Effects of the molecular structure on the electrochemical properties of naturally occurring α-hydroxyquinones. An electrochemical and ESR study

The electrochemical study of a series of naturally occurring α-hydroxyquinones (2-hydroxy-1,4-naphthoquinone, Perezone, Horminone and 7α-O-methyl-Conacytone), revealed that the reduction mechanism occurring at the first voltammetric reduction signal, is determined by the presence of selfprotonation reactions. Furthermore, the detailed voltammetric analysis of this signal, reveals that there is a dependence of the structure of the chemical compound bearing the α-hydroxy-moiety in the reduction mechanism. These differences are determined by the stability and basicity of the first electrogenerated anion radical for each compound. However, since this species disproportionates readily in solution, it was not possible to analyze its structural properties by coupled electrochemical – ESR experiments. As during the selfprotonation process an important amount of deprotonated quinone is produced, the spectroelectrochemical study at a potential region at the second reduction process for this type of compounds showed well resolved ESR spectra, corresponding to the dianion radical structures originated from the reduction of the deprotonated quinone. The differences in chemical behavior showed that the abietanic skeleton of naturally occurring quinones Horminone and 7α-O-methyl-Conacytone induces changes in the spin structure which prove valuable in determining the stability and reactivity of the electrogenerated dianion radical.