Electrochemically induced reactions

Reactions involving thermal or photochemical substitution of coordinated carbon monoxide in metal carbonyl complexes are well known (see Chapter 3) and related processes may be induced electrochemically. A number of review articles blS have dealt with the electrochemically-induced 

The replacement of carbon monoxide in the complex, [W(CO)4L2l (L2 = aromatic diamine), has been described.38 The reaction takes place under mild conditions in acetonitrile upon electrochenucal reduction of [W(CO)4L2]. The corresponding solvento-complexes are obtained in high yield.38 

In contrast, the thermal decarbonylation of the starting material is not selective, and the product contains P(OMe)3 ligands coordinated to each iron atom. 

The first reduction step is efficient when the reduced state (Substrate ) is more reactive than the starting material (Substrate) and the potential E2 is more negative than Ei. Indeed, if Ei E2, the process is not catalytic. 

Treichel, Dean and Douglas42 have studied the thermal substitution of the CO ligands in the bicapped tri-iron cluster, [Fe3(p3-PPh)2(CO)9]. The reaction leads to a mixture of products, Eq. 7.6 (n = 1,2 or 3)  

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The first is the catalytic efficiency of the electrocatalytic process, which in the case of the electrochemically induced reaction is called coulombic efficiency. It is determined by the number of product molecules formed per electron consumed. In our example, the consumption of 0.02 electrons per molecule indicates a coulombic efficiency of 50 molecules produced per electron consumed. 

The absence of dimerization in S l reactions indicates that it is not an important termination step (equation 8), which may be related to the low concentration of the radical R in the chain reaction. However, 17% of the dimerization product 1, l -biadamantyl was found in the reaction of 1-iodoadamantane with the less reactive carbanionic nucleophiles, such as acetone enolate ion in DMSO22. Likewise, under appropriate experimental conditions, the dimerization product 4,4 -dicyanobiphenyl (39%) was the principal product of the termination step in the electrochemical induced reaction of 4-chloroben-zonitrile with 2-pyridinethiolate ions in liquid ammonia23. 

Initiation by electrochemical induction may have the disadvantage of low yields of substitution due to the reduction of the radicals formed near the electrode, mainly in those cases in which the radical anion of the halide compound fragments at a considerably high rate. Redox catalysis, that is activation involving a suitable ET mediator, is an important means to avoid termination steps in electrochemically induced reactions. This approach has been extensively studied by the Saveant group15. A general equation has been proposed in order to predict the yield of ET-initiated S l chain reactions and related mechanisms under preparative electrochemical conditions in the presence of a redox mediator35. 

The electrochemically induced reaction of /j-chlorobenzonitrile with malononitrile anion in liquid ammonia, using 4,4 -bipyridine as a redox mediator, gave the substitution product 172 in 85% yield (equation 1 1 1)204. 

However, if E° [ArNu/(ArNu) ]>>E° [ArX/(ArX) ], the ET reaction is close to the diffusion limit from right to left (non-catalytic system), and the radical anion (ArNu) has to be oxidized by the anode208. A detailed study of the electrochemically induced reaction of /7-bromobenzophenone with CN" ions in liquid ammonia showed that this is a non-catalytic system208. 

J. Rohrs, G. Ludwig, and D. Rahner, Electrochemically Induced Reactions in Soils—A New Approach to the In-situ Remediation of Contaminated Soils Part 2 Remediation Experiments with a Natural Soil Containing Highly Chlorinated Hydrocarbons, Electrochim. Acta 47(9), 1405-1414, Feb. 15 (2002). 

Rohrs J, Ludwig G, Rahner D. (2002). Electrochemically induced reactions in soils-a new approach to the in-situ remediation of contaminated soils Part 2 Remediation experiments with natural soil containing highly chlorinated hydrocarbons. Electrochimica Acta 47 1405-1414. 

Rahner D, Ludwig G, Rohrs J, Neumann V, Nitsche C, Gnderitz I. (2001). Electrochemically induced reactions in soils—a new approach to the in-sitn remediation of contaminated soils In 3. Symposium on Electrokinetic Remediation (eds. K Czurda, R Hans, C Kappeler, R Zom). April 18-20,2001, Karlsruhe, Germany. Karlsruhe, Germany Schr. Angew. Geol.,... 

Electrochemically induced reactions have been a feature of selenium chemistry in the past two or three years. This year a potentially useful synthesis of ap-unsaturated aldehydes (279) has been reported and involves electrochemical generation of the active selenating species from diphenyl diselenide. Propargyl alcohols are thus selenated to give (279) in good to excellent yields. The ap-unsaturated aldehydes (279) are also formed when 1-lithioselenoalkenes, RCH=C(Li)SePh, react with DMF. Various other a-substituted alkenes are obtained in a similar fashion when DMF is replaced by other electrophiles. 

