Sacrificial anode process

Electrocatalysis with nickel-bpy complexes has been shown useful for synthetic applications,202,211 especially when used in combination with the sacrificial anode process in an undivided cell (Equation (45)).207,211 Under these very simple experimental conditions, efficient nickel catalysts can be also generated in the presence of the cheap pyridine ligand.212... 

Nickel associated with 2,2 -bipyridine has also been studied and the low valent complexes obtained by electroreduction are very efficient catalysts in the dimerisation of aryl compounds [26]. In addition, they have been used in combination with the sacrificial anode process which allows the running of such... 

This corresponds to the well-known sacrificial anode process that has been widely developed to achieve electro syntheses reactions in undivided cells2. This allows the in situ preparation of zinc(II) ions, thus avoiding the use of hygroscopic zinc halides. 

This sacrificial anode process compares favorably with Wakselman s classical Barbier procedure which requires activated zinc powder, and CF3Br under pressure, leading to lower yields with benzaldehyde (52% in DMF)21. These good results compared to those obtained from a mixture of CF3ZnBr and (CF3)2Zn allow one to illustrate the formation of a transient organozinc compound of the type CF3—Zn , which would be the really active species. This prompts the following question does the reduction of Zn2+ in Zn... 

Arylzinc species prepared via the sacrificial anode process and from aryl halides in the presence of a nickel 2,2 -bipyridine, as already reported in Section . .1, were found totally unreactive towards common electrophiles such as aldehydes, carboxylic anhydrides or activated alkyl halides. However, they react with some electrophiles when they are activated by the presence of a catalytic amount of copper salts (10 mol% Cul) together with tetramethylethylene diamine (1MEDA) as described by Knochel and Singer on the ArZnX—CuCN metal exchange47 or when the reaction is catalyzed by palladium complex. 

Aryl- or heteroarylzinc species have been successfully synthesized from the corresponding aryl or heteroaryl bromides or chlorides in high yields, on the laboratory scale (10 mmoles of substrate). The use of cobalt as catalyst allowed the synthesis of organozinc reagents using the sacrificial anode process with a wide variety of solvents. 

The reductive-trimethylsilylation, via either a chemical (Li/HMPA/THF) or an electrochemical (undivided cell/sacrificial anode) process, led to the synthesis of 1-(trimethylsilyl)alkylamines (RSMAs).191 Thus, l-(trimethylsilyl)f-amylamine was prepared in 67% yield from pivalaldehyde iV-(trimethylsilyl)imine, after hydrolysis of the intermediate silazane. 

This carbanionic species is associated to a metal cation either initially present in the medium as a dissociated salt (a supporting electrolyte) usually a quaternary ammonium salt or simultaneously electrogenerated from an easily oxidable metal used as the anode, e.g. Mg or A1 (Eq. (2)). This method has been called the sacrificial anode process. 

Then, the use of nickel(II) or cobalt(II) complexes as catalyst associated to the sacrificial anode process allows synthesis of functionalized mono- or diorganozinc species in a simple and efficient manner. Alternating Jt-conjugated copolymers, based on this electrochemical preparation of intermediate aryldizinc species and their subsequent palladium-catalyzed coupling with unsaturated dihalogenated compounds, can be synthesized. Furthermore, aromatic ketones are synthesized efficiently via cobalt-catalyzed cross-coupling reaction between arylzinc bromides and acid chlorides. 

Sacrificial anode systems operate without external power source. The anodes are reactive metals such as magnesium and zinc or aluminum alloys. The energy for the process is derived from the anode material. Careful design is required to match the output and lifetime of the anodes with the polarization and life-expectancy requirements of the plant. Sacrificial anode CP is used for offshore platforms, sub-sea pipelines and the inside of ballast tanks on tanker ships. 

Foods such as meat, fish, and some vegetables contain sulfur-bearing amino acids that form volatile sulfur compounds during processing and storage. When these compounds react with iron, a black precipitate forms on the container and in most instances darkens the food. A small piece of aluminum welded to the tinplate can has been used to prevent container corrosion and sulfide staining in commercially canned hams. In this case, the aluminum acts as a sacrificial anode and stops the reaction with tin and iron that otherwise could occur at the small exposed tinplate areas (14). 

For further details the reader is referred to the excellent paper by Chaussard et al. [149] Use of sacrificial anodes in electrochemical functionalization of organic halides and the paper by Silvestri et al. [178] Use of sacrificial anodes in synthetic electrochemistry. Processes involving carbon dioxide . 

The catalyst is not necessary either for the electrocarboxylation of aryl halides or various benzylic compounds when conducted in undivided cells and in the presence of a sacrificial anode of aluminum [105] or magnesium [8,106], Nevertheless both methods, i.e., catalysis and sacrificial anode, can be eventually associated in order to perform the electrocarboxylation of organic halides having functional groups which are not compatible with a direct electroreductive process. 

