Benzonitrile radical anion

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

The temperature dependence of kET(CR3> revealed only a moderate change (2.6-3.0 sec ) upon varying the temperature between 163 and 203 K [47]. The longest lifetime of the resulting charge-separated state (i.e., ferricenium ion Ceo radical anion pair) in frozen benzonitrile (PhCN) is determined as 0.38 sec [47], which is more than one order of magnitude louger than any other intramolecular... 

Other methods that use 55 anions as precursor for the synthesis of fullerene-derivatives usually involve chemical formation of the anion. Alkylation of 55 has been accomplished, e.g. by reduction with propanethiol and potassium carbonate in DMF [91,92], sodium methanethiolate in acetonitrile [93], the naphthalene radical anion in benzonitrile[94], potassium naphthalide [95] or simply with zinc [96]. 

D. The chronoamperometric results can also be used to ascertain the number of electrons involved in the formation of benzonitrile from p-chloro-benzonitrile. In order to translate the chronoamperometric data into a meaningful n value, a compound is selected that has a diffusion coefficient very similar to that of p-chlorobenzonitrile and that gives a stable, known product upon electroreduction. Tolunitrile, which satisfies these criteria, is known to be reduced to its radical anion at a diffusion-controlled rate. Since this one-electron process gives a value of 168 pA s1/2- M x cm 2 for it1/2/CA, the corresponding value of 480 pA s1/2 A/ 1 cm-2 for the reduction of p-chlorobenzonitrile to benzonitrile anion radical must represent an overall three-electron process. When we subtract the one electron that is required to reduce benzonitrile to its radical anion from this total, we immediately conclude that two electrons are involved in cleavage of the carbon-chlorine bond in p-chlorobenzonitrile. A scheme that is consistent with these data is described by Equations 21.1 to 21.6. 

The radical anion Cw, can also be easily obtained by photoinduced electron transfer from various strong electron donors such as tertiary amines, fer-rocenes, tetrathiafulvalenes, thiophenes, etc. In homogeneous systems back-electron transfer to the reactant pair plays a dominant role resulting in a extremely short lifetime of Qo. In these cases no net formation of Qo is observed. These problems were circumvented by Fukuzumi et al. by using NADH analogues as electron donors [154,155], In these cases selective one-electron reduction of C6o to Qo takes place by the irradiation of C6o with a Xe lamp (X > 540 nm) in a deaerated benzonitrile solution upon the addition of 1-benzyl-1,4-dihydronicoti-namide (BNAH) or the corresponding dimer [(BNA)2] (Scheme 15) [154], The formation of C60 is confirmed by the observation of the absorption band at 1080 nm in the near infrared (NIR) spectrum assigned to the fullerene radical cation. 

When the haloarene radical anion fragments rapidly, and close to the Na(Hg) surface, only dehalogenation was obtained. However, by using benzonitrile as a redox catalyst, good yields of substitution were found. Thus, the reaction of p-bromoanisole with Ph2P ions and benzonitrile gave 85% of the substitution product. 

Perhaps the most striking example of such behavior is the reduction of halonitrobenzenes and haloben-zonitriles [i]. Even when the reduction of haloben-zenes occurs at more negative potentials than that of nitrobenzenes or benzonitriles, in halonitrobenzenes and halobenzonitriles the acceptance of the first electron yields a radical anion, sufficiently stable to eliminate the halide ion. The resulting radical of nitrobenzene or ben-zonitrile accepts the second electron and a proton. This is followed by the usual reduction of the NO2 or CN group. Thus formally, the C-X bond is reduced before the NO2 or CN group. 

In a study of SRN1 photoreactions of halothiophenes with phenylthiolates anions, phenylthiothiophene radical anions are formed and the yield of the expected phenylthiothiophene product is limited by the bond rupture in this intermediate the thiophenylthiolate anion is formed and detected as thiomethyl ether after quenching of the reaction by methyl iodide. By adding benzonitrile as an extra electron acceptor, the bond rupture may be controlled and the selectivity of the reaction has been improved in favour of the thiophenyl ether but at the expense of the overall reaction rate [118]. 

