Some Electrochemical and Chemical Properties of Methoxatin and Analogous Quinoquinones

The present study establishes relationships between structure and reactivity for the pyrroloquinoline and phenanthroline quinones. The electrochemical reductions of 1,7- and 1,10-phenanthroline-5,6-quinones, like other quinones, are reversible and occur by 2e- transfer in a single step in aqueous solution and by two 1e--transfer steps in aprotic media. The electron-withdrawing pyridine moieties both increase their potentials and stabilize their aprotic semiquinones. The electrochemistry of the cofactor methoxatin and its trimethylester derivative is similar to the phenanthroline quinones in aqueous solution. However, the electrochemical reductions of methoxatin and its triester in aprotic solutions are characterized by at least three potentials, each accounting for less than 1e-. This has been explained by the proposal of semiquinone complexing with itself and with quinone. Despite an electron-donating pyrrole moiety, methoxatin and its trimethylester have relatively high potentials in aprotic solution. This is presumably due to stabilization of radical anions by the aforementioned complexing or by delocalization with carboxylic acid and ester groups. The reduction potential of methoxatin, in both aqueous and aprotic solvent, suggests that oxidation of methanol should be a thermodynamically favorable process. No evidence for an electrochemically reduced state lower than the quinol was found for any of the compounds. Chemical reactivity is influenced by the orientation of the pyridine nitrogen. The two quinones with a pyridine nitrogen peri to a quinone carbonyl add and oxidize nucleophiles most readily. Comparison of pKa values establishes that the pyridine nitrogen of the triester of methoxatin (Ib) is more electron deficient than are the pyridine rings of II and III. The same conclusion is expected for methoxatin (Ia), providing the carboxyl substituents remain undissociated. The reductions of the protonated quinones are reversible in aqueous solution. At pH values where their pyridine rings are not protonated, the reductions of Ia, Ib, and II, but not III, are reversible. The irreversibility of the reduction of neutral III is attributed to the self-complexing of reduced III by hydrogen bonding of its 1,10-pyridine nitrogens and 5,6-hydroxyl substituents. The reductions of the four quinones in water in the pH range examined involves a single IIH+ + 2e- step. Of the quinones, II has the greatest potential in aqueous solution. In aprotic media, II and III