The separator is often the weakest component in any electrochemical cell. There are also difficulties in employing ion-exchange diaphragms in aprotic media. Particularly with large industrial cells, it is advantageous to devise reaction conditions that allow the use of an undivided cell. One solution to these problems for an electrochemical reduction process employs a sacrificial anode of magnesium, alumin-... 

As concerns the mechanism of anodic oxidation of 5 at sacrificial anodes, it can be noted that the process occurs at potentials close to those of the oxidation of the corresponding diorganomagnesium compounds (1). For example, half-peak potentials for the oxidation of 5b and lb in THF containing 0.25 M TBAP at a lead electrode measured at a scan rate of 0.3 V s are equal to Jip/2 = —1.73 and —1.72 V vs. 0.01 M Ag /Ag, respectively. However, the oxidation mechanism for both compounds is different, as shown... 

Moreover, in the divided cell the exo.endo ratio of bromosilanes was 91 9 in the anode compartment bnt only 52 48 in the cathode compartment. Thus, the nature of the ultrasonic effect was explained assuming that beside the electrochemical silylation at the cathode, a parallel silylation process occurs at a magnesium anode, namely the silylation by 70 of an intermediate Grignard reagent produced from dibromide 69. It appears as a rare example of the anodic reduction However, the increase in the current density dnring electrolysis cansed a decrease in the apparent current efficiency. This observation indicates a chemical natnre of the anodic process. Of course, the ultrasonic irradiation facihtates the formation of the organomagnesium intermediate at the sacrificial anode and the anthors reported a similar ultrasonic effect for the nonelectrochemical but purely sonochemical... 

Another way of preventing iron corrosion is to use a sacrificial anode, a process which involves placing the iron with another metal that oxidizes even faster. 

The sacrificial zinc anode process has also been used to perform the electrochemical cyclopropanation of alkenes from dibromomethane or bromochloromethane in CH2C12/... 

An analogous non-electrochemical Ni(0)-catalysed process, exploited in a Mannich/ Reformatsky multicomponent process58, will be discussed in Section III (equation 41). In the third study, the a-bromoester Id is simply electrolysed in the presence of a carbonyl compound in DMF/THF in a 1 2 ratio using both indium and zinc rods as sacrificial anodes. While aldehydes afford the expected 3-hydroxyesters in high yield, aliphatic, aromatic and cyclic ketones, with the exception of acetone, directly afford /3-lactones,... 

If possible, the cell should be undivided to minimize the construction cost and also the energy consumption (see goal 1). The application of a controlled reaction at the auxiliary electrode taking place at low potential allows for the use of undivided cells in many cases. For oxidations, the cathodic process at the auxiliary electrode may be a proton reduction under formation of hydrogen. For reductions, the anodic process may be the oxidation of formate or oxalate under production of carbon dioxide [68] or the dissolution of sacrificial anodes [69] (see also Sec. V.B). 

All reactions described in this part were run in the presence of a Zn sacrificial anode. Two processes involving the anode can occur ... 

Anode material In aqueous solutions the anodic processes are either breakdown of the electrolyte solution (with oxygen evolution at an inert anode being favored) or the use of soluble anodes. The use of soluble anodes is limited by the passivation of many metals in aqueous solutions. In ionic liquids, however, the first option is not viable due to the cost and the nature of the anodic breakdown products. New strategies will therefore have to be developed to use soluble anodes where possible or add a sacrificial species that is oxidized to give a benign gaseous product. Preliminary data have shown that for some metals the anodic dissolution process is rate limiting and this affects the current distribution around the cathode and the current density that can be applied. 

Simple halides normally undergo direct electrochemical coupling in very low yields819. However, use of nickel complexes as catalysts has been used to synthetic advantage820. Also, the use of a sacrificial anode has much improved the procedure for electrochemical coupling and this process has been reviewed recently821. 

Chlorofluoro compounds were electroreductivelv added to aldehydes to give the corresponding fluorinated alcohols (231) (equation 124)214. In this process platinum or lead are employed as the cathode, under constant-current conditions. The starting dihalides or trihalides (halogen = F, Cl or Br) can be prepared on an industrial scale by the reaction of the corresponding tri- or tetrahalide at a sacrificial anode (Zn, A1 or Mg)220. 

If a divided cell is used, this reaction can also be carried out using BDD electrodes, avoiding sacrificial anodes and therefore simplifying workup procedure (Fig. 5.14) (Reufer et al. 2006). The MHA yield of this process is not very high but this is a remarkable example for the versatility of BDD electrodes in preparative electroorganic synthesis. 

In the EC process, the destabilization mechanism of the contaminants, particulate suspension, and breaking of emulsions may be summarized as follows. (1) Compression of the diffuse double layer around the charged species by the interactions of ions generated by oxidation of the sacrificial anode. (2) Charge neutralization of the... 

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