Solvated electron from aromatic anions

The results for phenolate and naphtholate show that internal transition may lead to solvated electron formation from aromatic anions. The fact that the products are the negatively charged solvated electron and a radical which is neutral (and not a positively charged one) may be partly responsible for the increased efficiency of anions over the undissociated molecules. Primary recombination may decrease in the absence of coulombic attraction. Moreover the ionization potential of the anion is lower. 

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). 

Rathore et al. (2006) studied the intramolecular single-electron transfer in anion-radicals formed from fluorenylidene derivatives. The derivatives used for the reduction were Me— Flu—CH2—Flu—CH2—Flu—CH2—Flu—Me and its deuterated analog, Me—Flu—CH2—(Flu-d8)— CH2—(Flu-dj)—CH2—Flu—Me. Each parent compound initially gave an anion-radical in which an unpaired electron was tunneled between the two internal Flu nuclei and then occured within the outer Flu nuclei. In the outer part, coordinative solvation of the anion-radical by HMPA proceeded much more effectively because of ready space accessibility. Such a solvation provides a driving force for electron tunneling. As the solution electron affinities of perdeuterated aromatic hydrocarbons are less than those of perprotiated hydrocarbons, the electron tunneling was found to be at least an order of magnitude faster only in the case of [Me—Flu—CH,—(Flu-do)—CH,— (Flu-d8)-CH2-Flu-Me]-. ... 

In general, from among the protic solvents, only liquid ammonia (the first used)1 is particularly useful, and is still used more than any other solvent despite the low temperature at which reactions have to be carried out (b.p. -33 °C) and the fact that solubilities of some aromatic substrates and salts (M+Nu-) are poor. Ammonia has the added advantage of being easily purified by distillation, being an ideal system for production of solvated electrons, and has very low reactivity with basic nucleophiles and radical anions, and aryl radicals. Also, poor solubilities can sometimes be ameliorated by use of cosolvents such as THF. In addition it can be used as a solvent for the in situ reductive generation of nucleophiles such as ArSe- and ArTe- ions, e.g. the formation of PhTe- from diphenyl ditelluride (equation 16).54 55... 

Alkali and alkaline earth metals dissolve in liquid ammonia with the formation of solvated electrons. These solvated electrons constitute a very powerful reducing agent and permit reduction of numerous conjugated multiple-bond systems. The technique, named for Birch provides selective access to 1,4-cydohcxiidicnes from substituted aromatics.8 In the case of structures like 21 that are substituted with electron-donating groups, electron transfer produces a radical anion (here 22) such that subsequent protonation occurs se lectively in the ortho position (cf intermediate 23) A second electron-transfer step followed by another protonation leads to com pound 24... 

Birch reduction11 is the partial reduction of aromatic rings by solvated electrons produced when alkali metals dissolve (and react) in liquid amines. Typical conditions are sodium in liquid ammonia or lithium in methylamine. These electrons add to benzene rings to produce, probably, a dianion 57 that is immediately protonated by a weak acid (usually a tertiary alcohol) present in solution. The anions in the supposed intermediate 57 keep as far from each other as they can so the final product is the non-conjugated diene 58. It is important to use the blue solution of solvated electrons before it reacts to give hydrogen and NaNH2. 

In the electroreduction of aromatic hydrocarbons, nitro compounds, and quinones in aptotic solvents, the first step is the transfer of an electron from the electrode to form a radical anion. Once the radical anion is formed, electron repulsion will decrease the facility with which a second electron transfer occurs. But solvation and ion pairing diminish the effect of electron repulsion and tend to shift the reduction potential for the addition of the second electron to more... 

Many aromatic compounds, including benzene, also have high rates of reaction with the solvated electron. However, the reaction does not typically lead to decomposition of the target compound. Instead, a radical anion is produced from the parent species, as shown in Eq. (82) ... 

One of the solvated electrons is transferred into an antibonding 7t -orbital of the aromatic compound, and a radical anion of type C is formed (Figure 17.82). The alcohol protonates this radical anion in the rate-determining step with high regioselectivity. In the case under scrutiny, and starting from other donor-substituted benzenes as well, the protonation occurs in the ortho position relative to the donor substituent. On the other hand, the protonation of the radical anion intermediate of the Birch reduction of acceptor-substituted benzenes occurs in the para-position relative to the acceptor substituent. 

Fig. 17.82. Birch reduction of benzenes give 1,4-cyclohexa-dienes. The radical anion C is formed by capture of a solvated electron in an antibonding n -orbital of an aromatic compound. The alcohol protonates this radical anion to the radical D, which captures another electron from the solution to form the carbanion E. The carbanion is protonated by a second equivalent of the alcohol, and the 1,4-dihydroaromatic compound results.

