Indirect Electrochemical Generation of Radicals at the Anode

Alcohols can be oxidized to carboxylic acids at the nickel hydroxide electrode. Here, nickel hydroxide acts as an immobile mediator at the surface of a nickel net anode. It is oxidized at a low potential to nickel(III) oxide hydroxide, which abstracts a hydrogen from the hydroxymethyl group to generate a hydroxymethyl radical, which is then further oxidized. The nickel(II) hydroxide thereby formed is reoxidized without going into solution. The electrode is selective for primary alco- 

Manganese(III) can oxidize carbonyl compounds and nitroalkanes to carboxy-methyl and nitromethyl radicals [186]. With Mn(III) as mediator, a tandem reaction consisting of an intermolecular radical addition followed by an intramolecular electrophilic aromatic substitution can be accomplished [186, 187). Further Mn(III)-mediated anodic additions of 1,3-dicarbonyl and l-keto-3-nitroalkyl compounds to alkenes and alkynes are reported in [110, 111, 188). Sorbic acid precursors have been obtained in larger scale and high current efficiency by a Mn(III)-mediated oxidation of acetic acid acetic anhydride in the presence of butadiene [189]. Also the nitromethylation of benzene can be performed in 78% yield with Mn(III) as electrocatalyst [190]. A N03 radical, generated by oxidation of a nitrate anion, can induce the 1,4-addition of aldehydes to activated olefins. NOj abstracts a hydrogen from the aldehyde to form an acyl radical, which undergoes addition to the olefin to afford a 1,4-diketone in 34-58% yield [191]. 

A comprehensive compilation of radical processes initiated by electron transfer at the electrode or by chemical redox reagents, that lead to transition metal complexes, has been provided by D. Astruc [207]. 

Nikishin, Russ. Chem. Bull. 1998, 47, 1133 M. N. Elinson, S. K. Feducovich, A. A. Zakhar-enkov, G. I. Nikishin, Mencleleev-Commun. 1999, 20. 

Fussing, H. Lund, S. U. Pedersen, Acta Chem. Scand. 1998, 52, 657 S. U. Pedersen, 

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