Cathodic reduction of organic halides

Generally, the two-electron reduction of organic halides produces carbanion species. In fact, cathodic reduction of organic halides under certain conditions gives the product derived from the corresponding carbanion intermediates. Silicon is known to stabilize the carbanion at the a position by dn-pn interaction. Therefore, we can expect that silicon promotes the electron transfer from carbon-halogen bonds and the formation of the carbanion at the a position. 

Quite often the resulting anionic species undergoes a fast follow-up reaction. Thus, homogeneous ET becomes a crucial step in many electrode reactions a well-known example is the cathodic reduction of organic halides. In mercury, heterogeneous ET to most halides is slow. This kinetic... 

Since a carbon-halogen bond is more easily reduced than a silicon-halogen bond, cathodic reduction of organic halides such as allyl, benzyl, aryl and vinyl halides in the... 

The combination of organic halides and metal halides is also effective for the reductive formation of the metal-carbon bond. This type of reaction proceeds by a mechanism involving the cathodic reduction of organic halides [Eq. (9)] because reduction potentials... 

For these reasons, we preferred a technique using an undivided cell with a sacrificial anode made of a rearhly oxidized metal. Anodic oxidation of metals has been previously shown to provide organometallic compounds from either preformed carbanions [22], or carbanions formed in situ by cathodic reduction of organic halides... 

A third catalytic route consists of using cathode materials with catalytic properties. The electrocatalytic activity of electrode materials towards the reduction of organic halides has been the object of many studies during the past few years [82], with silver having been shown to possess powerful electrocatalytic activities... 

Baizer and Chruma reported electrolytic reductive coupling, in a broad study in which reduction of organic halides was conducted in the presence of electrophiles as seen in equation 1034. Controlled potential electroreduction of reactive halides at a mercury cathode in the presence of olefinic substrates with electron-withdrawing groups (Michael receptors) gave moderate yields (50-75% in many cases) of carbanion addition products35. Current yields in excess of 100% in the case where chloroform was used as a cosolvent with carbon tetrachloride indicated the intervention of electrocatalytic reactions (equations 10 and 11). 

Electrochemical reactions serve as efficient and convenient methods for the synthesis of organoelemental compounds. There are four major methods for the formation of element (metal)-carbon bonds. The first method utilizes the anodic oxidation of organometallic compounds using reactive metal anodes. In the second method, the organic compounds are reduced using reactive metal cathodes. The third method involves the cathodic reduction of organic compounds in the presence of metal halides. The fourth one utilizes both the cathodic and the anodic processes. 

Organic halides serve as effective precursors of organic radicals, and the reduction of organic halides with reactive metal cathodes leads to the formation of organometallic compounds [Eq. (5)]. For example, the reduction of alkyl halides with a Pb cathode results in the formation of R4Pb [24-28]. The reduction at a Hg cathode results in the formation of R2Hg [29-32]. Sn is also effective as a reactive cathode to give R4 n [33-35]. 

Organotin compounds may be synthesized by the cathodic reduction of organic compounds in the presence of tin halides. For example, the reduction of allylic halides in the presence of chlorostannanes gives the corresponding allylstannanes in good yields [69]. Combination of this reaction with in situ palladium-catalyzed reaction with allylic halides leads to effective formation of the head-to-tail homo coupling products as shown in Eq. (17). 

Electrocatalytic cathodes, Pt, Rd, Ru, and Ag, do not follow the behavior of medium and high overpotential metals. Thus, electrogenerative reduction of vinyl fluorides and chlorides gives saturated halides oj nonhalogenated olefins and alkanes, depending on catalyst, potential, and electrolyte or reactant concentration 31). In contrast to this, conventional electroreduction is pH independent, while fluorides are not reduced 371). Other differences between electrogenerative, catalytic, and conventional electrochemical reduction of organic halides have been outlined recently 31). 

Due to the pronounced lack of reversibility of most electrode reactions of organic species, the pH effect is not always predictable, although one should always expect a cathodic (toward more negative values) shift of Ei/2 values upon increase of pH. The degree of reversibility of many electrode reactions is also affected by pH. In cases of total irreversibility (e.g., reduction of organic halides) E1/2 values are more or less pH independent. The effect of pH on E1/2 of three pairs of closely related compounds is shown in Figure 1. 

Pletcher and associates [155, 159, 160] have studied the electrochemical reduction of alkyl bromides in the presence of a wide variety of macrocyclic Ni(II) complexes. Depending on the substrate, the mediator, and the reaction conditions, mixtures of the dimer and the disproportionation products of the alkyl radical intermediate were formed (cf. Section 18.4.1). The same group [161] reported that traces of metal ions (e.g., Cu2+) in the catholyte improved the current density and selectivity for several cathodic processes, and thus the conversion of trichloroacetic acid to chloroacetic acid. Electrochemical reductive coupling of organic halides was accompanied several times by hydrodehalogena-tion, especially when Ni complexes were used as mediators. In many of the reactions examined, dehalogenation of the substrate predominated over coupling [162-165]. 

Various activated olefins can also be employed instead of organic halide for the formation of a carbon-silicon bond. Thus, cathodic reduction of a,j -unsaturated esters, nitriles... 

Electrochemical studies are usually performed with compounds which are reactive at potentials within the potential window of the chosen medium i.e. a system is selected so that the compound can be reduced at potentials where the electrolyte, solvent and electrode are inert. The reactions described here are distinctive in that they occur at very negative potentials at the limit of the cathodic potential window . We have focused here on preparative reductions at mercury cathodes in media containing tetraalkylammonium (TAA+) electrolytes. Using these conditions the cathodic reduction of functional groups which are electroinactive within the accessible potential window has been achieved and several simple, but selective organic syntheses were performed. Quite a number of functional groups are reduced at this limit of the cathodic potential window . They include a variety of benzenoid aromatic compounds, heteroaromatics, alkynes, 1,3-dienes, certain alkyl halides, and aliphatic ketones. It seems likely that the list will be increased to include examples of other aliphatic functional groups. 

A mercury cathode mostly functions only as an electron donor numerous examples of such simple reductions have been reported. In some cases, however, the electrode is attacked with the formation of organic mercury compounds, e.g., by radicals during the reduction of a halide. When using mercury, suitable precautions must be taken to avoid mercury poisoning and environmental pollution. 

CATHODIC REDUCTION OF PERFLUORINATED AND POLYFLUORINATED ORGANIC HALIDES... 

The basic mechanism for the reduction of alkyl halides on mercury cathodes is essentially the same in aqueous and in organic media. In general the mechanism can be written... 

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