Electrochemical aspects of the reduction of biologically active 2-hydroxy-3-alkyl-1,4-naphthoquinones

A series of natural and synthetic 2-hydroxy-3-alkylnaphthoquinones, several of them with relevant biological activities, showed typical voltammograms, on Hg and glassy carbon electrodes, in aprotic medium (DMF–0.1 mol l−1 TBAP). Two main pairs of peaks, the first one corresponding to irreversible and the second one to quasi-reversible processes, were observed. Intermediate shoulders were also present. They showed strong dependence on scan rate, concentration and pKa of added proton sources. Lapachol was chosen for detailed investigations, on Hg and glassy carbon electrodes. A self-protonation mechanism and hydrogen-bonded intermediates explain the nature of the first peaks and shoulders. The last pair of peaks is related to the quasi-reversible two-electronic reduction of the respective anion of the 2-hydroxynaphthoquinones (NQOH). Addition of proton sources of different strength caused pKa-dependent modifications of voltammograms. Electrochemical effects are demonstrated over the full reduction range, from protonation of unreduced quinones to hydrogen-bonding of reduced dianions. Strong acids like trifluoroacetic acid cause protonation of the original quinone and consequent easier reduction. Benzoic acid is strong enough to cause the merging of the waves into one pair of irreversible waves. Phenol is able to modify only the more basic electrogenerated intermediates. H-free derivatives were prepared and their voltammetric behavior observed. They showed the expected quinone reduction pattern.