Self-protonation mechanism in the electrochemical reduction of jatropholone

The electrochemical reduction mechanism of jatropholone (JOH) and derivativ es (JOAc) was investigated by cyclic voltammetry, polarography, coulometry and controlled potential electrolysis. It involves at the first wave potent ial, a self-protonation mechanism, whereby the two-electron reduction produ ct (JOHH(-)), an aromatic ketone, is formed together with the conjugate bas e of the former compound, JO(-). The unusual stoichiometry of the cathodic process at the first wave, 1 mol electron mol(-1), indicates that JOHH(-) i s not further protonated. The decreased basicity of the anion is due to res onance stabilisation. The second wave is related to the reduction of JO(-) and further reduction of JOHH(-), that has a reminiscent electroactive grou p. In the latter case, the background electrolyte is liable to be the proto n donor, through Hoffman elimination. The complete reduction involves 4 e(- ) and 4 protons. Electrolysis performed near the first wave potential led t o the almost exclusive reduction of the exocyclic double bond, without dime rization. (C) 1999 Elsevier Science S.A. All rights reserved.