Electrochemical Substitution

Anodic substitution has some interesting features compared to ordinary substitution reactions. The net reaction is formally between an organic compound and a nucleophile, as in Eq. (6). 

In this equation, E normally corresponds to hydrogen but can also be another group, e.g. -OCH3. The denotation E is used to show the interplay between nucleophiles and electrophiles in electroorganic reactions. Thus, due to the oxidative nature of the process, a nucleophile can be used as a reagent in an otherwise impossible substitution reaction. As an example of an anodic substitution process, electrochemical cyanation of an aromatic hydrocarbon can be mentioned  

Similarly, electrophiles can be brought into reaction with an organic compound with substitution as a result in a cathodic process. This is schematically depicted in Eq. (7), in which Nu corresponds to a leaving nucleophilic group of some kind. The electrophile could be Hi , C02, S02, etc. 

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The electrochemical substitution of either stereoisomer of the A/-acyl-a-amino carboxylic acid (109) led to product (110) in 75% yield with a trans/cis ratio of 77/23 (Scheme 39) [126, 127], which is in contrast to the stereoselectivity (>97.5% ds) observed in the substitution of the methoxy group of (110) with dibenzoylmethane in trifluoroacetic acid. 

As discussed above, a chemical transformation which occurs during the ac electrolysis does not require the intermediate formation of excited states. The chemical reaction may take place in the reduced and/ or oxidized form of a compound. Nevertheless, in this case the electrolysis may still lead to the same products as those of the photolysis due to the obvious relationship between electronic excitation and redox processes. It will be then quite difficult to elucidate the mechanism of electrolysis. This reaction type may apply to the electrochemical substitution of Cr(CO) (59). 

The substitutionally labile complex may be generated not only by reduction but by oxidation as well. An immediate relationship of such a reaction to the ac electrolysis proceeding without generation of excited states can be recognized. The initial production of the substitutionally labile oxidation state of ML can be achieved electrochemically (67-76), chemically (75-77) or photochemically (78). In the electrochemical experiments reduction or oxidation was accomplished by a direct current. In most cases these processes are catalytic chain reactions with Faradaic efficiencies much larger than unity. Electrochemical substitution of M(CO), with M = Cr, Mo, W was carried out by cathodic reduction to M(CO) which dissociates immediately to yield M(CO). Upon anodic reoxidation at the other electrode coordinatively unsaturated M(CO), is formed and stabilized by addition of a ligand L to give M(CO)5L (68). 

Similarly, the memory of chirality was observed in the electrochemical substitution of... 

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