In the S l mechanism of aromatic substitution the initiating step is the formation of a radical anion. In order to distinguish the process from the route described above (SR+N1) in which a radical cation plays a crucial role, the symbol S l has been used17. Creation of the radical anion can occur by several procedures. The reaction can be electrochemically initiated, a solvated electron in a solution of alkali metal in liquid ammonia may be involved or a radical anion may be used as the source of electrons. The most common source of electrons is, however, the nucleophile itself involved in the substitution reaction. In many cases the electron transfer from nucleophile to substrate is light-catalysed and the process is then sometimes referred to as S l Ar. Although the nucleofugic group in S l... 

A solution of sodium in ammonia may be considered as a source of solvated electrons. The alcohol functions as a proton source. The aromatic molecule accepts an electron from the solution to form a radical anion 1, protonation of which by the alcohol forms the radical 2 (Scheme 11.2). Acceptance of a second electron generates a new carbanion, which is also protonated and gives the 1,4-diene 3. The overall transformation is reduction of the aromatic compound to the 1,4-diene. 

Spectra and kinetics were also determined for many other species. The solvated electron was observed and its spectrum was determined in a wide variety of solvents, from ethers and alcohols to hydrocarbons and even supercritical fluids. Other radicals, including the benzyl radical, the first species studied in pulse radiolysis, were observed. Excited states, both singlet and triplet, anions and cations, were determined for aromatic species. The number and variety of species is large. The importance of these studies was that it was now possible to observe the intermediate states in the radiation-chemical reactions and thus confirm or refute reaction mechanisms that had been proposed based on product yield data. 

For reduction, relevant data from polarographic and cyclic voltammetric experiments are summarized in Tables 1 and 2, respectively. For the results in Table 1 the variety of solvents and reference electrodes used makes comparisons difficult. It is clear, however, that even with the activation of a phenyl substituent (entries 6,7,9-14) reduction occurs at very cathodic potentials. In this context it is worth noting that in aprotic solvents at ca. — 3 V vs. S.C.E.) it becomes difficult to distinguish between direct electron transfer to the alkyne and the production of the cathode of solvated electrons. Under the latter conditions the indirect electroreductions show many of the characteristics of dissolving metal reductions (see Section II.B). Even at extreme cathodic potentials it is not clear that an electron is added to the triple bond the e.s.r. spectra of the radical anions of dimesitylacetylene and (2,4,6,2, 4, 6 -hexa-r-butyldiphenyl)acetylene have been interpreted in terms of equal distribution of the odd electron in the aromatic rings . 

Hoijtink et al. [27] also developed an alternative method of generating anionic species, which was improved by Szwarc et al. [28]. The technique involves potentiometric titration of aromatic compounds with a standard solution of Na-biphenylide. The extremely negative reduction potential of biphenyl assures that most of the common aromatics can be reduced to at least their respective radical anions. The values of the thermodynamic reduction potentials are generally obtained from the potentiometric titration curve. As all experiments are usually carried out in ethereal solutions, such as tetrahydrofuran (THF) or dimethoxyethane, problems of follow-up processes are less severe. Later, Gross and Schindewolf [29] reported on the potentiometric titration of aromatics using solvated electrons in liquid ammonia. 

The radical nature of the anion radical (X) has been established from electron spin resonance spectroscopy and the carbanion nature by its reaction with carbon dioxide to form the carboxylic acid derivative. The equilibrium in Eq. (8.13) depends on the electron affinity of the aromatic hydrocarbon and the donor properties of the solvent. Tetrahydrofuran (THF) is a useful solvent for such reactions. This fairly polar solvent (dielectric constant = 7.6 at room temperature) promotes transfer of the s electron from the alkali metal to the aromatic compound and stabilization of the resultant complex, primarily via solvation of the cation. Sodium naphthalenide is... 

The halogenated biphenyls represent a class of compounds where reductive conditions [51] (solvated electron, aromatic radical anions, ketyl radicals) answer the purpose much better than the OH radical. However, this is to be seen in the context that the toxicity of the halogenated biphenyls exceeds that of biphenyl itself by such a wide margin that the latter compound is considered as relatively harmless. Complete removal of the pollutant would still have to rely on the oxidative pathway. A similar situation exists with respect to nitro-aromatics [52] which are also subject to reductive attack, indirectly by the OH radical via a-hydroxyethyl radical generated from the additive ethanol [53]. 

In polar solvents, such as acetonitrile, organic donor-acceptor systems such as those listed in Table 6.2 show only the fluorescence due to A no new fluorescence appears as in exciplex formation. Flash spectroscopy shows absorption spectra characteristic of the hydrocarbon radical anion and the amine radical cation. The product in these solvents is either an ion-pair or two free ions, stabilised no doubt by solvation, and the reaction is a complete transfer of an electron from one molecule to another, rather than exciplex formation. The reaction goes effectively to completion, and so (with only one fluorescence lifetime to be considered) the kinetic equations for the intensity and lifetime reduce to the simple Stem-Volmer forms (Equations (6.16) and (6.19)). The rate constants for the reactions of aromatic hydrocarbons with various amines in acetonitrile are found to be correlated with the free-... 

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