Review: Hydrogen Bond and Protonation as Modifying Factors of the Quinone Reactivity

The redox chemistry of different quinones is focused to understand how the reduction products can be stabilized by supramolecular interactions or how they can react through different proton transfer mechanisms. This reactivity depends not only on the electronic properties of the substituents, but also on different structural effects related to the formation of intra and intermolecular hydrogen bonds. In order to explore a wider spectrum for covalent and non-covalent interactions, several proton donors and substituted quinone structures have been tested in two aprotic solvents as dimethyl sulfoxide and acetonitrile. The presence of an amino group between the quinone system and a substituted aromatic ring, allows the smooth transmission of the substituent electronic effect provoking a gradual modification of the quinone redox and basicity properties. It has been shown, in α-phenolicnaphthoquinones ¸ that intramolecular hydrogen bond association stabilizes both electrogenerated radical-anion and dianion, which modifies the reactivity of both intermediaries. Regarding the presence of external hydrogen-bond donors, it has been proposed that the stoichiometry and thermodynamics of the intermolecular hydrogen bond association process can be related to: the basicity properties of the quinone and the acidity constant values of both, electroreduced quinone intermediaries and proton donor species. In the same way, the number of α-hydrogens contained in the quinone nucleus and the geometry of the association complexes involving the intermediaries are both related to their stability, which alternatively is totally modified when direct protonation reactions appear in the reaction pathways. The self-protonation reactions of quinones (α-hydroxy quinones) afford a very illustrative example to do this kind of modifications. From the examples discussed here, it was concluded that the different strategies reviewed could allow tuning the reactivity of a specific quinone system, in order to test and to predict new molecules for biological applications.