Electrolysis/tris amine

The use of a chemical mediator can alter the chemoselectivity of an electrochemical reaction. In the reaction illustrated in Scheme 2, -methylstyrene was oxidized using both direct electrolysis and mediated conditions [10]. The current density, amount of charge passed, temperature, and other variables were all kept constant. The only difference was the addition of 6.4 mole percent of tris(4-bomophenyl)amine to the mediated reaction. The direct electrolysis tended to afford the product of a four-electron oxidation (7). When 3.5 F mole of electricity was passed, a 55% yield of (7) was obtained along with a 45% yield of (6). With additional current (6.4 F mole ), a 75% yield of (7) was obtained. The mediated process led to a preponderance of the product from the two-electron oxidation. When 3.5 F mole of electricity was passed in the experiment using the triarylamine mediator, a 93% yield of (6) was obtained along with only 6% of the four-electron oxidation product. 

Thus, as compared with direct electrolysis, not only a gain in energy can be obtained but also the selectivities can be enhanced. The selectivity of the reaction in this case is determined by the potential differences between the different functional groups of the substrate and the mediator in combination with the rate of the respective follow-up reaction. A typical example of this type is the cleavage of a carbon-sulfur bond using tris(4-bromophenyl)amine as organic mediator ) (Eq. (12)). 

On the basis of the polarographic studies, the direct electron transfer polymerizations of the monomers, of which half-wave potentials could be measured, were conducted keeping the potential at a level where the monomer alone was reduced and the electrolyte was not affected. During the electrolysis of a-methyl-styrene, for example, the red color of the carbanion was observed around the cathode, but dissipacted and vanished quickly. Only low polymers were found in the cathodic compartment, but no polymer in the anolyte. In the polymerization of other monomers almost identical results were obtained. From the fact that tri-n-butyl-amine was detected in the catholyte and analysis of the end group of polymers obtained, two possible termination steps were postulated ... 

Anodic oxidation of azomethine, hydrazone, oxime, formazane, and semicarbazone structures has been used to initiate the intramolecular cyclization [119] under formation of heterocycles like triazoles [126,127], oxadiazoles [128,129], triazolinones [129], benzoxa-zoles [130,131], benzimidazoles [130,131], pyrazoles [132], indazoles [133], furoxanes [134], and tetrazolium salts [135] (see Chapter 18). Some of these reactions can be performed advantageously by indirect electrolysis using tris(4-bromophenyl)amin or 2,3-dihydro-2,2-dimethylphenothiazine-6(l/7)-one as mediators [119,136]. Two examples are given in Eqs. (19) and (20). 

In the example given above, experimental conditions and the control of the electrical charge may orientate the reaction selectively to the mono-, di-, and tri substitution. The action of Lewis acid on substituted amine functions allows one to get back the iminium intermediates [38, 39]. Thus, the nucleophilic action in the course of the electrolysis could be seen as an iminium protection , and renders possible easy synthesis of the heterocycles as outlined below ... 

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