Electrochemical reduction of 2-fluorenecarboxaldehyde: A mechanism with a grandparent–grandchild reaction

The electrochemical reduction of 2-fluorenecarboxaldehyde, 1, has been investigated, principally in N,N-dimethylformamide. The initially formed anion radical undergoes an irreversible dimerization reaction with a rate constant of 2600 M−1 s−1. The very basic dimer dianion is protonated by the starting material. This “grandparent–grandchild” reaction well accounts for the amount of conjugate base of 1 that is formed. The effect of adding various OH, NH and CH acids was investigated. The enhancement of the dimerization rate observed with the OH and NH acids is interpreted in terms of the formation of hydrogen-bonded complexes with the anion radical which in turn undergo more rapid dimerization reactions than does the uncomplexed anion radical.