Carbanions electron transfer from

The red solution of polystyryl carbanions can be kept for days without change in color or viscosity. No changes are observed on addition of further amounts of naphthalene to the red solution. These observations raise some questions. An electron transfer, say for example, between naphthalene" and phenathrene, is a reversible process and it leads eventually to an equilibrium between naphthalene , naphthalene, phenathrene-, and phenanthrene. Is the reaction involving styrene irreversible Now, the initial process of electron transfer from naphthalene to styrene that produces... 

This oxidative process has been successful with ketones,244 esters,245 and lactones.246 Hydrogen peroxide can also be used as the oxidant, in which case the alcohol is formed directly.247 The mechanisms for the oxidation of enolates by oxygen is a radical chain autoxidation in which the propagation step involves electron transfer from the carbanion to a hydroperoxy radical.248... 

E0 = 40 kJ mol-1 at AH=0) is substituted by a few consecutive fast reactions with electron transfer. Russel [284-291] studied a few reactions of oxidation of alkylaromatic hydrocarbons in the presence of strong bases. He proved the chain mechanisms of these reactions. One of them includes a few stages with addition of dioxygen to carbanion. Another includes the electron transfer from carbanion to dioxygen. 

This reaction was found to be accelerated by the addition of electron acceptors such as nitrobenzene and m-trifluoromethylnitrobenzene. These electron acceptors accelerate the electron transfer from the carbanion to dioxygen. 

Generally, the two-electron reduction of organic halides produces carbanion species. In fact, cathodic reduction of organic halides under certain conditions gives the product derived from the corresponding carbanion intermediates. Silicon is known to stabilize the carbanion at the a position by dn-pn interaction. Therefore, we can expect that silicon promotes the electron transfer from carbon-halogen bonds and the formation of the carbanion at the a position. 

Perhaloalkanes have been found to scramble halogen atoms via consecutive halophilic reactions following carbanion generation by halophilic attack by base. S l reaction of an allylsilane has been applied in a stereocontrolled synthesis of ( )-dihydronepetalactone, and functionalized aryl and arylmethyl carbanions have been generated by reductive cleavage of aryl and arylmethyl alkyl ethers by electron transfer from alkali metals. ... 

On the other hand, five other possibilities exist (b) the rearrangement is influenced by the Co through a loose electronic influence in the radical pair (c) there is an electron transfer from the radical to Co(II) to produce Co(I) and a carbonium ion and (d) there is an electron transfer from the Co(II) to the radical to produce Co(III) and a carbanion. In cases (c) and (d), the ionic substrate derivative would rearrange (e) the Co(II) binds to the substrate radical to form a o-bonded species, and then this new organocobalt species rearranges or generates... 

Intramolecular addition of vinyl radicals to olefins as a method for heterocycle synthesis has been examined. The vinyl radicals can be conveniently generated from vinyl bromides and samarium(II) diiodide [95JOC7424], The intermediate radical after cyclization undergoes a further electron transfer from samarium to furnish a carbanion which is quenched at the end of the reaction. A samarium(II) diiodide mediated aryl radical cyclization onto a dihydrofuran has been reported [95T8555],... 

Garrone et al. (168) have shown that the sensitization of MgO to electron donation by preadsorption extends to propene, butene, and acetylene. Ultraviolet reflectance measurements show bands characteristic of the carbanions, and the protons are assumed to react with 02c to form OH c. The addition of oxygen then leads to an electron transfer from the carbanion to form 02, whereas the radical then formed can oxidize or dimerize. They suggest that in this way OJ can be formed without the need for electron transfer from the solid to form a preexisting radical. However, it is clear that the oxide surface is involved and without the presence of 0 c the reaction will not proceed. The function of the Owould be to abstract a proton from the adsorbed molecule alternatively an electron could be donated to the hydrocarbon molecule and then the 0 c would abstract a hydrogen to form OH c. 

The reason for the E selectivity lies in the mechanism of the elimination. The first step is believed to be two successive electron transfers from the reducing agent (sodium metal) to the sulfone. Firstly, a radical anion is formed, with one extra unpaired electron, and then a dianion, with two extra electrons and therefore a double negative charge. The dianion fragments to a transient carbanion that expels acetate or benzoate to give the double bond. 

The possibility of a thermally activated electron transfer from an anion to an acceptor is not always excluded. As explained by Bordwell [106], this will of course depend on the oxidation potential of the anion or on its related basicity and this author has shown that the electron transfer reactivity of carbanions rapidly decreases with a decrease of basicity. It is thus possible that among the dark SRN1 reactions some of them are activated by an initial ground state electron transfer from the anion to the accepting substrate. 

