Anthracene, electrochemical reduction

Fig. 2 Reduction of /-BuBr in DMF. Left electrochemical reduction. Right reduction by anthracene anion radical. O experimental data, oblique lines theoretical predictions.

In several examples the reductive halide-hydrogen exchange has been studied on a preparative scale. For example, the indirect electroreduction of 2-chloropyridine in DMF using anthracene as mediator gives pyridine in 83-86 % yield 2 . For the dehalogenation of 1-chlorohexane (80% yield), naphthalene is applied as redox catalyst. Similarly, 6-chloro-hexene yields 1-hexene (60%) and methylcyclopentane (25%), which is the product of the radical cyclization . The indirect electrochemical reduction of p- and y-bromocarboxylic esters forms coupling and elimination products besides the dehalogenated products... 

Repeated cycling through the RUB reduction wave resulted in a decrease in size of the catalytic current. This occurred even when the solution was stirred between cycles. This behavior implies that a blocking or filming of the electrode occurred during the reduction process. Repeated cycling over the oxidation wave removed the film and reactivated the electrode. The electrochemical reduction of 9,10-diphenylanthracene (DPA), 1,3,6,8-tetraphenylpyrene (TPP), anthracene (ANT), fluoranthene FLU) and 2,5-diphenyl-l,3,4-oxadiazole (PPD) in the presence of S Os - all showed similar cathodic waves. 

Scheme 1.7 Electrochemical reduction of anthracene with phenol as proton donor.

Figure 17.7 Electrocatalysis of O2 reduction by Pycnoporus cinnabarinus laccase on a 2-aminoanthracene-modified pyrolytic graphite edge (PGE) electrode and an unmodified PGE electrode at 25 °C in sodium citrate buffer (200 mM, pH 4). Red curves were recorded immediately after spotting laccase solution onto the electrode, while black curves were recorded after exchanging the electrochemical cell solution for enzyme-fiiee buffer solution. Insets show the long-term percentage change in limiting current (at 0.44 V vs. SHE) for electrocatalytic O2 reduction by laccase on an unmodified PGE electrode ( ) or a 2-aminoanthracene modified electrode ( ) after storage at 4 °C, and a cartoon representation of the probable route for electron transfer through the anthracene (shown in blue) to the blue Cu center of laccase. Reproduced by permission of The Royal Society of Chemistry fi om Blanford et al., 2007. (See color insert.)...

A different electrochemical approach was applied to the cathodic reduction of sulfones in W,JV-dimethylformamide (Djeghidjegh et al., 1988), for example t-butyl phenyl sulfone, which is reduced at a more negative potential ( pc = -2.5 V) than is PBN (-2.4 V). Thus, the electrolysis of a mixture of PBN and the sulfone would possibly proceed via both true and inverted spin trapping. If a mediator of lower redox potential, such as anthracene (-2.0 V), was added and the electrolysis carried out at this potential, it was claimed that only the sulfone was reduced by anthracene - with formation of t-butyl radical and thus true spin trapping was observed. It is difficult to see how this can be reconciled with the Marcus theory, which predicts that anthracene - should react preferentially with PBN. The ratio of ET to PBN over sulfone is calculated to be 20 from equations (20) and (21), if both reactions are assumed to have the same A of 20 kcal mol-1. 

The addition of alkyl halides to aromatic anion radicals, generated by alkalimetal reduction in ethereal solvents, was already known in the 1950s [201] and was reviewed by Garst in 1971 [202]. The first electrochemical analogue was observed by Lund etal. [203]. These authors cathodically reduced hydrocarbons such as naphthalene, anthracene, stilbene [145, 146], and pery-lene [147-150] in the presence of alkyl halides and isolated hydrogenated and alkylated products. Similar reactions are observed when the halides are replaced by ammonium or sulfonium [204]. 

Fig. 3 Electrochemical and homogeneous standard free energies of activation for self-exchange in the reduction of aromatic hydrocarbons in iV.A -dimethylformamide as a function of their equivalent hard sphere radius, a. 1, Benzonitrile 2, 4-cyanopyridine 3, o-toluonitrile 4, w-toluonitrile 5, p-toluonitrile 6, phthalonitrile 7, terephthalonitrile 8, nitrobenzene 9, w-dinitrobenzene 10, p-dinitrobenzene 11, w-nitrobenzonitrile 12, dibenzofuran 13, dibenzothiophene 14, p-naphthoquinone 15, anthracene 16, perylene 17, naphthalene 18, tra 5-stilbene. Solid lines denote theoretical predictions. (Adapted from Kojima and Bard, 1975.)...

A photoexcited anion also plays a role in the photoelectrocatalytic reduction of 4-chlorobiphenyl using the anion radicals of anthracene and 9,10-diphenylanthracene409. The anion radicals were electrochemically generated and excited by means of visible light. Formation of the aryl anion radical then takes place either by direct electron transfer or by... 

In another example rather similar electrochemically generated anion radicals of anthracene and 9,10-diphenylanthracene were excited by visible light and found to give a greater than 10-fold increase in the rate of reductive dechlorination of 4 chlorobiphenyl. The kinetic results suggested a fast light-assisted reduction pathway implying either the photoexcited anion radical or an electron ejected from it [183]. 

Before moving on, one important exception to the SSR is worth noting, which arose from the work of Bewick, Pons and coworkers [92, 93] on the adsorption of acrylonitrile, and the later work of Ko-rzeniewsld and Pons [94-96]. In essence, the work showed that the vibrations of an adsorbed molecule that are parallel to the electrode surface may become activated as a result of the electric field, and this was termed the electrochemical Stark effect [94-96] as would be expected, this effect depends very strongly on the nature of the adsorbed molecule. Thus, for example, Pons and coworkers [96] observed a bipolar band centered near 1600 cm in the in situ infrared spectrum of anthracene on reducing it to its radical anion the band was attributed to the Ag C—C symmetric stretch of the anthracene shifting to lower V on reduction. As the molecule adsorbs... 

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