiation should be slow to produce a low steady concentration of radical-anions. An initiator which produces an almost instantaneous and complete electron transfer to monomer produces a high radical concentration which will ensure their rapid mutual termination. 

The work of Wooding and Higginson (18) provides further evidence for the fast reduction of styrene by a solution of potassium in liquid ammonia, a reduction that proceeds more readily than the electron transfer process. These workers found that the anionic polymerization of... 

The potassium salt of the phthalodinitrile (ort/zo-dicyanobenzene) anion radical also reacts with an electrophile according to the electron transfer scheme. If the electrophile is tert-butyl halide, the reaction proceeds via the mechanism, including at the first-stage dissociative electron transfer from the anion radical to alkyl halide, followed by recombination of the generated tertiary butyl radical with another molecule of the phthalodinitrile anion radical. The product mixture resulting in the reaction includes 4-tert-butyl-1,2-di-cyanobenzene, 2-tert-bytylbenzonitrile, and 2,5-di(tert-butyl)benzonitrile (Panteleeva and co-authors 1998). 

One curious case of the effect of light on electron-transfer equilibrium involves the reduction of ,p-di(t-butyl)stilbcnc with potassium in DME. The reaction leads directly to a diamagnetic dianion a solution of this dianion remains ESR silent unless subjected to ultraviolet irradiation by a Hg/Xe lamp. The anion radical of a,(3-di(t-butyl)stilbene then formed from the dianion by the loss of an electron. The electron reverted within 5-10 min after ultraviolet irradiation was turned off, transforming the anion radical into the dianion (Gerson et al. 1996). This case deserves to be clarified. Maybe the light effect consists simply in singlet-triplet transformation of the dianion, with the formation of some more or less stable biradical state of the dianion, which possesses two unpaired electrons and can even be a paramagnetic one. 

Photoinduced electron transfer from the carboxylate ion to the excited triplet phthalimide (fcT = 293-300 kj mol-1) appears to be followed by a rapid protonation of the radical anion and cyclization via a biradical recombination reaction (Scheme 9.1). Acetone (which also acts as photosensitizer) containing a small amount of water is the solvent of choice, whereas potassium carbonate is the ideal base to enhance cyclization versus simple decarboxylation and ring opening of the phthalimide [4]. 

The same Anal product 175 is obtained in the reaction of spirostiborane 150 with liquid sodium-potassium alloy, most probably via initial one-electron-transfer generating the anion radicals 176a and/or 176b131. ... 

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