Anion radicals, electrolytic production

Controlled one-electron reductions transform l,2,3,4-tetraphenyl-l,3-cyclopentadiene or 1,2,3, 4,5-pentaphenyl-l,3-cyclopentadiene into mixtures of the dihydrogenated products and the corresponding cyclopentadienyl anions (Famia et al. 1999). The anion-radicals initially formed are protonated by the substrates themselves. The latter are thermodynamically very strong acids because of their strong tendency to aromatization. As with the cyclopentadiene anion-radicals, they need two protons to give more or less stable cyclopentadienes. The following equations represent the initial one-electron electrode reduction of l,2,3,4,5-pentaphenyl-l,3-cyclopentadiene (CjHAtj) and explains the ratio and the nature of the products obtained at the expense of the further reactions in the electrolytic pool ... 

On one-electron rednction, aldehydes and ketones give anion-radicals. It is the carbonyl group that serves as a reservoir for the unpaired electron Ketones yield pinacols exclusively. Thus, acetophenone forms 2,3-diphenylbutan-2,3-diol as a result of electrolysis at the potential of the first one-electron transfer wave (nonaqueous acetonitrile as a solvent with tetraalkylammonium perchlorate as a supporting electrolyte) (van Tilborg and Smit 1977). In contrast, calculations have shown that the spin densities on the carbonyl group and in the para position of the benzene ring are equal (Mendkovich et al. 1991). This means that one should wait for the formation of three types of dimer products head-to-head, tail-to-tail, and head-to-tail (cf. Section 3.2.1). For the anion-radical of acetophenone, all of the three possible dimers are depicted in Scheme 5.21. 

Similar redox-combined processes have been reported. For example, it has been clarified by control experiments using a photoirradiated semiconductor electrode that the photocatalytic production of indazoles from substituted azobenzenes is based on the condensation of two intermediates formed through oxidation and reduction.38 40) In the case of oxidation of substituted olefins a similar redox combined mechanism is assumed cation and anion radicals are formed by the reaction of olefin with positive hole and of 02 with excited electron, respectively, and they react to produce a 4-membered ring intermediate, a dioxethane, to undergo bond cleavages into the desired products.4l) In the photocatalytic reactions, a positive hole and excited electron must react at the neighboring surface sites of a small semiconductor particle, enabling the combination of reduction and oxidation without the addition of an electrolyte, which is an indispensable component in electrolysis. However, in the particulate system the recombination of positive hole and electron is also likely, as well as... 

Besides the effect of the electrode materials discussed above, each nonaqueous solution has its own inherent electrochemical stability which relates to the possible oxidation and reduction processes of the solvent,the salts, and contaminants that may be unavoidably present in polar aprotic solutions. These may include trace water, oxygen, CO, C02 protic precursor of the solvent, peroxides, etc. All of these substances, even in trace amounts, may influence the stability of these systems and, hence, their electrochemical windows. Possible electroreactions of a variety of solvents, salts, and additives are described and discussed in detail in Chapter 3. However, these reactions may depend very strongly on the cation of the electrolyte. The type of cation present determines both the thermodynamics and kinetics of the reduction processes in polar aprotic systems [59], In addition, the solubility product of solvent/salt anion/contaminant reduction products that are anions or anion radicals, with the cation, determine the possibility of surface film formation, electrode passivation, etc. For instance, as discussed in Chapter 4, the reduction of solvents such as ethers, esters, and alkyl carbonates differs considerably in Li or in tetraalkyl ammonium salt solutions [6], In the presence of the former cation, the above solvents are reduced to insoluble Li salts that passivate the electrodes due to the formation of stable surface layers. However, when the cation is TBA, all the reduction products of the above solvents are soluble. 

The accessible potential region at a certain electrode is dependent on the choice of supporting electrolyte. The alkali metal cations are reduced at about —2.0 volts (SCE), whereas tetraalkylammonium ions can be used until about - 2.5 volts (SCE). These ions also interact with anion radicals to a lesser extent than the smaller alkali metal cations. This is of importance when using electrolytic reactions for the production of radicals for ESR measurements.35 The tetraalkylammonium ions, however, are more strongly adsorbed at the electrode than the metal ions, and this may influence the kinetics of the reaction. 

One should also mention the work on electroreduction of t-nitrobutane in AN, DMF, DMSO and pyridine by Corrigan and Evans [182]. The results obtained were not in agreement with the dielectric continuum theory. They explained their results by ion-pairing between the anion radical product and cations of the background electrolyte. 

The uv-vislble absorption spectra for the anion radical products were compared with those for the products from controlled potential electrolytic reduction of 3,5-di-tert-butyl-o-benzoquinone (Xmaxz 340 and 380 nm), dehydroascorblc acid (Xmax/ 360 nm), phenazine, lumiflavin (Xmax, 420 nm), and azobenzene (Xmax 410 nm). 

