Hydrocarbons radical anions and dianions

Hydrocarbon Radical Anion and Dianion Alkali-Metal Compounds.— The orystal structure of (Ph—Ph), K+(tetraglyme)2 is essentially the same as that of the Rb analogue the cations are surrounded by ten oxygens of the two tetraglyme molecules. The effect of pressure on the ion-pair equilibria, solvent-separated ion-pair (s.s.i.p.)v contact ion-pair (c.i.p.), for Naph, Na+ in THF was studied as was the volume change for the conversion c.i.p. -> s.s.i.p. Disproportionation equilibria [equation (1)] were variously studied, e.g. for ArH = perylene (Pe), ... 

Lithium arene radical anions and dianions, [ArH2] Li and [ArH] 2 Li", can react with acidic hydrocarbons , alkyl halides or alkyl sulfides to form organolithiums via electron transfers. 

Various thermodynamic and kinetic problems concerning the chemistry and physics of radical anions and dianions were investigated quantitatively by electron photoejection. The approaches described allow the determination of the relative electron affinities of various aromatic hydrocarbons and the thermodynamics and kinetics of disproportionation of their radical anions into dianions. These approaches also allow the observation of unstable radical anions and their isomerization or dimerization. 

The thermodynamics of disproportionation can be readily studied with the device shown in Figure 6 (8). The device is composed of two bulbs linked by a narrow tube closed on both ends by sintered plates to prevent diffusion of solutions from one bulb to the other. One bulb contains a 50/50 solution of the hydrocarbon investigated and its radical anion in an ethereal solvent, whereas the other bulb contains a 50/50 solution of the radical anion and its dianion in the same solvent. Two platinum wires inserted in the bulbs act as electrodes, and the connecting tube, forming a liquid junction, is filled with a concentrated solution of a salt that contains the same cation as the solutions of the radical anions and dianions. The whole unit is evacuated and immersed in a Dewar flask containing a liquid kept at constant tern-... 

Holy, N.L., 1974. Reactions of radical-anions and dianions of aromatic-hydrocarbons. Chem. Rev. 74 (2), 243-277. 

For some halides, it is advantageous to use finely powdered lithium and a catalytic amount of an aromatic hydrocarbon, usually naphthalene or 4,4 -di- -bu(ylbiphcnyl (DTBB).28 These reaction conditions involve either radical anions or dianions generated by reduction of the aromatic ring (see Section 5.6.1.2), which then convert the halide to a radical anion. Several useful functionalized lithium reagents have been prepared by this method. In the third example below, the reagent is trapped in situ by reaction with benzaldehyde. 

It was in 1990 that Kratschmer et al. [217,218] reported the first macroscopic preparation of in gram quantities by contact-arc vaporization of a graphite rod in a 100 Torr atmosphere of helium, followed by extraction of the resultant soot with toluene. Fullerene ions could also be detected by mass spectrometry in low-pressure hydrocarbon flames [219]. The door was opened by, Kratschmer and co-workers preparative success to extensive studies of the electrochemical behavior of the new materials. Cyclic voltammetry of molecular solutions of Ceo in aprotic electrolytes, e.g., methylene chloride/quatemary ammonium salts, revealed the reversible cathodic formation of anionic species, the radical anion, the dianion, etc. (cf. [220,221]). Finally, an uptake of six electrons in the potential range of 1-3.3 V vs. SHE in MeCN/toluene at — 10°C to form the hexavalent anion was reported by Xie et al. [222]. This was in full accordance with MO calculations. A parametric study of the electroreduction of Cgo in aprotic solvents was performed [223]. No reversible oxidation of C o was possible, not even to the radical cation. However, the stability of di- and trications with special counterions, in the Li/PEO/C 3 MoFf cell, was claimed later [224]. 

The kinetics of disproportionation is conveniently studied by flash photolysis, A flash of visible light leads to the photoejection of electrons from radical anions or dianions (II). Consider an equilibrated system involving an aromatic hydrocarbon, its radical anion, and its dianion. A flash of light ejects electrons from the dianions and radical anions to convert the dianions into radical anions and the radical anions into the parent hydrocarbon. The ejected electrons are rapidly captured, mainly by the hydrocarbons this process converts the hydrocarbons into radical anions in less than a few milliseconds. The following cases should be considered ... 

The kinetic studies of the electron exchange between the radical anion and the dianion of 3, compared with the reaction between the radical anion and the neutral molecule [67c], provide insight into the structure of the radical anion. The reaction rate was found to be dependent on the counter-cation, in the order Li > Na > K. The electron exchange reaction occurs rapidly between the radical anion and the dianion, and much more slowly between the radical anion and the neutral molecule. This behavior is different from that observed in most aromatic hydrocarbons. Naphthalene shows extremely fast exchange between the radical anion and the neutral molecule, and a slower one with the dianion [69]. The exchange rates of the radical anion of 3 suggest that its carbon skeleton is similar to that of the dianion, and not that of the neutral molecule [67c]. 

