Electrooxidation of substituted ferrocenes: indirect oxidation of the side chain

Anodic oxidation of ferrocenes bearing an electron-withdrawing substituent with a H-atom in the α-position (ferrocenylacetic acid 1a, ferrocenylacetonitrile 2a) has been investigated by cyclic voltammetry and by macroscale electrolysis in a flow cell fitted with a porous graphite felt anode. Oxidation of ferrocenes (1a, 2a) into their ferrocenium cations (1a+, 2a+) increases the acidity of the H-atom in the α-position to the cyclopentadienyl ligand. The deprotonation is favoured by a basic medium to give a radical (1b, 2b). According to the substrate, the radical can be oxidised (1b) or can dimerise (2b). Exhaustive electrolysis of 1a leads to ferrocenecarboxaldehyde after multi-step reactions (redox reactions and nucleophilic attack by H2O). Electrolysis of 2a affords a dimer which was further oxidised into 1,2-diferrocenyl-1,2-dicyano ethylene.