Electrochemical reactions involving oxidative addition

In 1966, a method for the synthesis of organotin compounds by electrolysis of alkyl halides using a tin anode and a magnesium cathode was developed.94,95 Such electrochemical reactions have been studied extensively by Tuck and coworkers.9 These studies have been reviewed and a general mechanism proposed for this class of electrochemical reactions.9 The reactions of organic halides with a metal anode resemble oxidative addition reactions, Eqs. 7.19 - 7.20  

These reactions are reminiscent of those used in the preparation of Grignard reagents and of Frankland s synthesis of organozinc halides. The products, RMX and R2MX2, are often extremely reactive and disproportionate readily. They can be stabilized, however, by complex formation with other ligands 

The compounds, [M(R)XL2] (M = Ni, Pd R = alkyl, aryl X = halide), are very well known and are generally prepared by reaction between [MX2L2] and the appropriate alkyllithium, LiR, or Grignard reagent, RMgX. 8 Habeeb and Tuck have reported a direct electrochemical synthesis.99 For example, the compounds, [M(C6Fs)Br(PEt3)2] (M = Ni, Pd), have been prepared, Eq. 7.21  

Although applicable for cases where M = Ni, Pd, such reactions do not occur for M = Pt. Organonickel cyanides have simUarly been prepared via C-CN bond cleavage of organonitriles at a dissolving nickel electrode in the presence of PEts, Eq. 7.22  

The mechanisms of these electrochemical reactions involve radical intermediates.  

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The three basic steps in the palladium-catalysed Suzuki-Miyaura reaction involve oxidative addition, transmetalation, and reductive elimination. A systematic study of the transmetalation step has found that the major process involves the reaction of a palladium hydroxo complex with boronic acid, path B in Scheme 3, rather than the reaction of a palladium halide complex with trihydroxyborate, path A. A kinetic study using electrochemical techniques of Suzuki—Miyaura reactions in DMF has also emphasized the important function of hydroxide ions. These ions favour reaction by forming the reactive palladium hydroxo complex and also by promoting reductive elimination. However, their role is a compromise as they disfavour reaction by forming of unreactive anionic trihydroxyborate. A method for coupling arylboronic acids with aryl sulfonates or halides has been developed using a nickel-naphthyl complex as a pre-catalyst. It works at room temperature in toluene solvent in the presence of water and potassium carbonate. ... 

The basic relationships of electrochemical kinetics are identical with those of chemical kinetics. Electrochemical kinetics involves an additional parameter, the electrode potential, on which the rate of the electrode reaction depends. The rate of the electrode process is proportional to the current density at the studied electrode. As it is assumed that electrode reactions are, in general, reversible, i.e. that both the anodic and the opposite cathodic processes occur simultaneously at a given electrode, the current density depends on the rate of the oxidation (anodic) process, ua, and of the reduction (cathodic) process, vc, according to the relationship... 

Modern electrochemical methods provide the coordination chemist with a powerful means of studying chemical reactions coupled to electron transfer and exploiting such chemistry in electrosynthesis. In addition, the electrochemical generation of reactive metallo intermediates can provide routes for the activation of otherwise inert molecules, as in the reduction of N2 to ammonia,50 and for electrocatalyzing redox reactions, such as the reduction of C02 to formate and oxalate,51 the oxidation of NH3 to N02-,52 and the technologically important oxidation of water to 02 or its converse, the reduction of 02 to water.53 Electrochemical reactions involving coordination compounds and organometallic species have been extensively reviewed.54-60... 

In the particular case of lithiation or delithiation of cathode materials used in lithium secondary batteries, the calculation of the electrochemical equivalent involves an additional parameter related to the reaction of intercalation of lithium cations into the crystal lattice of the host cathode materials. Consider the theoretical reversible reaction of intercalation of lithium into a crystal lattice of a solid host material (e.g., oxide, sulfide) ... 

Electrochemical reactions that involve oxidative addition are possible not only for organohalides but also for other substrates. For example, the synthesis of [CdBr(SnPh3)(bipy)], which contains a Sn-Cd bond, can be accomplished electrochemically, Eq. 7.23 ... 

In addition to metals, other substances that are solids and have at least some electronic conductivity can be used as reacting electrodes. During reaction, such a solid is converted to the solid phase of another substance (this is called a solid-state reaction), or soluble reaction products are formed. Reactions involving nomnetaUic solids occur primarily in batteries, where various oxides (MnOj, PbOj, NiOOH, Ag20, and others) and insoluble salts (PbS04, AgCl, and others) are widely used as electrode materials. These compounds are converted in an electrochemical reaction to the metal or to compounds of the metal in a different oxidation state. 

The mechanism of the Zn chloride-assisted, palladium-catalyzed reaction of allyl acetate (456) with carbonyl compounds (457) has been proposed [434]. The reaction involves electroreduction of a Pd(II) complex to a Pd(0) complex, oxidative addition of the allyl acetate to the Pd(0) complex, and Zn(II)/Pd(II) transmetallation leading to an allylzinc reagent, which would react with (457) to give homoallyl alcohols (458) and (459) (Scheme 157). Substituted -lactones are electrosynthesized by the Reformatsky reaction of ketones and ethyl a-bromobutyrate, using a sacrificial Zn anode in 35 92% yield [542]. The effect of cathode materials involving Zn, C, Pt, Ni, and so on, has been investigated for the electrochemical allylation of acetone [543]. 

Chalcogenolato complexes of mercury can be prepared by a variety of methods. Early preparations involve the reactions of thiols with mercury cyanide,1 the reaction of mercury salts with alkali chalcogenolates, electrochemical methods,2 and the oxidative addition of dichalcogenides to metallic mercury.3 The last method is very convenient for the preparation of complexes with sterically undemanding ligands, but becomes less facile as the... 

Silyl enolates are a class of electron-rich, non aromatic compounds which can be described as masked enols or enolates since hydrolysis following their reaction yields ketones they can be purified by distillation or chromatography, and then converted back to the enolate anion. The electron-rich character of these species can be used for oxidation reactions and examples have been described in the preceding sections. In this section, additional examples of chemical, PET and electrochemical redox reactions involving silyl enolates will be discussed, for a better appreciation of these interesting species in organic synthesis. 

For these syntheses the 7i-bond is formed via an elimination reaction involving either two protons (electrochemical oxidation), or one proton and one or two heteroatoms. In addition, the a- and n-bonds can be forced simultaneously by a coupling reaction of two carbenes. 

The electrochemical reaction may involve, in addition to diffusion and charge transfer, chemical reactions in which the oxidant or reductant is involved and/or adsorption of the electroactive species. Sometimes the magnitude of the current is limited by the rate of a chemical reaction or an adsorption process. 

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