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). 

The delocalised radical formed by protonation of the radical-anion is more easily reduced than the starting arene. For some polycyclic aromatic hydrocarbons, the redox potential for this radical species can be determined using a cyclic voltammetry technique [10]. Reduction in dimethylformamide is carried out to the potential for formation of the dianion. The dianion undergoes rapid monoprotonation and on the reverse sweep at a fast scan rate, oxidation of the monoanion to the radical can be observed. The radical intermediate from pyrene has E° = -1.15 V vs. see in dimethylformamide compared to E° = -2.13 V vs. see for pyrene,... 

A number of anion-radicals and dianions of, e.g., aromatic hydrocarbons, heterocycles, and carbonyl and nitro compounds have been used as mediators in cleavages22-27 such mediators must be used in aprotic media such as DMF. 

The most common electrochemical effects exerted in bulk solution are related to association (solvation, ion-pairing, complex formation, etc.) with the electroactive substance or electrochemically generated intermediates [4,19]. The importance of solvation can be gauged by comparing calculated and measured values of the parameter AE1/2 (defined as the difference, in volts between the half-wave potentials of the first and second polarographic waves) exhibited by polycyclic aromatic hydrocarbons (PAH) in dipolar aprotic solvents [46,47], It can be shown that AE1/2 is related to the equilibrium constant for disproportionation of the aromatic radical anion into neutral species and dianion, that is,... 

The protonation of anion radicals and dianions derived from aromatic hydrocarbons has been studied in some detail by Hoijtink and co-workers 113-11 s). it was shown that apart from the reactions given above (Eqs. (42)—(46)) other disproportionation equilibria also play an important role. These are different for different anion radicals, making the whole picture very complex. Kinetic studies on the disproportionation of the nitrobenzene anion radical and some of its derivati-ves 116,u 7) jlave s 10wn that in aqueous solution at a pH > 11.5, reaction (41) is of great importance, whereas the protonated radical ion and the radical ion are the kinetically active species in the pH interval between 3.2 and 11.5. 

The mechanism of the reduction of aromatic hydrocarbons was actually established early by Hoijtink and his co-workers (Hoijtink, 1970) as an ECE mechanism (p. 25) (see also Given and Peover, 1960 Santhanam and Bard, 1966). The two one-electron waves due to formation of anion radical and dianion in an aprotic medium change in a characteristic way upon addition of incremental amounts of a proton donor the height of the first wave increases at the expense of the second one until at sufficiently high concentration only a single two-electron wave is obtained. This behaviour in combination with the HMO calculations referred to above clearly show that the radical anion is protonated to a neutral radioed which is reducible at a less negative potential than the substrate [reaction... 

A radical anion of an aromatic hydrocarbon was implicated as early as 1866, when Berthelot obtained a black dipotassium salt from naphthalene and potassium [41]. This reaction must have proceeded via the naphthalene radical anion as a more or less fleeting intermediate. Again, Schlenk and co-workers captured the essence of such an intermediate. In the case of anthracene they noticed the existence of two different species, a purple dianion and a blue transient species with a banded spectrum [42]. They identified this intermediate as a monosodium addition product which contains trivalent carbon . Further details were revealed only with the advent of electron paramagnetic resonance spectroscopy. 

Since electrode measurements involve low substrate concentrations, reactive impurities have to be held to a very low level. The physical data and purification methods for several organic solvents used in electrode measurements have been summarized (Mann, 1969). But even when careful procedures for solvent and electrolyte purification are employed, residual impurities can have profound effects upon the electrode response. For example, the voltam-metric observation of dications (Hammerich and Parker, 1973, 1976) and dianions (Jensen and Parker, 1974, 1975a) of aromatic hydrocarbons has only been achieved during the last ten years. The stability of radical anions (Peover, 1967) and radical cations (Peover and White, 1967 Phelps et al., 1967 Marcoux et al., 1967) of aromatic compounds was demonstrated by cyclic voltammetry much earlier but the corresponding doubly charged ions were believed to be inherently unstable because of facile reactions with the solvents and supporting electrolytes. However, the effective removal of impurities from the electrolyte solutions extended the life-times of the dianions and dications so that reversible cyclic voltammograms could be observed at ambient temperatures even at very low sweep rates. 

For radical cations this situation is typically observed when deprotonation of the dimer dication is slow and for radical anions under conditions that are free from electrophiles, for example, acids, that otherwise would react with the dimer dianion. Most often, this type of process has been observed for radical anions derived from aromatic hydrocarbons carrying a substituent that is strongly electron withdrawing, most notably and well documented for 9-substituted anthracenes [112,113] (see also Chapter 21). Examples from the radical cation chemistry include the dimerization of the 1,5-dithiacyclooctane radical cations [114] and of the radical cations derived from a number of conjugated polyenes [115,116]. 

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