Concerted one-electron reductions

Concerted One-Electron Reductions. Reduction of 02 in the presence of excess zinc cation [Znu(bpy)2+], (tetraphenylporphinato)iron(III) ion [(H20)+FemTPP], and cuprous ion [Cu MeCN)/] results in formation of metal-dioxygen adducts. Figures 9.13 and 9.14 illustrate the cyclic voltam-mograms for (FemTPP)Cl and tCuI(MeCN)4]C104, respectively, in the absence and presence of 02. Reaction schemes for the three metal-02 systems are outlined ... 

Of conrse, the cyclic cation-radical formed should be less stable than the alkene cation-radical (which contains a double bond that is favorable for the spin-charge scattering). However, the cation-radical product and corresponding nentral species are generated in a concerted process. The process involves simultaneous covalent bond formation and one-electron reduction of the cyclic product (Karki et al. 1997). Similar to other branched-chain processes, the cation-radical dimerization is characterized by an activation enthalpy that is not too high. These magnitudes are below 20 kJ mol for the pair of cyclohexadiene and trani-anethole (p-MeOCgH4CH=CHCHMe, Z-form Lorenz and Bauld 1987). It is clear that the cation-radical variant of cyclodimerization differs in its admirable kinetic relief. For cyclohexadiene and tran -anethole, catalytic factors are 10 and 10, respectively (Bauld et al. 1987). 

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. 

In aprotic solvents, the radical anion, RX , for aryl halides has been detected as intermediate. In cyclic voltammetry of aryl halides, though an irreversible two-electron reduction occurs at low scan rate, a reversible one-electron reduction occurs at high scan rate. Thus, it is possible to get the values of the standard potential ( °) for the RX/RX couple and the rate constant (k) for RX -> R (therefore, the lifetime of RX ). In Fig. 8.18, the relation between ° and log k for aryl bromides in DMF is linear with a slope of 0.5 [5If], It is apparent that the lifetime of RX , obtained by 1/k, increases with the positive shift of E0. In contrast, the existence of RX for alkyl monohalides has never been confirmed. With these compounds, it is difficult to say whether the two processes, i.e. electron transfer and bond cleavage, are step-wise or concerted (RX+e -> R +X ). According to Sa-veant [5le], the smaller the bond dissociation energy, the larger the tendency for the concerted mechanism to prevail over the step-wise mechanism. 

In this regard the electrochemical reduction behavior of arylmethylalkylsulfo-nium salts is of interest. One-electron reduction produces C—S bond cleavage by a concerted process. We believe that the sulfuranyl radical may be an intermediate in the one-electron reduction of sulfonium salts only when the cation-radical and radical species are extremely unstable. It has been concluded that as the electron enters the C—S a orbital bond cleavage occurs concomitantly [89],... 

One problem with this simple picture is that it seems inherently unlikely that pairs of electrons should act in concert—a pair of electrons in a single orbital spend as much time as far away from each other as possible, so why should a pair of electrons act together to move from one bond into another It seems more reasonable for them to move one at a time. The transfer of one electron from one molecule to another is well known—it is the basis for one-electron oxidation and one-electron reduction, with many examples in electrochemistry, in sodium-in-ammonia reductions, and in inorganic redox reactions—but is it a common pathway in ionic organic chemistry, or something that only happens in favourable circumstances ... 

Casanova and Rogers [59] as well as Fry [69] postulate that the reduction of vicinal dihalides is a concerted process in which both carbon-halogen bonds are partially cleaved as a carbon-carbon double bond starts to form. Nonelectrochemical evidence [70] suggests that a vicinal dihalide undergoes one-electron reduction to a radical anion, which loses the first halide ion to form a neutral radical, after which the neutral radical accepts an electron to become a carbanion that eliminates the second halide ion to yield an olefin. From a study of the behavior of meso- and c/,/-l,2-dibromo-l,2-diphenylethane, Fawell and coworkers [71,72] concluded that the reduction of vicinal dihalides is a stepwise process. Andrieux and coworkers [73] have examined the reductive elimination of vicinal dibromides at carbon in MeCN. 

A number of one-electron reductions has now been reported in which the uptake of an electron by the organic compound is concerted with, or followed by, the cleavage of a covalent bond. 

Successful systems have used colloidal platinum as an efficient catalyst for the multi-electron reduction process by which hydrogen is produced. The platinum acts as a charge pool in that electrons from one-electron processes are trapped, to be later delivered to the substrate in a concerted manner, thus avoiding formation of high-energy intermediates (Figure 12.12). 

As an analogous example, the behavior of sulfonium salts can be mentioned. At mercury electrodes, sulfonium salts bearing trialkyl (Colichman and Love 1953) or triaryl (Matsuo 1958) fragments can be reduced, with the formation of sulfur-centered radicals. These radicals are adsorbed on the mercury surface. After this, carboradicals are eliminated. The carboradicals capture one more electron and transform into carbanions. This is the final stage of reduction. The mercury surface cooperates with both the successive one-electron steps (Scheme 2.23 Luettringhaus and Machatzke 1964). This scheme is important for the problem of hidden adsorption, but it cannot be generalized in terms of stepwise versus concerted mechanism of dissociative electron transfer. As shown, the reduction of some sulfonium salts does follow the stepwise mechanism, but others are reduced according to the concerted mechanism (Andrieux et al. 1994). 

In the electrochemical reduction of aryl diethyl phosphates, the initial one-electron addition is to the aryl ring and this is concerted with expulsion of the diethyl phosphate anion leaving an aryl radical. Further electron addition and protonation leads to the reaction product in 43-73 % yields. Examples of this electro-... 

In spite of this progress, the gaps in our knowledge of the molecular mechanisms of the participation of flavins in one-electron transfer reactions are enormous. Whether the reduction of flavins by obligatory two-electron donors occurs by a concerted two-electron process or by sequential one-electron transfers remains a matter of controversy and is a subject under current active investigation. It is hoped that this review will convince the reader of the usefulness and necessity of redox potential measurements in the understanding of electron transfer reactions in flavoenzymes. These type of measurements have become more numerous in recent years however, more information of this type is needed. We have seen that the apoprotein environment can alter the one-electron potentials of their respective bound flavin coenzymes by several hundred millivolts, yet virtually nothing is known, on a molecular basis, of how this is achieved. 

The second alternative to bypass a difficult RX reduction consists of using redox catalysis [29], Thus, the reduction of RX can be performed at the much less negative one-electron reversible reduction potential (Equation 12.18) of an adequate redox mediator M, which delivers the electron to RX through an homogeneous electron transfer when RX does exist (Equation 12.19), or for a concerted bondbreaking RX reduction (Equation 12.20) ... 

Chlorostannate and chloroferrate [110] systems have been characterized but these metals are of little use for electrodeposition and hence no concerted studies have been made of their electrochemical properties. The electrochemical windows of the Lewis acidic mixtures of FeCh and SnCh have been characterized with ChCl (both in a 2 1 molar ratio) and it was found that the potential windows were similar to those predicted from the standard aqueous reduction potentials [110]. The ferric chloride system was studied by Katayama et al. for battery application [111], The redox reaction between divalent and trivalent iron species in binary and ternary molten salt systems consisting of 1-ethyl-3-methylimidazolium chloride ([EMIMJC1) with iron chlorides, FeCb and FeCl j, was investigated as possible half-cell reactions for novel rechargeable redox batteries. A reversible one-electron redox reaction was observed on a platinum electrode at 130 °C. 

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