Single electron transfer aromatic anion reactions

An example of transfer of electron from nucleophile to substrate is seen in the formation of the radical anions (observable by ESR) of 5-halo-2A/,3W-benzotf>]thiophene-2,3-diones on treatment with nucleophiles.20 It has been proposed in some cases, that this single-electron transfer step takes place through a charge transfer complex between the nucleophile and the aromatic substrate.21-22 Some reactions occur spontaneously, i.e. without any catalysts or reagents other than the substrate and the nucleophile, but the initiation process is usually, although not invariably, photostimulated (near-ultraviolet radiation, 300-... 

Reactions with reductively and oxidatively generated [26] perfluoroalkyl radicals have also been successfully used for perfluoroalkylation of aromatic compounds (Scheme 2.103). For the reductive initiation, the single electron transfer (SET) necessary for formation of the radical anion priming the reaction sequence can be provided either by a reductive reagent (for example HOCH2SO2Na) [27] or by an electron-rich aromatic substrate itself [28]. The oxidatively induced variant enables the perfluoroalkylation of more electron-deficient aromatic substrates, for example quinoline. 

Aromatic aldehydes can be reduced to their corresponding alcohols over irradiated TiOj [22], The reaction involves the formation of an a-hydroxyl radical via a single electron transfer from excited TiOj to the aldehyde followed by protonation of the radical anion. The hydroxyl radical then is reduced by a... 

The photo-NOCAS reaction was first described by McMahon and Arnold and is a photonucleophilic Sfj2Ar aromatic substitution between dicyanobenzene and an olefin in the presence of electron donor photosensitizers (phenanthrene or biphenyl) in acetonitrile-methanol solutions. This reaction system has been researched extensively in recent times. As shown in Scheme 6, the single electron transfer from olefin to photo-excited electron-deficient dicyanobenzene forms the cation radical of the olefin, which initiates a quenching reaction with nucleophile solvent methanol molecules and forms the methoxyalkyl radical. Addition of an electron transfer photosensitizer (phenanthrene or biphenyl) to the reaction mixture increases the efficiency of the reaction simply by absorbing more Hght. The excited state of the photosensitizer donates an electron to dicyanobenzene to give the photosensitizer radical cation and dicyanobenzene radical anion. The photosensitizer radical cation then oxidizes the olefin. 

ScoOiOPorph/RCoooporph-1 The principle of the method is illustrated in Fig. 18 with the example of reaction (142). The rate constants obtained with the investigated nucleophiles (or with single electron donors—that is the question ) are compared to those of the reaction of a series of anion radicals with the same alkyl halide in the same medium. As discussed on p. 59, aromatic anion radicals behave in this reaction as outer sphere electron donors and the alkyl halide undergoes a dissociative electron transfer. For... 

The feasibility of some of these radical pathways has been examined using Marcus theory to obtain rate constants for comparison with the experimental data (Eberson, 1984). For some relevant anions, including hydroxide, methoxide, t-butoxide, the anion of benzaldehyde hydrate and di-2-propyl-amide, the necessary E°(RO-/RO) values are available or can be estimated with sufficient accuracy. For the reaction of t-butoxide with benzophenone in THF, or the benzaldehyde hydrate anion with benzaldehyde in aqueous dioxan, direct electron transfers between the anion and the neutral are not feasible the calculated rate constants are orders of magnitude too low to be compatible with the observed reduction rates. Any radicals observed in these reactions must arise by some other more complex mechanism. The behaviour of an aromatic aldehyde hydrate dianion has not been examined in this way, but MNDO calculation (Rzepa and Miller, 1985) suggests that such a species could easily transfer either a single electron or a hydrogen atom to an accepting aldehyde. 

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