Lu, Y. King, F. L. Duckworth, D. C. Electrochemically-induced reactions of hexafluorophosphate anions with water in negative ion electrospray mass spectrometry of undiluted ionic liquids./. Am. Soc. Mass Spectrom. 2006,17,939-944. 

Elofson and Gadallah (1971) showed that electrochemically induced Pschorr and related reactions give almost quantitative yields in several cases. 

Reactive radical ions, cations and anions are frequent intermediates in organic electrode reactions and they can serve as polymerization initiators, e.g. for vinylic polymerization. The idea of electrochemically induced polymerization of monomers has been occasionally pursued and the principle has in fact been demonstrated for a number of polymers But it appears that apart from special cases with anionic initiation the heterogeneous initiation is unfavorable and thus not competitive for the production of bulk polymers A further adverse effect is the coating of electrodes... 

Polarisation modulation infrared rejiection-absorption spectroscopy (PM-IRRAS or JRRAS). Potential modulation IR studies rely on switching the potential at a reflective electrode between rest and active states, generating difference spectra. However, the EMIRS technique has several drawbacks the relatively fast potential modulation requires that only fast and reversible electrochemical process are investigated the absorption due to irreversibly chemisorbed species would be gradually eliminated by the rapid perturbation. Secondly, there is some concern that rapid modulation between two potentials may, to some extent, in itself induce reactions to occur. 

In the ruthenium frA-bipyridine system, an orange emission at 610 nm arises when the excited stated [Ru(bpy)32+] decays to the ground state. Ru(bpy)32+ is the stable species in the solution and the reactive species—Ru(bpy)33+—can be generated from Ru(bpy)32+ on the electrode surface by oxidation at about +1.3 V. Adding Ru(bpy)32+ to the electrolyte and using an end-column electrode to convert the Ru(bpy)32+ into the active Ru(bpy)33+ form allow a simple and sensitive ECL detection mode. The reaction lends itself to electrochemical control due to the electrochemically induced interconversion of the key oxidation states ... 

The original work was on ionic reactions in normal micelles in water, but subsequently there has been extensive work on reactions in reverse micelles (O Connor et al., 1982, 1984 Kitahara, 1980 O. A. El Seoud et al., 1977 Robinson, et al., 1979). There also has been a great deal of work on photochemical and radiation induced reactions in a variety of colloidal systems, and microemulsions have been used as media for a variety of thermal, electrochemical and photochemical reactions (Mackay, 1981 Fendler, 1982 Thomas, 1984). 

Pschorr s synthesis of phenanthrene (1893) in five steps with the essential dediazoniation and ring closure of 2-diazonio-a-phenylcinnamic acid giving, on addition of copper powder, phenanthrene-9-carboxylic acid, is today still the highest yielding one of all the reactions discussed in this section, Pschorr was able to get 93% yield, and today electrochemically induced Pschorr and related reactions141 give almost quantitative yields in several cases. 

Under similar reaction conditions, some ketones such as (53) gave products via an electrochemically induced Favorskii rearrangement (Eq. 10) [89, 90]. 

Electrolysis of amides in MeOH containing the bromide ion efficiently led to products of the Hofmann rearrangement (for example, 119 to 120 Scheme 43) [131]. This reaction, named the electrochemically induced (E-I) Hofmann rearrangement, is achieved without any bromine and base under mild and neutral reaction conditions. 

Historically, the concept of EG Bs was developed in relation to electrochemically induced Wittig reactions [45], Co-electrolysis of the phosphonium salt and the carbonyl compound was carried out using the carbonyl compound as the solvent and gave reasonable yields of the alkene [45]. Most of these reactions can be rationalized within Scheme 19, in which the phosphonium ion participates both as the probase (PB) and as the acidic substrate [46]. 

Rossi, 1983), light-induced and solvated-electron-induced reactions (Bun-nett, 1978), electrochemical induction (Saveant, 1980a), synthetic aspects (Beugelmans, 1984 Norris, 1990 Wolfe and Carver, 1978), photochemical induction of the reaction at heteroaromatic carbon centres (Lablache-Combier, 1988). There is no point in repeating here the extensive lists of reactions contained in these review articles. We shall just underline a few points and report recently available findings. 

Scheme 4.4. Electrochemically induced ionic chain reaction between aldehydes and chloroform.

Electrochemically Induced Radical Cyclization Reactions of Aryl Halides... 

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