Electrogenerated proton

Electrogenerated protons (h + ionsj, from the anode reaction, migrate through the membrane towards the cathode under the influence of the applied electric field. These protons are typically produced by the oxidation of water which also forms... 

Carbonyl compounds are reduced to symmetrical ethers, probably by way of reduction of some of the starting material to a silyl ether (9), reaction to form the mixed ketal (10) and then reductive replacement of the silyloxy group. Some hydrocarbon may be obtained as a by-product by reduction of (9 Scheme 4). Among the acid partners that have been used are trifluoroacetic acid, trityl perchlorate (with aldehydes) and electrogenerated protons. With Nafion resin symmetrical ethers are obtained from aldehydes, but silyl ethers are obtained from ketones. ... 

In most cases, the behaviour of sulphones at the cathodic interface (obviously rendered basic when insufficiently buffered due to the accumulation of electrogenerated bases) may be strongly modified by the presence of vicinal CH groups in the a position to the S02 group. These acidic groups may transfer protons and are often responsible for low yields of the cleavage processes or for undesirable isomerization reactions. 

Increased acidity of this hydrogen atom may lead to self-protonation reactions with electrogenerated bases (in aprotic solvents or in unbuffered solutions). 

It has been found50 that such a multielectron step does not exist with 58, which exhibits a classical two-electron scission. In general, allylic sulphones (59) without an unsaturated system in a suitable position are not reducible. Thus, they do not exhibit a cathodic step in protic solutions. However, in aprotic media the isomerization may be base catalyzed, since small amounts of electrogenerated bases from electroactive impurities, even at low concentration, may contribute to start the isomerization. Figure 10 shows the behaviour of t-butyl allylic sulphone which is readily transformed in the absence of proton donor. On the other hand, 60 is not isomerized but exhibits a specific step (Figure 10, curve a) at very negative potentials. 

Several other polypyridyl metal complexes have been proposed as electrocatalysts for C02 reduction.100-108 For some of them HCOO- appears as the dominant product of reduction. It has been shown for instance that the complexes [Rhin(bpy)2Cl2]+ or [Rh n(bpy)2(CF3S03)2]+ catalyze the formation of HCOO- in MeCN (at —1.55 V vs. SCE) with a current efficiency of up to 80%.100,103 The electrocatalytic process occurs via the initially electrogenerated species [RhI(bpy)2]+, formed by two-electron reduction of the metal center, which is then reduced twice more (Rlr/Rn Rh°/Rh q. The source of protons is apparently the supporting electrolyte cation, Bu4N+ via the Hoffmann degradation (Equation (34)). 

The hydrogenation of a double bond during cathodic reductions generally results from addition of protons to an electrogenerated basic species. It is worth noting that partial and selective hydrogenation can take place according to the nature of the heterocycles and the experimental conditions. 

Electrogenerated acids (EGAs) are strong Bronsted acids arising from protons produced directly or indirectly in an anodic... 

Two types of electrogenerated carbon bases have commonly been used (1) dianions derived from activated alkenes, and (2) carbanions formed by reductive cleavage of halogen compounds or by direct reduction of weak carbon acids. In both cases, the efficiency of the proton transfer reaction relies on a thermodynamically favored proton transfer or a fast follow-up reaction of the deproto-nated substrate. 

Results of a study of polar, steric and structural influences on the kinetics of proton transfer (ylide formation) from phosphonium ions to electrogenerated bases have been interpreted with caution. ... 

Reduction of phenyl vinyl sulphones in dimethylformamide containing phenol as proton donor causes loss of phenylsulphinate ion. The reaction probably involves a series of electron and proton addition steps [74]. In absence of a proton source, phenyl vinyl sulphone radical-anion undergoes a dimerization reaction discussed on p. 57. Reactions of alkyl substituted vinyl sulphones are complicated by alkene migration in the presence of electrogenerated bases. Dimers are formed and further reduction leads to loss of phenylsulphinate ion [81] (Scheme 5.3). 

The electrochemical measurements were carried out in the presence of one equivalent of a weak acid (acetanilide) to ensure protonation of the electrogenerated terf-butoxy anion. This was necessary to avoid the interference of the father-son reaction between t-BuO and the perbenzoate, leading to the corresponding ester. The initial one-electron reduction proceeds with 0—0 bond cleavage leading to the formation of t-BuO and ArCOO according to a stepwise (equations 71, 72) or concerted (equation 73) mechanism. At the working potentials, f-BuO is reduced (equation 74) to the anion t-BuO (E° = —0.23 V)and thus the overall process is a two-electron reduction. 

The reduction scheme (equations 85-88) may be complicated by other reactions induced by the initial ET. The most important of these is the selfprotonation, a reaction between an electrogenerated species and the starting material. Self-protonation is the most common case of the so-called father-son reactions. This type of reaction has been characterized both theoretically and experimentally for the case of an initial slow ET by Vianello and co-workers in a study concerning the reduction of Ph2CH-SPh, as shown in equation (89). ... 

It may not always be clear from the conditions for electrochemical generation which species is the effective EGB. In some cases a possible complication is fast disproportionation of radical-anion to dianion (Scheme 12). This can mean that for electrogeneration at, say, the first reduction potential E Jl) it is possible for either the radical-anion or the dianion to act as base, depending on the relative rates of protonation by acid HA (k and kp, the value of the disproportionation constant (Kj), and the rate at which equilibrium between radical-anion and dianion is attained. In principle, of course, it is also possible that electrogeneration at E p2) could lead to a situation where radical-anion was the effective base as a consequence of rapid reproportionation causing it to be present in high concentration, thus offsetting its probably much lower kinetic basicity. These points are discussed in more detail on p. 157. 

If a tetraalkylammonium salt is used as supporting electrolyte, this process is either reversible or quasi-reversible and occurs at around -0.8 V vs aqueous SCE in various aprotic solvents and with various electrode materials (Hg, Pt, GC). If a Bmisted acid is added to the solution, the first step is converted to a two-electron process 0 produced in the first step is protonated to form 02H, which is more reducible than 02. Thus, 02H is further reduced to 02H at the potential of the first step. According to detailed polarographic studies in H20-DMS0 mixtures, about 30% v/v water is needed to convert the one-electron process to the two-electron process [41]. A metal ion, M+, interacts with 02 to fonn an ion-pair M+-02 (often insoluble) and shifts the half-wave potential of the first wave in a positive direction [42]. Electrogenerated superoxide 02 can act either as a nucleophile or as an electron donor and has been used in organic syntheses [43],... 

By analogy with [17] the C60n" anion radicals, which are generated on the cathode under the action of voltage, can reduce the toluene molecules present in the solvate shell. On the other hand, with protons in the medium, protonization of the Cm/1 anion radicals, which are electrogenerated on the cathode, to hydrofullerenes can proceed in the following reaction... 

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