The reduction of CO2 at metallic cathodes has been studied with almost every element in the periodic table °. This reaction can be driven electrochemi-cally or photochemically " and semiconductors have been used as cathodic materials in electrochemical or photoelectrochemical cells . The aim of these studies has been to find cathodes that discriminate against the reduction of H2O to H2 and favor the reduction of CO2 and also to find a cathode selective for one product in the reduction of CO2. A fundamental requirement is that the latter process occurs at a lower overpotential on such electrodes. However the purposes mentioned before in metallic cathodes depends on a series of factors such a solvent, support electrolyte, temperature, pressure, applied overpotential, current density, etc. (we will see the same factors again in macrocyclic electro-catalysis). For instance when protons are not readily available from the solvent (e.g., A,A -dimethylformamide), the electrochemical reduction involves three competing pathways-oxalate association through self-coupling of COj anion radicals, production of CO via O-C coupling between and COj and CO2, and formate generation by interaction of C02 with residual or added water. ... 

ESR spectroscopy as products generated during electrochemical oxidation of PR3 compounds in solution the initially-formed phosphinium radical cation reacts rapidly with a further molecule of phosphine Phosphonium radical anions are the least well characterised of all phosphorus radicals. Electrolytic reduction of PPh3 in CH3CN, at a dropping mercury cathode, produced biphenyl and diphenylphosphonic add. The radical anion was presumed to be an intermediate ... 

Finally, the oxidation of nitrate anions leads to nitrate radicals (NOs ) that add to olefins (Scheme 14) [37]. These oxidations were carried out at a platinum anode using constant current conditions, an undivided cell, a mixed MeCN H2O Et20 solvent system, and LiN03 as the electrolyte. The initial oxidation led to a nitrate product that was not stable and hence... 

A series of bicyclo[3.3.0]octanols are accessible by electroreductive tandem cyclization of linear allyl pentenyl ketones 189, as shown by Kariv-Miller et al. [189]. The electrolyses are carried out with an Hg-pool cathode and a Pt-flag anode. As electrolyte, tetrabutylammonium tetrafluororborate is used. The reaction is stereoselective, yielding only two isomers 192 and 193. In a competing reaction, a small amount of the monocyclic alcohol is formed. Since all the monocycles have the 1-allyl and the 2-methyl group in trans geometry it is assumed that this terminates the reaction. The formation of a bicyclic product requires that the first cyclization provides the cis radical anion which leads to cis-ring juncture [190] (Scheme 37). 

M (CO)6 complexes all undergo irreversible electrochemical reduction in nonaqueous electrolytes at peak potentials close to —2.7 V versus SCE in tetrahydrofu-ran (THF) containing [NBu4][Bp4]. The product of the reductions are the din-uclear dianions [M2(CO)io] although under some conditions polynuclear products can also been obtained, Sch. 3 [2]. It was initially proposed that the primary step involved a single-electron transfer with fast CO loss and subsequent dimerization of the 17-electron radical anion [M(C0)5] [34]. A subsequent study showed that a common intermediate detected on the voltammetric timescale was the 18-electron species [M(CO)5] and that the overall one-electron process observed in preparative electrolysis arises by attack of the dianion on the parent material in the bulk solution, Sch. 2 [35]. 

Electrolytic polymerization or electrolytically initiated polymerization, or shortly electro-initiated polymerization or electropolymerization, generally means initiation by the electron transfer processes which occur at the electrodes of an electrolytic cell containing monomer and electrolyte, in that by controlling the electrolysis current it is possible to control the generation of initiating species. Under appropriate conditions it may proceed by a free radical, anionic or cationic mechanism. In addition to the electrolytic addition polymerization, production of polymers through condensation reaction by electrolytic means should also be covered. Examples of each of these propagation mechanisms have now been reported in the literature. 

Eggins and McNeill compared the solvents of water, dimethylsulfoxide (DMSO), acetonitrile, propylene carbonate, and DMF electrolytes for C02 reduction at glassy carbon, Hg, Pt, Au, and Pb electrodes [78], The main products were CO and oxalate in the organic solvents, while metal electrodes (such as Pt) which absorb C02 showed a higher production for CO. In DMF, containing 0.1 M tetrabutyl ammonium perchlorate and 0.02 M C02 at a Hg electrode, Isse et al. produced oxalate and CO with faradaic efficiencies of 84% and 1.7%, respectively [79], Similarly, Ito et al. examined a survey of metals for C02 reduction in nonaqueous solution, and found that Hg, Tl, and Pb yielded primarily oxalate, while Cu, Zn, In, Sn, and Au gave CO [80, 81]. Kaiser and Heitz examined Hg and steel (Cr/Ni/Mo, 18 10 2%) electrodes to produce oxalate with 61% faradaic efficiency at 6 mA cm-2 [82]. For this, they examined the reduction of C02 at electrodes where C02 and reduction products do not readily adsorb. The production of oxalate was therefore explained by a high concentration of C02 radical anions, COi, close to the surface. Dimerization resulted in oxalate production rather than CO formation. 

The anode acetoxylation of aromatic compounds in solutions of acetic acid carrying alkali or tetraalkylammonium acetates takes the same route. As shown (Eberson 1967 Eberson Jonsson 1981), the process starts with one-electron oxidation at the anode and then passes through the same stages as on oxidation with cobalt trifluoroacetate. The reaction takes place at potentials sufficient to oxidize the substrate but not sufficient to convert the acetate ion into the acetoxy radical. Interestingly, the acetoxyl group comes to the product not from acetic acid (a solvent) but from the acetate ion (a conducting electrolyte) Salts with the tosylate or perchlorate anions stop the acetoxylation in the solution of acetic acid. 

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