Radical anions acidic hydrocarbons

In practice, both the cyciopentadienyl cation and the radical are highly reactive and difficult to prepare. Neither shows any sign of the stability expected for an aromatic system. The six-77-electron cyciopentadienyl anion, by contrast, is easily prepared and remarkably stable. In fact, cyclopentadiene is one of the most acidic hydrocarbons known, with p/C, = 16, a value comparable to that of water Cyclopentadiene is acidic because the anion formed by loss of H+ is so stable (Figure 15.5). 

Formally related reactions are observed when anthracene [210] or arylole-fines [211-213] are reduced in the presence of carboxylic acid derivatives such as anhydrides, esters, amides, or nitriles. Under these conditions, mono- or diacylated compounds are obtained. It is interesting to note that the yield of acylated products largely depends on the counterion of the reduced hydrocarbon species. It is especially high when lithium is used, which is supposed to prevent hydrodimerization of the carboxylic acid by ion-pair formation. In contrast to alkylation, acylation is assumed to prefer an Sn2 mechanism. However, it is not clear if the radical anion or the dianion are the reactive species. The addition of nitriles is usually followed by hydrolysis of the resulting ketimines [211-213]. 

Decay of arene hydrocarbon radical-anions formed during preparative scale electrochemical reduction in the presence of general acids involves protonation as... 

Aromatic 7c-systems bearing two positive charges can accept one electron to form a delocalised radical-cation, which is isoelectronic with the radical-anion from the corresponding aromatic hydrocarbon. The phenanthrene analogue 4 is one such example [30]. Pyrazine is bis-protonated and reduced in acid solution to the... 

Lewis acids such as A1C13, SbCl5, or PFS have been used successfully to generate a variety of radical cations. Antimony pentachloride was first used with hydrocarbons such as benzene or anthracene [22, 23]. Salts obtained from aromatic amines with this reagent were found to be paramagnetic [24] eventually, well resolved ESR spectra identified the formation of radical cations [25,26]. Although an electron transfer mechanism must be involved, the fate of the complementary radical anions and details of their decay are poorly understood. Once again, it appears doubtful that Lewis acids are suitable oxidants for the study of the sometimes delicate substrates discussed in this review. 

With substrates bearing electron-withdrawing groups (EWG) the radical anions are formed in sufficiently high concentrations that protonation by ammonia proceeds rapidly. However, with hydrocarbons and aromatic ethers etc., it is necessary to displace the initial equilibrium between the substrate and the radical anion through protonation by a stronger acid than ammonia (pA(a ca. 35). Alcohols (p (a 16-20) are normally used for this purpose. 

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]. 

Esters of aromatic acids and alcohols are usually on reduction cleaved to aryl car-boxylate and alkyl radical, which may be reduced to the anion and protonated to the hydrocarbon [43,44]. The influence of EGB should be considered when an acid is used as protecting group. Acyl derivatives of phenols as a rule cleaves between the carbonyl group and oxygen to phenolate and acyl radical, and may thus be used for protection of a phenol. The fate of the acyl radical is not clear a dimerization to a diketone has been suggested [45,46]. Such a ketone would be more easily reduced than the ester and possibly attacked by the EGB [47]. The radical anion of some easily reducible esters (e.g., esters of 4-nitrobenzoic acid) cleaves very slowly, but the dianion usually cleaves fast [46]. 

The better defined participation of carbonyl radical anions is evident [56] in the electrocatalyzed reaction between aromatic carbonyl compounds (or other easily reduced carbonyl compounds) and dialkyl phosphonates (Scheme 18). In similar vein, and also in Scheme 18, radical anions generated from aromatic aldehydes may abstract a proton from an added acidic hydrocarbon such as fluorene (pXa22.6) or indene (pXa20.1), and the resulting carbanion adds to unreduced aldehyde. The chain reaction is propagated by protonation of the addition product by another molecule of hydrocarbon [57]. Reaction is by controlled potential coelectrolysis in THE, at the aldehyde reduction potential, and substantial yields are only obtained with 2,6-dichlorobenzaldehyde. 

Those EGBs for which proton-transfer rates are easily measured are radical anions derived by one-electron electrochemical reduction from azobenzenes (Sec. III.A.l), aromatic (Sec. III.C.3), and heteroaromatic hydrocarbons (Sec. III.A.3), and dioxygen (Sec. III.B.l). In those cases the protonated EGB is removed in a fast disproportionation reaction (cf. Sec. II.B, Eq. 2-4), and the proton-transfer step therefore is made effectively irreversible. In CV experiments with addition of an acidic substrate, protonation of the already mentioned radical anions is observed as an increase in the cathodic peak current (change from a one-electron to a two-electron process) and a decrease in the anodic peak current. Where the proton transfer reaction is fast compared to the time scale of the CV experiment, the cathodic peak current is doubled and the anodic peak completely vanishes. If the CV at low scan rates is unchanged after addition of (an excess of) acidic substrate, the EGB is too weak a base to deprotonate the substrate at a reasonable rate. 

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. 

Other typical reactions of acidic hydrocarbons with alkali metals are given in Table 1. Example 4 (Table 1) shows that, in addition to hydrogenation of multiple bonds, reductive cleavage may be a complicating side reaction. In the reaction of triphenylmeth-ane this side reaction may be eliminated by addition of butadiene which, as its radical anion, acts as proton acceptor. 

Certain aromatic hydrocarbons which contain moderately acidic methylene groups react with oxygen in basic solution to give ketyl radical-anions, e.g. 

Since the primary source of oxygen species (ROS) is the superoxide radical anion O [1] and the most reactive free radical is the hydroxyl radical (HO") a wide spectrum of various oxidants damage the cellular components. The hydroxyl radical, for example, reacts very quickly with all major components of cells, e.g., proteins, hydrocarbons, nucleic acids, and hpids [ 1 ]. 

Carbon monoxide reacts at atmospheric pressure with the radical anions of condenses aromatic hydrocarbons in ether solvents, e.g., the sodium-naphthalene-tetrahydro-furan system, is used for promoting the reduction of CO, to give products with new carbon-carbon bonds. Among the products with the naphthalene radical anion, dihydronaphthalene dicarboxylic acids and oxalic acid are isolated after oxidation with air and treatment with water. 

In multielectron transfer processes, the reduction of CO2 can yield formic acid, carbon monoxide, formaldehyde, methanol, or methane that is, the primary electrochemical process supplies Ci compounds. These reactions can proceed at reasonable reduction potentials between —0.24 and —0.61 V (NHE) (Equations (6.12-6.16) the reduction potentials, E°, refer to pH 7 in aqueous solutions versus NHE), while the formation of the C02 radical anion is estimated to take place at —2.1 V.104 Reduction of CO (in the presence of H + ) supplies CH2" radicals that may yield methane directly or leads to higher hydrocarbons (e.g., ethene or ethane) by recombination.24,105 Efficient formation of ethene (together... 

Recently, the Langmuir-Blodgett (LB) technique has been recognized as a useful way of tailoring thin films of molecular thickness. Interest in the LB technique has led to a number of investigations of different types of materials. For example, the classic long-chain fatty acids and alcohols as well as polymerizable molecules, e.g. diacetylenic acids [211-213], aromatic hydrocarbons, e.g. substituted anthracenes [214, 215], TCNQ radical anion salts [216, 217] and charge transfer complexes [218-220] and dye substances can now all be produced as monomolecular layers. 

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