Therefore, a molecule in the TICT state can be regarded as a rigidly linked radical anion-radical cation pair. Experimental proof for this expectation can be gained from transient absorption spectroscopy. In the simplest case, the absorption spectrum of the TICT excited state is expected to be the sum of the individual ion spectra. This was indeed found in a few cases, with some perturbations which can be explained by the interaction of the closely-spaced radical ions. Thus, the transient absorption spectra of DMABN [135], of DMABK (or DMABA) [137] and of BA [58] resemble the spectrum of benzonitrile and acetophenone radical anion (the absorption of the dimethyl-amino radical cation is expected to be situated in the UV region and could not be observed) and to the sum of anthracene anion and cation absorption spectra, respectively. 

The radical anion and the radical cation of ZnBPur were identified by transient absorption and EPR. The EPR spectra were also consistent with O2 forming a complex with the photosensitizer in glassy benzonitrile. ZnBPur 5uelded a = 0.58, much smaller than a = 0.94 of PdBPur but higher than 0 = 0.33 of H2BPur (81,82). This is consistent with the expected heavy-atom effect and imrelated to a compensation between (Pa and (Pq, as expected from the fact that O2 is not generated in a competitive reaction mechanism. 

The efficiency of electron-transfer reduction of Cgo can be expressed by the selfexchange rates between Coo and the radical anion (Ceo ), which is the most fundamental property of electron-transfer reactions in solution. In fact, an electrochemical study on Ceo has indicated that the electron transfer of Ceo is fast, as one would expect for a large spherical reactant. This conclusion is based on the electroreduction kinetics of Ceo in a benzonitrile solution of tetrabutylammonium perchlorate at ultramicroelectrodes by applying the ac admittance technique [29]. The reported standard rate constant for the electroreduction of Ceo (0.3 cm s ) is comparable with that known for the ferricenium ion (0.2 cm s l) [22], whereas the self-exchange rate constant of ferrocene in acetonitrile is reported as 5.3 x 10 s , far smaller than the diffusion limit [30, 31]. 

The Srn 1 reaction has been applied to heterocycles. Among five-membered ring compounds, halothiophenes have been the most studied they have been shown to be susceptible to both electron-stimulated and photostimulated reactions, and have been converted to the corresponding acetonitriles [150], acetones [151], and phenyl-sulfides [ 152] in low to medium yields. In the study with the benzenethiolate anion it has been shown that the yield is low because of fragmentation of the adduct radical anion it can be increased by adding an electron acceptor, e.g. benzonitrile which prevents decomposition. Further applications include the thermally activated SrnI reaction between 3-iodobenzothiophene and enolates [153] and the photo-stimulated reaction of 3-halo-2-aminobenzothiophenes [154]. 

Reactions of /)-benzoquinone and its derivatives with alkoxide ions (RO R = H, Me, Et, i-Pr, PhCH2) in MeCN also result in formation of the corresponding semiquinone radical anions accompanied by the formation of RO-substituted p-benzoquinones, which are the oxidized products of p-benzoquinones [360], Detailed product and kinetic analyses of the reactions indicate that RO-adduct anion of /)-benzoquinone is an actual electron donor and that RO is acting as a very strong base or nucleophile rather than a one-electron reductant in an aprotic solvent, such as MeCN [360], Similarly, the reaction of C o with methoxide anion (MeO ) in benzonitrile (PhCN) results in the disproportionation of Cgo to yield both C(,o and the methoxy adduct [360]. Spectroscopic and kinetic studies also indicate that a methoxy adduct anion of Ceo is a real electron donor and that MeO is acting as a very strong base or nucleophile rather than an electron donor in PhCN [360],... 

Figure 2. (A) Dependence on A of the logarithm of the rate constants of reoxidation of the following radical anions (with increasing A ) biphenyl, 1-methylnaphthalene, naphthalene, 2-methylphenanthrene, phenanthrene, / -etrphenyl, and benzonitrile in the presence of chlorobenzene. From Ref. [7]. (B) Rate constants of the homogeneous electron exchange between chlorobenzene and redox catalysis as functions of the half-wave potential difference and of the standard potential difference (at room temperature in the considered solvent =). With increasing AE biphenyl, naphthalene, dibenzothiophene, phenanthrene, / -toluonitrile, m-toluonitrile, /7-terphenyl, and benzonitrile. From Refs 1 and 2.

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