ABSTRACT
In general, the direct anodic oxidation of simple aliphatic primary alcohols to aldehydes and acids is often difficult to accomplish due to their high oxidation potentials, which are on the order of 2.5-2.7V versus fcrrocene/ferrocenium couple [108,109]. In contrast, for benzylic alcohols, oxidation of the ring rr-system to the usual radical cation can occur, making transformations at the benzylic carbon more accessible. In the case of primary benzylic alcohols, direct anodic oxidation in acetonitrile gives benzaldehyde, apparently via an ECEC mechanism [Eq. (49)] [110,111]. The initially formed radical cation loses a proton to give the a-hydroxybenzyl radical, which undergoes subsequent oxidation and proton loss. Addition of bases like pyridine or sodium carbonate is helpful, though their role is not entirely clear beyond the likely suppression of acid-catalyzed ionization of the starting alcohol to benzyl cation, which can be trapped by solvent in a Ritter-type reaction. Overoxidation of the aldehyde to the acid also occurs, presumably via the hydrate, which may form via interception of the ahydroxybenzyl carbocation by adventitious water [111,112]. Better results are usually obtained by indirect oxidation with mediators such as triarylamines [113]. N-oxyl radicals [114,115], chelated osmium complexes [116], and alkylammonium hypochlorites under phase-transfer catalysis conditions [117]. Such indirect methods are treated in detail elsewhere in this edition.
