Homo- and Heterocoupling of Cathodically Generated Radicals

Radicals can be generated by cathodic reduction from carbocations, protonated C=X bonds, and the reduction of halides or onium salts. The reduction potentials of carbocations range from 1.87 V (vs nhe) for NCCH(4-CN-C6H4)+, 0.97 V for the benzylcation, 0.33 V for the tert-butyl cation, and 0.0 V for the methoxymethyl-cation to -0.88 V for Et2N=CHCH3+ [143], 

Stable cations as tropylium salts [144], cyclopropenylium salts [145], alkyl-pyridinium salts (Eq. 18) [146], and pyrylium salts [147] have been dimerized at the cathode and radicals are proposed as intermediates. 

The cathodic dimerization of aromatic aldehydes and ketones in slightly acidic medium via ketyls or hydroxymethyl radicals is a powerful method for the preparation of pinacols in good to excellent yields, e.g. 98 [148a], 99 [148b] or 100 [148c]. Correspondingly, the heteroanologous azomethines, pyrimidines, quinazolines and immonium ions can be hydrodimerized, presumably also via radicals [149], 

Electrohydrocyclization of bis-imines provides a short access to piperazines, e.g. 101 [150], Heterocoupling between acetaldehyde and methyl acrylate in slightly acidic medium affords 102 in high yield, and probably involves a hydroxymethyl radical as intermediate, which adds to the acrylate [151], 

The large number of synthetically useful intermolecular hydrodimerizations and intramolecular cyclizations of activated olefins to complex carbon skeletons involves in most cases radical anions as key intermediates [152]. 

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