Electron transfer, from radical anions

FIGURE 3.2. Variation of the rate constants of dissociative electron transfer from aromatic anion radicals to butyl and benzyl halides as a function of steric hindrance. Data points from reference 10. Solid lines, best-fit parabola dashed lines, prediction of the Morse curve model, logAf-1 s-1). Adapted from Figure 3 of reference 6b, with permission from the American Chemical Society. 

Introduction of nitrobenzene sulfenates into the same mixture of trichlorosilane and tributylamine results in the evolution of hydrogen. As proven by Todres and Avagyan (1978), trichlorosilane with tributylamine yields the trichlorosilyl anion and tributylammonium cation. This stage starts the process involving one-electron transfer from the anion to a nitrobenzene sulfenate. At that time, nitrobenzene sulfenate produces the stable anion-radical with the tributylammonium counterion. The anion-radical gives off an unpaired electron to the proton from the counterion (see Scheme 1.14). 

The carbon dioxide anion-radical was used for one-electron reductions of nitrobenzene diazo-nium cations, nitrobenzene itself, quinones, aliphatic nitro compounds, acetaldehyde, acetone and other carbonyl compounds, maleimide, riboflavin, and certain dyes (Morkovnik and Okhlobystin 1979). The double bonds in maleate and fumarate are reduced by CO2. The reduced products, on being protonated, give rise to succinate (Schutz and Meyerstein 2006). The carbon dioxide anion-radical reduces organic complexes of Co and Ru into appropriate complexes of the metals(II) (Morkovnik and Okhlobystin 1979). In particular, after the electron transfer from this anion radical to the pentammino-p-nitrobenzoato-cobalt(III) complex, the Co(III) complex with thep-nitrophenyl anion-radical fragment is initially formed. The intermediate complex transforms into the final Co(II) complex with the p-nitrobenzoate ligand. 

Methods of electron spectroscopy are widely used to follow the electron-transfer process. Thus, the progress of electron transfer from naphthalene anion-radical to cup-stacked carbon nanotubes is easily detected by monitoring the UV absorption spectrum. Namely, the absorption band around 500-900 nm due to naphthalene anion-radical completely disappears after reduction of the nanotubes. At the same time, the reduced nanotubes exhibit ESR spectrum characterized with g-factor of 2.0025 (Saito et al. 2006). This g-value is close to the free spin g-factor of 2.0023 that is diagnostic of the delocalized electron on carbon nanomaterials (Stinchcombe et al. 1993). It should be parallelly... 

The [sym-dodeca-13C] benzophenone (equation 313) has been synthesized via Friedel-Crafts phenylation of carbon tetrachloride and subsequent dehalogenation of the product using [per-13C] benzene. Ci2Dio13CO and Ci2Hio13CO were applied637 in the EPR and MS study of the electron transfer from ketyls to isotopically substituted ketones (equation 311-313). The equilibrium constants Keq at —75°C, for electron transfer from the anion radicals of polyatomic hydrocarbons (A -) to their isotopic analogues (B),... 

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). 

The decisive point of the novel scheme is the amortization of CioHgLi by the elimination of the metal hydride. The authors admit that the weakness of their scheme is the lack of evidence for the formation of alkali metal hydride and for the formation of H2 from the (supposed) reaction between the protonating agent and the alkali metal hydride. However, the main sense of this scheme consists of its better agreement with the observed stoichiometry. As to the first step of Scheme 1-27 (the proton landing), it can certainly have a more intimate mechanism, say, electron transfer from the anion radical to the protonating... 

The equilibrium constant for the reaction of the electron transfer from the anion radical salts of aromatic compounds (with the usual isotope content) to neutral molecules of the same compounds containing heavier isotopes is less than unity (entries 1-10 in Table 2-1). This means that for heavier compounds (enriched with neutrons), the electron affinity is smaller. This difference is conserved at different temperatures and reaction mediums (including those favorable to the destruction of ionic pairs—in HMPA and in THF containing 18-crown-6). 

The electron transfer from aromatic radical anions to various electron acceptors takes place efficiently in solution. Likewise, when a second solute, pyrene, is added to the MTHF solution of PVB, the electrons transfer from polymer anions to pyrene occurs [50]. The rate constant determined by pulse radiolysis is approximately a third of that of the electron transfer from biphenyl anion to pyrene. 

The study of the photochemistry of aryl carbanions has been restricted to aryllithiums with only a limited number of studies available. Hence, a general picture of their photochemistry is not available at this time. Photolysis of phenyllithium in the presence of aromatic hydrocarbons such as naphthalene, biphenyl, phenylene, etc. in diethyl ether results in electron transfer from the phenyllithium to the aromatic hydrocarbon, with production of the corresponding hydrocarbon radical anion, as observed by ESR spectroscopy [6-8] (Eq. 1). Photolysis of phenyllithium or 2-naphthyllithium alone gave the corresponding biaryl products and metallic lithium [9-10]. For this reaction, it is possible to write a mechanism which does not require electron transfer from the anion [9,10],... 

