Radicals anion, aromatic

The sonochemical formation of arene-metal compounds appeared in the literature in 1957, and a number of examples were found (Fig. 9). Benzoquinoline-sodium was prepared in 45 min at room temperature instead of a 2-day reflux in ether. 

In most of the recent work, cleaning baths were found to be sufficient to ensure a satisfactory reaction. A series of aryl and hetero-aryl radical anions and aromatic dianions were prepared. scheme describes the experimental set-up. In the presence of N,N -tetramethylethane- or N,N -tetramethyl-propanediamine, non-ethereal solvents can be used.77/78 

77 Sugahara, K. Fujita, T. Watanabe, S. Hashimoto, H. /. Ghent. Technol. Biotechnol. 1987,37, 95-99 Ghent. Abstr. 1988,108, 21317x. 

Radical anions from aromatics which are intermediates in Birch-type reductions were prepared sonochemically. Pyridine, quinoline, and indole sonicated with lithium in THF in the presence of trimethylsilyl chloride yield the bis-TMS dihydroaromatics, which can be reoxidized, by air or benzoquinone, in a rapid and easy method to prepare silyl-substituted aromatics. The procedure was extrapolated to phenols (Eq. 6).  

---

Aromatic Radical Anions. Many aromatic hydrocarbons react with alkaU metals in polar aprotic solvents to form stable solutions of the corresponding radical anions as shown in equation 8 (3,20). These solutions can be analyzed by uv-visible spectroscopy and stored for further use. The unpaired electron is added to the lowest unoccupied molecular orbital of the aromatic hydrocarbon and a... 

Sodium naphthalene [25398-08-7J and other aromatic radical anions react with monomers such as styrene by reversible electron transfer to form the corresponding monomer radical anions. Although the equihbtium (eq. 10)... 

Monomers which can be polymerized with aromatic radical anions include styrenes, dienes, epoxides, and cyclosiloxanes. Aromatic radical anions... 

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, are efficient difunctional initiators (eqs. 6,7) (3,20,64). However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization, since the unique abiUty of lithium to provide high 1,4-polydiene microstmcture is lost in polar media (1,33,34,57,63,64). Consequentiy, a significant research challenge has been to discover a hydrocarbon-soluble dilithium initiator which would initiate the polymerization of styrene and diene monomers to form monomodal a, CO-dianionic polymers at rates which are faster or comparable to the rates of polymerization, ie, to form narrow molecular weight distribution polymers (61,65,66). 

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). 

Table 3 presents the hyperfme splitting constants of some sulfur-containing aromatic radical anions. The series studied included the monoxides and dioxides of dibenzothio-phene 1, thioxanthene 2, thioxanthone 3, dibenzo[b,/] thiepin 4 and dithienothiophene dioxide 5. 

TABLE 3. Hyperfine splitting constants (gauss) of some sulfur-cpntaining aromatic radical anions obtained in 1,2-dimethoxyethane with K metal as the reductant at — 80 °C... 

First reported by Fredenhagen in 1926 F3, F4), the graphite-alkali-metal compounds possess a relative simplicity with respect to other intercalation compounds. To the physicist, their uncomplicated structure and well defined stoichiometry permit reasonable band-structure calculations to be made S2,12) to the chemist, their identity as solid, "infinite radical-anions frequently allows their useful chemical substitution for such homogeneous, molecular-basis reductants as alkali metal-amines and aromatic radical anions N2, B5). 

A number of synthetic procedures are available (Ai2). (2) For precisely defined stoichiometries, the isobaric, two-bulb method of Herold is preferred H5, H6, H2). (2) To generate compounds suitable for organic synthesis work, graphite and alkali metal may be directly combined, and heated under inert gas (Pl, lA). (5) Electrolysis of fused melts has been reported to be effective iN2). 4) Although alkali metal -amine solutions will react with graphite, solvent molecules co-inter-calate with the alkali metal. Utilization of alkali metal-aromatic radical anion solutions suffers the same problem. 

Table 2.1 Hyperfine parameters and spin densities for aromatic radical anions. (Data from ref. 11.)...

The sonochemistry of the other alkali metals is less explored. The use of ultrasound to produce colloidal Na has early origins and was found to greatly facilitate the production of the radical anion salt of 5,6-benzo-quinoline (225) and to give higher yields with greater control in the synthesis of phenylsodium (226). In addition, the use of an ultrasonic cleaning bath to promote the formation of other aromatic radical anions from chunk Na in undried solvents has been reported (227). Luche has recently studied the ultrasonic dispersion of potassium in toluene or xylene and its use for the cyclization of a, o-difunctionalized alkanes and for other reactions (228). 

Scheme 12.4. Opening of epoxides in the presence of aromatic radical anions.

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-... 

The solvent effects on the C-Cl bond cleavage in the aromatic radical anions of 9-chloroanthracene, 3-nitrobenzyl chloride, and 3-chloroacetophenone were described by applying the Saveant model. The results showed that the bond dissociation energy is not strongly solvent dependent. 

It has been shown that when nucleophilic aromatic photo-substitution reactions are carried out in non-deoxygenated solutions of aprotic solvents, such as DMSO and acetonitrile, destructive superoxide anions may be formed from aromatic radical anions. Such solvents are best avoided. There has been a review of mechanistic aspects of photo-substitutions of the cyano group in aromatic compounds. ... 

Scliemc 4.1. Dissociative electron transfer using an aromatic radical-anion as electron relay... 

Rate constants in excess of 10 M s are determined by pulse-radiolysis methods [4, 5]. High-energy irradiation of a solution containing the substrate and an excess of the aromatic species, generates the aromatic radical-anion. The decay of this by electron transfer to the substrate is followed using uv-spectroscopy and affords a rate constant for the second-order process. Slow rates of electron transfer are determined by adding the substrate to a solution of the aromatic radical-anion and following the reaction by conventional methods. 

Figure 43. Rates of disociative electron transfer from aromatic radical-anions to the peroxide 2 measured in dimethylformamidc by cyclic voltammetry. Data from ref. [9],...

Figure 4.4. Homogeneous electrocatalytic redticiion of 2-bromopyridine in ditnethylfomiamide. Forward electron transfer rate constant vs. standard potential of the aromatic radical-anion donor. Data from ref, [29],...

Figure 4.5, Potential energy diagrams for the homogeneous electron transfer reaction between an aromatic radical-anion and a second aromatic with a frangible R-X bond, (a) The situation where back electron transfer and bond cleavage have similar free energy of activation, (b) The situation where the RX radical-anicm has high energy and the R-X bond has low dissociation ertergy.

Figure 4.6. Homogeneous dissociative electron transfer reaction between aromatic radical-anions and (a) di-(4-cyanophenyl) disulphide, (b) diphenyl disulphide in dimethyl formamide. Data from ref [31J.

One approach to this problem [40, 41] generates the alkyl radicals by homogeneous electron transfer to an alkyl halide from an aromatic radical anion. These... 

Aromatic radical-anions react as nucleophiles towards alkyl halides and carbon dioxide. With alkyl halides, the rate-determining step is an outer sphere one-electron transfer to generate the alkyl radical and halide ion. The radical then adds... 

Aromatic radical-anions will react with carbon dioxide. The redox process for carbon dioxide has E° = -2.2 V vr. see [108], so the high yield reactions with the... 

---