Magnesium sacrificial anode

Electroreductive coupling of ben2otrifluorides with sacrificial aluminum or magnesium anodes in the presence of acetone, carbon dioxide, or /V, /V-dimethylformamide provides a novel route to ArCF2-derivatives (310). 

Cathodic protection (CP) is an electrochemical technique of corrosion control in which the potential of a metal surface is moved in a cathodic direction to reduce the thermodynamic tendency for corrosion. CP requires that the item to be protected be in contact with an electrolyte. Only those parts of the item that are electrically coupled to the anode and to which the CP current can flow are protected. Thus, the inside of a buried pipe is not capable of cathodic protection unless a suitable anode is placed inside the pipe. The electrolyte through which the CP current flows is usually seawater or soil. Fresh waters generally have inadequate conductivity (but the interiors of galvanized hot water tanks are sometimes protected by a sacrificial magnesium anode) and the conductivity... 

New combined (or binary) alloy sacrificial anodes have been developed . An aluminium anode, for example, might have attached to it a short-life supplementary magnesium anode, or anodes, for quick polarisation of the structure. The overall reduction in structure current requirements is claimed to result in an anode weight saving of 35-50% . 

Where sacrificial anode technology is employed, typically zinc or magnesium anodes are provided, although zinc anodes cannot be employed at over 140°F because of a reversal of potential above that temperature. In the boiler waterside circuits, they preferentially corrode,... 

Quite recently, Troupel et al. have developed an effective synthesis of gem-difluoro-/ -oxonitriles using an electrogenerated base derived from phenyl bromide and a sacrificial magnesium anode together with a nickel cathode coated with a small deposit of cadmium as shown in Scheme 4.5 [34]. 

Alkyl alkanoates are reduced only at very negative potentials so that preparative scale experiments at mercury or lead cathodes are not successful. Phenyl alkanoates afford 30-36% yields of the alkan-l-ol under acid conditions [148]. Preparative scale reduction of methyl alkanoates is best achieved at a magnesium cathode in tetrahydrofuran containing tm-butanol as proton donor. The reaction is carried out in an undivided cell with a sacrificial magnesium anode and affords the alkan-l-ol in good yields [151]. In the absence of a proton donor and in the presence of chlorotrimethylsilane, acyloin derivatives 30 arc formed in a process related to the acyloin condensation of esters using sodium in xylene [152], Radical-anions formed initially can be trapped by intramolecular addition to an alkene function in substrates such as 31 to give aiicyclic products [151]. 

A number of electroorganic syntheses based on the use of sacrificial magnesium anodes have been described. However, in only a few reactions, listed below, was the formation of intermediate organomagnesium compounds postulated or documented. 

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... 

This last electrochemical process is carried out in an undivided electrolysis cell fitted with a sacrificial magnesium anode and a nickel foam as cathode. The reaction is conducted in dimethylformamide in the presence of both NiBr2(bpy) as the catalyst and dried ZnBr2 (1.1 molar equivalents with respect to bromothiophene), which is used both as supporting electrolyte and as a zinc(II) ion source. The other conditions are the same as those described in the section concerning the aromatic halides. The yield of 3-thienylzinc bromide was roughly 80%, as determined by GC analysis after treatment with iodide (equation 34). 

Quite recently, stereoselective electrochemical synthesis of silyl enol ethers using a sacrificial magnesium anode was reported, as shown in equation 100126. 

Nickel-2,2-bipyridine complexes are also used for the preparation of unsymmetrical biaryls such as 4-methoxy-4 -trifluoromethylbiphenyl by electroreduction of two aryl halides, one of which has electron-donating and the other electron-withdrawing groups in the aromatic ring as shown in equation 110. The reaction was carried out in N-methylpyrrolidinone at constant current in an undivided cell fitted with a sacrificial magnesium anode and excess of 2,2 -bipyridine167. 

Anode, sacrificial — a rather easily oxidizable metal, e.g., zinc, magnesium, aluminum, electrically connected with a metal construction to be protected from corrosion. Due to the formation of a -> galvanic cell the sacrificial anode is oxidized instead of the metal to be protected. Sacrificial anodes are the oldest and simplest means for electrochemical corrosion protection. 

A Barbier modification has been described in which aldehydes are prepared by electrolysis of organic halides in DMF with a sacrificial magnesium anode [10]. 

A somewhat similar cell design has been used in the equipment for electrolytic reductions in liquid ammonia for both static and circulating media in this case the center electrode is a sacrificial magnesium anode [54]. 

When using sacrificial magnesium anodes in DMF, an overconsumption of metal is observed it has been shown that electrochemically scouring of magnesium induces a chemical reduction of DMF, possibly primarily involving a reductive dimerization [176]. 

In some older papers the reduction of some compounds by anodically generated Mg [180,181] has been described. It has not been investigated whether Mg" " (or finely divided Mg ) plays any role during electrolysis using sacrificial magnesium anodes. 

Electrochemical Si-Si bond formation has been known since the work of E. Hengge and G. Litscher in 1976, using a divided cell and a mercury anode [5]. Later, T. Shono developed a new undivided cell design employing magnesium as sacrificial anode, which is oxidized and subsequently converted to MgCL by the chloride from the chlorosilane [6]. 

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