Electrode reaction Electrohydrodimerization

Again the dimerization or coupling reactions will involve free radicals or radical ions, but the intermediates are generated cathodically. Suitable substrates are electrophiles such as activated olefins and carbonyl compounds. In this section, it is intended to focus only upon the electroreduction of activated olefins, partly because the electrode reactions of aldehydes and ketones has been discussed earlier. However, it is the electrohydrodimerization of an activated olefin that has become a successful, commercial electro-organic process, i.e., the electroreduction of acrylonitrile developed by Monsanto ... 

Kinetic and mechanistic studies on reductive couplings have accumulated in the last decades, and particularly electrohydrodimerizations and dimerization of aromatic systems have been much studied. The level of the mechanistic discussions in this chapter reflects the somewhat uneven level of knowledge accumulated for the different types of coupling reactions. In some cases where little mechanistic work has been done, the mechanistic rationalizations presented are based on evaluations made by the present authors. No attempts have been made to bring reduction potentials on a common scale since differences in solvent, supporting electrolytes, added acids, electrode material, etc. may lead to considerable differences in the measured potentials. This is particularly so when it comes to values of reduction peak potentials measured under conditions where the electrogenerated intermediate is consumed in a fast follow-up reaction. In some cases, however, the relative values may be of interest in a mechanistic discussion. Unless stated otherwise the cited potentials have been measured versus SCE. 

The most powerful illustration to date of the ability of steady-state TG/SC measurements to identify short-lived intermediates in electrode processes comes from studies in which the radical anion of acrylonitrile (AN) was detected in the electrohydrodimerization reaction (8). Prior to these investigations, the mechanism of the AN electrohydrodimerization process was unclear. On the basis of SECM measurements, it was shown that the reaction proceeded analogously to other activated olefins, including those highlighted in Sec. III.B. Electrogeneration of the radical anion, R , is followed by dimerization and then protonation to yield the hydrodimer ... 

Intramolecular cyclization has been observed in several electrohydrodimerization reactions,another indication of the versatility of this type of electrode process. High yields of cyclopropanes, cyclopentanes, and cyclohexanes have been obtained from a,/8-unsaturated esters. 

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