Cathodic reduction, halides

The anionic intermediates generated by the cathodic reduction of CHT and some of its derivatives such as 1-MeO- and 3-MeO-CHTs are regioselectively alkylated with alkyl halides to give 6-alkyl-l,3-cycloheptadiene and l-MeO-6-alkyl-l,3-cycloheptadiene as the main products, respectively21. 

Neither reaction excludes the other, nor are these the only routes possible. The oxidation of alkylanions on a metallic anode is likely. Other processes are more clearly indicated when, instead of Cd, Zn is used as a cathode. In the latter case, zinc alkyls R2Zn and RZnI are formed by cathodic reaction of RI. This allows an efficient preparation of R4M where M = Pb, Sn and R = propyl, butyl or pentyl39, in contrast to the cathodic reduction of alkyl halides which is practically limited to the methyl and ethyl groups. 

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... 

Because the reduction potential of ether is usually more negative than that of halides, examples that belong to this category are rather rare. Generally, cathodic reduction of ethers is similar to that of alcohols, and nonactivated ethers are not reducible under the conditions of electroreduction. Activated ethers such as benzylic and allylic ethers are elec-trochemically reduced to a limited extent (Scheme 7) [1, 15, 16]. Reduction of epoxides is usually difficult however, electroreductive cleavage of activated epoxides to the corresponding alcohols is reported [17, 18]. The cleavage of the C—O bond of ethers is more easily accomplished in anodic oxidation than in cathodic reduction, which is stated in Chapter 6. 

Benzenecarhodithioesters and carbo-thio-S-esters were shown to yield diphenylacetylene by the cathodic reduction in aprotic media. Thus, the formation of diphenylacetylene involves [202] two molecules of a substrate. The cathodic reactivity of thioamides involving a similar alkylation of the C=S group in the presence of primary alkyl halides was reviewed [199-201]. 

Cathodic substitution stands for C,C bond or C, heteroatom bond formation with cathodically generated anions. The question of regioselectivity is encountered in the reaction of such anions with allyl halides (path a) or in the reaction of allyl anions generated in an ECE process from allyl halides (path b). Cathodic reductive silylation of an allyl halide proceeds regioselectively at the less substituted position (Fig. 15) [91]. From the reduction potentials of the halides it is proposed that the reaction follows path b. 

Electrochemical oxidation of aldoximes using halide ions as mediators afforded the corresponding nitrile oxides in the anode compartment, which were simultaneously reduced to nitriles by cathodic reduction (equation 15). Sodium chloride affords the best result among the supporting electrolytes (Cl > Br > 1 > C104 > TsO ). Accordingly, the electrochemical reaction of oximes carried out in the presence of dipolephiles yielded isooxazolines (equation 16). 

The square-planar Ni(II) complexes of Lie and L17 catalyze the cathodic reduction of several alkyl halides in aprotic solvents (76). In the presence of activated olefins such as CH2=CHCN or CH2=CH COOC2H5, the reduction of alkyl bromide leads to mixtures of products that are compatible with those formed by radical addition to the double... 

The indirect electrochemical reduction of alkyl halides is also possible by use of nickel(I) complexes which may be obtained by cathodic reduction of square planar Ni(n)-complexes of macrocyclic tetradentate ligands (Table 7, No. 10, 11) 2 4-248) Comparable to the Co(I)- and Ni(O)-complexes, the Ni(I)-species reacts with the alkyl halide unter oxidative addition to form an organo nickel(III) compound. The stability of the new nickel-carbon bond dominates the overall behavior of the system. If the stability is low, the alkyl group is lost in form of the radical and the original Ni(II)-complex is regenerated. A large number of regenerative cycles is the result. 

