Synthesis, spectroscopic, structural and electrochemical studies of carboxyl substituted 1,4-naphthoquinones

Two carboxyl substituted quinones and their ethyl esters were prepared by alkylation of 2-methyl-1,4-naphthoquinone (MNQ), also known as menadione or vitamin K₃. All products were characterized by spectroscopic (¹H NMR, ¹³C NMR, IR) and electrochemical (cyclic voltammetry) methods, and the crystal structure of the two carboxylic derivatives was also determined. Both carboxyl substituted quinones crystallize in the system as hydrogen bonded dimers. In MeCN, the cyclic voltammograms of the ester derivatives present two reversible one-electron redox waves very similar to those of the parent quinone, MNQ. However, in the same solvent, the corresponding carboxyl substituted quinones show one cathodic and one anodic additional irreversible waves at more positive potentials and a decrease in current intensity of the two quinone reduction waves accompanied by loss of the quasi-reversible character of the second wave. These results show that the presence of the carboxylic substituent does not greatly modify the redox behaviour of the quinone, except for a small anodic shift of the potentials, but the associated presence of H⁺ ions in solution causes an important perturbation to the system, stabilizing the electrogenerated semiquinones by intermolecular self-protonation and/or hydrogen bonding.