Electrochemical generation of radicals

An electrode is inexpensive when compared with most chemical reagents. It is immobile, and thus causes less environmental and solubility problems than most chemical oxidants and reductants. It can change the polarity of reagents by oxidation or reduction ( Redox-Umpolung ) and in this way can shorten synthetic sequences. Controlled potential electrolysis allows the selective conversion of one out of several electrophores in a molecule. A technical scale-up causes in most cases lesser problems than the scale-up of a chemical reaction. These advantages and the wide choice of conversions have made electrolysis today at least for those that take the small effort to assemble an electrolysis cell and connect it to a d.c. power supply - to an attractive alternative and supplement for chemical synthetic methods. 

1 Electrochemical C,C-Bond Formation and Functional Group Interconversion 

At the anode, anions can be oxidized to radicals (la), neutral substrates to radical cations that can either lose a proton or react with a nucleophile to form a radical (lb), and the radicals can be further oxidized to carbocations (Ic). At the cathode, the mirror image reactions occur. Carbocations formed either by dissociation (Id) or by protonation of a C=X double bond are reduced to radicals (Id), neutral substrates are reduced to radical anions that can expel a leaving group or can be pro-tonated to a radical (le,f), and the radicals can be further reduced to anions (Ig). 

Pathways (la) and (Id) are the most frequent routes employed for the electrochemical generation of radicals. The radicals can be used in homocoupling, heterocoupling and addition reactions. In some cases these reactions have to compete with further oxidation or reduction of the radicals (see Sections 2.6.3.5 and 2.6.4.3). Electrogenerated radical ions (lb), (le), (If), cations (Ic) and anions (Ig) have been very efficiently used in electroorganic synthesis, e.g. for cathodic hydrodimerization, anodic dehydrodimerization, anodic substitution, cathodic cleavage or ring closure reactions [2, 5, 6]. These conversions are not treated in this review. 

In functional group interconversions (FGI), where the oxidation number of the substrate is changed, the electrode is the reagent of choice. Thereby mostly charged 

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Second-order irreversible chemical reaction following a reversible electron transfer dimerization. It is quite common in chemical reactions that newly formed radicals couple to each other. This also often happens in the electrochemical generation of radicals according to a dimerization process that can be written as ... 

C-C Bond Formation. Electrochemical generation of radicals (e.g., carban-ions) has been shown to give rise to stereoselective intramolecular cyclization. 

Within the last two decades Electron Spin Resonance-(ESR) spectroscopy has become a standard experimental technique in electrochemical research. The main interest was in the field of electrochemical generation of radicals to characterize their structure by ESR spectroscopy or to prove their presence in electrode reactions. The studies have been extended to the kinetics of radical reactions and the set up of reaction mechanism, to the solvation phenomena in radical electron densities and to radical conformation and ion complex structure. The latest development is the study of the electrode materials and their surface layers in electrochemical systems by simultaneous ESR spectroscopic and electrochemical measurements, e.g., of polymer modified electrodes. 

The anodic substitution reaction is another electrochemical approach to induce the Sn and Sn reactions. The term anodic addition reaction is used in the literature, since an anodic process is exploited for the initial electrochemical generation of radical-cation intermediates [81,82]. Hence, anodic addition involves... 

Fig. 2. Diagram of cell for internal electrochemical generation [32], W is a wave guide R is the resonant cavity M is the magnet C is the cell H is the cell holder E i, E2, E3 are the electrodes for electrochemical generation of radical anions. E3 is connected to the mercury drop, the surface of which forms the cathode.

In work by Bezugl3d and his co-workers [30] it was shown that in the electrochemical generation of radical ions from anthraquinone in methanol and isopropanol solutions the radical ions can only exist... 

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