Solutions of alkali metals in ammonia have been the best studied, but other metals and other solvents give similar results. The alkaline earth metals except- beryllium form similar solutions readily, but upon evaporation a solid ammoniste. M(NHJ)jr, is formed. Lanthanide elements with stable +2 oxidation states (europium, ytterbium) also form solutions. Cathodic reduction of solutions of aluminum iodide, beryllium chloride, and teUraalkybmmonium halides yields blue solutions, presumably containing AP+, 3e Be2, 2e and R4N, e respectively. Other solvents such as various amines, ethers, and hexameihytphosphoramide have been investigated and show some propensity to form this type of solution. Although none does so as readily as ammonia, stabilization of the cation by complexation results in typical blue solutions... 

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 cathodic reduction of benzyl chlorides in the presence of a chlorosilane using a divided cell provides the corresponding benzylsilanes while the electrolysis in an undivided cell gives chlorinated benzylsilanes (equations 70 - 71)98. Similarly, vinyl halides provide the corresponding vinylsilanes as stereoisometic mixtures (equation 12)" 100. 

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... 

In aprotic eatholytes, the cathodic reduction of alkyl halides at Sn cathodes leads to organotin compounds 427 428) ... 

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. 

On the other hand, an electroreductively induced anionic chain reaction system has been found to be highly useful as a synthetic reaction 35). In this reaction system, cathodic reduction of a halide AX forms an anion A- in the first step, A- attacks an electrophile B to yield an intermediate AB, and AB abstracts a proton from AH to regenerate the first anion A-. 

Tetralelluride anions were generated by cathodic reduction of tellurium at — 1.4 V in a cell consisting of a tellurium cathode and a platinum-net anode. AIM solution of dry sodium perchlorate in dimethylformamide served as the catholy te. The tetratelluride ions were then reacted with alkyl halides to yield the corresponding dialkyl ditellurium5. 

Cathodic reduction of tellurium in the presence of methyl iodide in a 1.0 M solution of sodium perchlorate in anhydrous dimethylformamide produced trimethyl telluronium iodide in 80% yield1. This reaction does not appear to proceed with other alkyl halides. 

The anions obtained by two-electron cathodic reduction of cations 8 and 9 undergo alkylation in the presence of an alkyl halide (80MI3 90ACS524). It has been suggested that the reaction between 2,4,6-triphenylthiopyranyl anion and terz-butyl bromide takes place via a rate-determining electron transfer from the anion to the alkyl halide, followed by combinations of the radicals (90ACS524). 

Many of the electrode theories have assumed that the anodic reaction is rate-limiting and that the cathodic reduction of silver ions from silver halide is not rate-limiting and might not present any limitations to the process of development. Hamano et al. [112] contend that there are instances where the cathodic process does influence development. They use the Butler-Volmer equation as the basis for their development rate model and derive Eq. (83),... 

TABLE 5. Electrochemical stereospecificity in the cathodic reduction of optically active cyclopropyl halides... 

Probably the only sub-halide of the group is Ag2F, made by the cathodic reduction of aqueous AgF at low current density. The bronze-coloured solid. 

The Birch reduction of aromatic hydrocarbons or of double bonds with alkali metals in liquid ammonia or amines [37] resembles in yield and selectivity the cathodic reduction with lithium halide as supporting electrolyte (for example, Ref. 38). 

III. Cathodic reduction of esters, lactones, anhydrides, and acyl halides... 

This chapter is concerned with the cathodic reduction of carboxylic acids and their derivatives, that is, esters, amides, anhydrides, acyl halides, hydrazides, nitriles, and corresponding thio derivatives. Cyclic derivatives of substituted carboxylic and polycarboxylic acids, such as lactones, lactams, imides, and anhydrides, are also included. Only those transformations in which the functional group itself is involved are discussed. Reductive coupling of carboxylic acids and derivatives is covered in Chapter 22, and there is some overlap with reduction of heterocycles in Chapter 18. 

CATHODIC REDUCTION OF ESTERS, LACTONES, ANHYDRIDES, AND ACYL HALIDES... 

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. 

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... 

---