Intramolecular Hydrogen Bonding/Selfprotonation Processes Modulated by the Substituent Effect in Hydroxyl-substituted Naphthoquinones

In this work, an electrochemical and spectroelectrochemical ESR study for a series of hydroxyl substituted 1,4-naphthoquinones is presented. Results show that the electrochemical behaviour is dependent on the relative positions of these groups within the molecules. For compounds which have hydroxyl groups at the annellated benzene ring, intramolecular hydrogen bonding (a substituent field effect) determines the energy of reduction of the system. When hydroxyl functions are located at the C-2 or C-3 positions, a selfprotonation process occurs. The electrogenerated dianion or trianion radicals, derived from deprotonated quinones, show that intramolecular hydrogen bonding has a significant effect both in the spin density distribution and in the energy required for the formation of these radical species. The difference observed in the slope for EpIc vs. log v function for 2,3,5,8-tetrahydroxy-1,4-naphthoquinone suggest that, for this compound, the proton transfer step does not occur as a single, but as a two-step sequence, where an hydrogen bonded adduct could be present as a stable intermediate. These processes could help to explain the discrepancies observed earlier performing linear free energy relationships these compounds.