The anions obtained by two-electron cathodic reduction of cations 8 and 9 undergo alkylation in the presence of an alkyl halide (80MI3 90ACS524). It has been suggested that the reaction between 2,4,6-triphenylthiopyranyl anion and terz-butyl bromide takes place via a rate-determining electron transfer from the anion to the alkyl halide, followed by combinations of the radicals (90ACS524). 

Certain suitably substituted maleic esters, such as 1, give dicyclopropyl derivatives, 2, by the successive two-electron transfer from naphthalenide anion radicals and intramolecular... 

Some nucleophilic displacement reactions (particularly those involving ketone enolates and other carbon nucleophiles) proceed routinely in low yield, with poor material balance. There is increasing evidence that these reactions are actually radical chain processes catalyzed, in principle, by a single electron transfer from the anion to the heterocycle. In such cases... 

Cation-with-anion reactions are rare in organic chemistry. One known example is a cyclopropenyl cation reacting with a cyclopropenyl anion. The reaction is not even direct C—C bond formation, but first an electron transfer from the anion to the cation, followed by combination of the two radicals, see R. W. Johnson, T. Widlanski and R. Breslow, Tetrahedron Lett., 1976, 4685. 

A few further examples of nucleophilic displacement of halogen from halogenothiophenes by a radical chain SrN, mechanism have been reported. The photostimulated reaction of 2-halo or 3-halothiophene with excess of tetrabutylammonium benzenethiolate in MeCN takes place by an SrN mechanism <87JOC5382>. The 2- or 3-(phenylthio)thiophene is formed in modest yields. The initiation and propagation steps are shown in Scheme 119. The first step may be the photostimulated electron transfer from thiolate anion to the halothiophene (ThX). The reaction does not take place in the dark. 

In alkaline solution, the anion of acetone will be produced by proton transfer from the radical of 2-propanol to the hydroxide ion (R3). Not only electron attachment of the solvated electron (R7), but also electron transfer from the anion of acetone (R4) will be a significant process to form the anion of the halogenated carbons. The radical of 2-propanol will be reproduced due to hydrogen abstraction by radicals formed by the dissociation of an anion of the halogenated carbons (R6). This is a chain reaction and causes effective degradation. On the contrary, dissociative electron attachment is the only process to decompose halogenated hydrocarbons in the pure alcohol solution. 

Aromatic nucleophilic substitution by superoxide occurs by a mechanism different from that encountered in aliphatic nucleophilic substitution reactions of this ion. Thus, reaction of enriched potassium superoxide with l-bromo-2,4-dinitro-benzene catalyzed by dicyclohexyl-18-crown-6 in benzene saturated with unlabeled oxygen results in 2,4-dinitrophenol almost devoid of label. The loss of label in this reaction rules out a direct displacement mechanism. This result is consistent with electron transfer from superoxide anion to the arene to form an intermediate aromatic anion radical which reacts with oxygen (from all sources) to yield phenol. This mechanism is formulated in equation 8.12. Examples of this reaction are presented in Table 8.7. 

Electrocatalytic dehalogenation of halobiphenyls in dry DMF proceeds by the pathway in Scheme 121 2. The first step is fast transfer of an electron from electrode to catalyst A (Equation 1), and the rate-determining step (rds) in DMF is homogeneous electron transfer from the anion radical of the catalyst to halobiphenyl ArX (Equation 2). This rds is thermodynamically unfavorable, and the overall reaction is driven by rapid cleavage of the anion radical of the halobiphenyl in Equation 3. 

Toshiyuki Ishikawa, Masayo Sato, Yuko Itoh, Hrroki (Research Center, Mitsubishi Chemical Corporation, Yokohama, Japan). J. Photopolym. Sci. Technol. (1999), 12 (5), 711-716. Quenching of imidazoyl radical (Im ) produced in 2-[p-(diethyl-amino)styryl]naphtho[l, 2-d]thiazole (NAS) sensitized photolysis of 2,2 -bis(2-chlorophenyl)-4,4, 5,5Gtetraphenyl-l,l -bi-lH-imidazole (BI) in PMMA film in the presence of 2-mercapto-benzothiazole (MBT) was studied by laser flash photolysis using a total reflection cell. It was obsd. that MBT acted as an accelerator increasing the initial concn. of Im by slowing down the back electron-transfer from BI anion to NASA cation, and also quenched Im radical by hydrogen transfer. 

---