Electrolytic coupling, catalysts

For further contributions on the dia-stereoselectivity in electropinacolizations, see Ref. [286-295]. Reduction in DMF at a Fig cathode can lead to improved yield and selectivity upon addition of catalytic amounts of tetraalkylammonium salts to the electrolyte. On the basis of preparative scale electrolyses and cyclic voltammetry for that behavior, a mechanism is proposed that involves an initial reduction of the tetraalkylammonium cation with the participation of the electrode material to form a catalyst that favors le reduction routes [296, 297]. Stoichiometric amounts of ytterbium(II), generated by reduction of Yb(III), support the stereospecific coupling of 1,3-dibenzoylpropane to cis-cyclopentane-l,2-diol. However, Yb(III) remains bounded to the pinacol and cannot be released to act as a catalyst. This leads to a loss of stereoselectivity in the course of the reaction [298]. Also, with the addition of a Ce( IV)-complex the stereochemical course of the reduction can be altered [299]. In a weakly acidic solution, the meso/rac ratio in the EHD (electrohy-drodimerization) of acetophenone could be influenced by ultrasonication [300]. Besides phenyl ketone compounds, examples with other aromatic groups have also been published [294, 295, 301, 302]. 

There are a few reports of poly(naphthalene) thin films. Yoshino and co-workers. used electrochemical polymerization to obtain poly(2,6-naphthalene) film from a solution of naphthalene and nitrobenzene with a composite electrolyte of copper(II) chloride and lithium hexafluoroarsenate. Zotti and co-workers prepared poly( 1,4-naphthalene) film by anionic coupling of naphthalene on. platinum or glassy carbon electrodes with tetrabutylammonium tetrafluoroborate as an electrolyte in anhydrous acetonitrile and 1,2-dichloroethane. Recently, Hara and Toshima prepared a purple-colored poly( 1,4-naphthalene) film by electrochemical polymerization of naphthalene using a mixed electrolyte of aluminum chloride and cuprous chloride. Although the film was contaminated with the electrolyte, the polymer had very high thermal stability (decomposition temperature of 546°C). The only catalyst-free poly(naphthalene) which utilized a unique chemistry, Bergman s cycloaromatization, was obtained by Tour and co-workers recently (vide infra). 

Ni(0) obtained from electrochemical reduction of NiBr2 in THF/HMPA acts as an efficient catalyst for the electroreductive coupling of ethylene with aryl halides to give 1,1-diarylethanes 217 (equation 109). By proper control of reaction conditions, such as reduction potential, solvent and supporting electrolyte, it can be shown that substituted olefins can be prepared from aromatic halides and alkenes164. 

According to the third catalyst-membrane coupling possibility, represented in Fig. 5c, the surface of the membrane is deposited with some catalytic material. This setup is typical of solid-electrolyte membranes, where the catalyst is also playing the role of the electrode, necessary to drive the permeation of ions throughout the membrane at a desired rate. Problems may arise here concerning the fact that the catalyst per unit membrane surface is limited to some extent, and that several catalytic materials (e.g., metal oxides) are poor electricity conductors [26]. 

Molybdenum redox couples have been used in two ways for the oxidation of methanol (106,107)—use of a Pt-Mo alloy as the catalyst or use of molybdates in the electrolyte. The electrode reaction is the oxidation of the Mo + to Mo + ion and the chemical regeneration of Mo + ion is effected with methanol. 

Using nickel-2,2 -bipyridine complex as the catalyst, electroreductive coupling of 5-bromoindole gave rise to the bis-indole shown using NaBr as the electrolyte and iron and the sacrificial electrode [219]. 

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