Photoinduced electron transfer from carbanions were observed by Tolbert who started working in this area with the conviction that the use of low energy chromophores and visible light should minimize troublesome oxidation-reduction reactions [130]. His objective was successfully achieved in certain cases [6], but a substantial part of his work was however concerned with the electron donating propensity of excited carbanions. 

In experiments devoted to the photodecarboxylation of benzannelated acetic acids, the corresponding carbanions were shown to be formed [154], During photolysis in water (pH > 7) and in the presence of p-nitrobenzoic acid, ESR signals assignable to the p-nitrobenzoate radical anion (radical dianion) were observed. This was attributed to an electron transfer from the carbanion. As the authors do not know if the carbanions are adiabatically formed, the question of an electron transfer from an excited or from a ground state carbanion remains open in this case. 

On photoreduction of 9,9 -biacridine in ethanol, Niizuma et al. obtained the radical-cation 91 (R = H) and analyzed its ESR spectrum.319 The corresponding radical 91 (R = Me) has been prepared by electron transfer from carbanions to lucigenin.260 Its ESR spectrum has been analyzed by Janzen and co-workers.259... 

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],... 

Perhaps the most common use of carbanions in organic photochemistry is in the synthetically useful SRN1 reaction. The reaction proceeds via a radical chain mechanism, which requires the transfer of an electron in an initiation step. Photoinduced electron transfer from a carbanion, which also serves as the nucleophile, is a convenient and mild method of initiation. A generalized mechanism is shown in Scheme 9. The excited state anion, with its enhanced... 

It is clear from this review that the topic of photoinduced electron transfer from carbanions is well-developed, with important synthetic applications, as exemplified by the SRN1 reactions. There is sufficient data available to indicate that electron transfer from photoexcited carbanions is a reasonably general process. It is now possible to predict with some certainty which systems will undergo PET. This area will see continued development especially with respect to the details of reaction. Much less is known with respect to PET to carbocations. However, it is clear that this is a developing area and the examples presented provide us with new opportunities for exploratory studies. Whereas neutral molecules have been traditional substrates for PET studies in the past, it is clear that both carbanions and carbocation can also serve as substrates for such investigations, which may lead to interesting results. 

Electron flow through flavocytochrome bz has been extensively studied in both the S. cerevisiae (Tegoni et al., 1998 Daff et al., 1996a Chapman et al., 1994 Pompon, 1980) and H. anomala (CapeillEre-Blandin et al., 1975) enzymes. The catalytic cycle is shown in Figure 3. Firstly, the flavin is reduced by L-lactate a carbanion mechanism has been proposed for this redox step (Lederer, 1991). Complete (two-electron) reduction of the flavin is followed by intra-molecular electron transfer from fully-reduced flavin to heme, generating flavin semiquinone and reduced heme (Daff et al.. 

It would appear then that the redox properties of flavocytochrome 4>2 are well understood. While this is generally true, there are a number of aspects which remain controversial and it is these that will form the main focus of this article. There are three major questions which will be addressed (i) Does the transfer of redox equivalents from lactate to flavin really involve a carbanion intermediate (ii) What controls the intramolecular electron transfers from flavin to heme (iii) Where, on the surface of flavocytochrome 4>2> does cytochrome c bind prior to inter-molecular electron transfer ... 

In such systems, dimerization of the carbanion or electron transfer from another alkali metal atom can produce a di-anion that adds monomer at both ends, complicating kinetics [75],... 

In case the anion is the polymeric species (anionic polymerization) a carbanion or an alkoxide anion forms the active chain end. Initiation is achieved by direct attack of organometaUic compounds, or by electron transfer from alkali metals, alkali metal complexes, or ionizing radiation. In case the cation is the polymeric species (cationic polymerization) a... 

The conversion of L-lactate to pyruvate is a two-electron redox process. One could consider this occurring as two one-electron steps (a radical mechanism) or as one two-electron step. There are two options for a single two-electron step, and these are hydride transfer (H ) or proton (H+) abstraction followed by a two-electron transfer from a carbanion intermediate. These two alternatives for lactate are shown formally in Eqs. (1) and (2) for hydride transfer and the carbanion mechanism, respectively. 

Fig. 12. Alternative routes of electron transfer from the substrate carbanion to the FMN. Oxidation of substrate can occur by a two-electron transfer via a covalent intermediate (A -I- E), by formation of a radical pair followed by a second one-electron transfer to give the covalent intermediate (B + C + E), and by formation of a radical pair directly followed by a second one-electron transfer to give reduced FMN (B + D).

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