Aromatic anions, solvated electron

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. 

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). 

Alkali metals in liquid ammonia can transfer an electron to the solvent, leading to so-called solvated electrons. These can add to the aromatic substrate 1 to give a reduced species, the radical anion 3 ... 

In the light of the success of the Birch conditions for reducing organic compounds it is not surprising that epoxides can be opened by solvated electrons [6-9]. The initially formed radical is then further reduced to give carbanionic species, which do not display the reactivity of radicals. This concept has been extended by Bartmann [10], Cohen et al. [11], Conrow [12], and Yus et al. [13,14] who employed aromatic radical anions as the reduc-... 

Aromatic radical-cations are generated by pulse-radiolysis of benzene derivatives in aqueous solution. Radiolysis generates solvated electrons, protons and hydroxyl radicals. The electrons are converted by reaction with peroxydisulpbate ion to form sulphate radical-anion, which is an oxidising species, and sulphate. In another proceedure, electrons and protons react with dissolved nitrous oxide to form hydroxyl radicals and water, Hydroxyl radicals are then made to react with either thallium(i) or silver(i) to generate thallium(ii) or silver(ll) which are powerfully... 

The measurement of the solvation of an aromatic anion and the measurement of the solvation of an electron in alcohols have quite a bit in common. They both observe the... 

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... 

The two free hydroxy groups are First protected with acetic anhydride. In a second step the acetyl group is reductively cleaved by a Birch reduction with lithium in liquid ammonia.19 Lithium dissolves in the ammonia with the formation of solvated electrons. Stepwise electron transfer to the aromatic species (a SET process) leads first to a radical anion, which stabilizes itself as benzylic radical 38 with loss of the oxygen substituent. A second SET process generates a benzylic anion, which is neutralized with ammonium chloride acting as a proton source (see Chapter 12). 

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... 

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. 

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. 

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... 

The reduction process of polycycles by lithium metal converts the neutral atoms to anions. The electron transfer is best achieved in ethereal solvents. This enables the stabilization of the lithium cation by coordination to the oxygen atoms of the solvent. The hydrocarbon anion and the cation are linked together by electrostatic forces in which the solvent molecules are also involved, therefore the ion-solvation equilibrium should be considered8. The limiting cases in this equilibrium are free ions and contact ion-pairs (CIP), and in between there are several forms of solvent separated ion-pairs (SSIP)9. In reality, anionic species of aromatic hydrocarbons in ethereal solvents exist between CIP and SSIP. Four major factors influence the ion-solvation equilibrium of lithium-reduced 7T-conjugated hydrocarbons, as observed by H and 7Li NMR spectroscopies8,10. 

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 . 

The radiation chemistry of 2-propanol is analogous to that of methanol, that is, the main reactive species are Cs and (CH3)2 COH. In alkaline solution, (CH3)2 COH deprotonates to (CH3)2CO . In the presence of N2O or acetone, es is converted to (CH3)2 C0H/(CH3)2C0 by the reactions in Eqs. 30 and 18, or the reaction of Eq. 20, respectively. The solvated electron in 2-propanol has been utilized to study electron-transfer reactions between aromatic radical anions (donor) and aromatic molecules (acceptor) [16]. The donor-acceptor pairs studied were pyrene-anthracene, pyrene-9,10-dimethylanthracene and w-terphenyl-/ -terphenyl. In the first two cases an equilibrium was established and the parameters forward and kback were measured this was the first example of the measurement of an equilibrium constant by use of pulse radiolysis. The rate constants for the electron-transfer reactions were examined in terms of the Marcus theory [17]. 

The reduction is initiated by addition of a solvated electron to the aromatic system to generate a radical anion, which is then protonated by an alcohol cosolvent to furnish a pentadienyl radical. Addition of another electron leads to a pentadienyl... 

---