Quinolines and quinolones : tautomerism and visible-light photocatalyzed reductions

Abstract

Many pharmaceuticals, corrosion inhibitors, colorants, materials for LEDs and OLEDs, as well pH and fluorescence indicators contain as a key core structure the quinoline or quinolone motive. The chemical modifications of the quinoline and quinolone core are as important as their synthesis for the development of new materials. In order to achieve the desirable diversity the protection group chemistry, as well the functional group transformations, play a significant role in the quinoline and quinolone chemical modifications. Additionally, in many cases the tautomeric state of the hydroxyquinoline is of paramount importance for the correct functioning of the corresponding pharmaceuticals.

The literature review part of the thesis deals with the quinoline and quinolone syntheses and their chemical modifications. In addition to this, the rudimentary concept of tautomerism is introduced, together with the tautomerism in hydroxyquinolines. Moreover, the basic concept of visible-light photocatalysis is presented with highlights of the reductive quenching cycles.

The experimental results presented in this thesis have been published in three peer-reviewed journals. The first one covers tautomeric locking and switching and the second and third ones report visible-light photoreductions of functionalized quinolines.

The design and synthesis of hydroxyquinolines equipped with two side arms with hydrogen bond accepting/donating properties and or metal chelating properties, allow us to achieve a fragile tautomeric equilibrium. This equilibrium was subject to controllable adjustment to one or the other side by metal chelation and locking or solvent addition.

Deprotection of O-benzylated quinolines to the corresponding quinolones was achieved by visible-light photoreduction. The described method merges the tautomeric abilities of the masked quinolones and modern photoredox catalysis, allowing us to achieve regio- and chemoselectivity.

Reduction of nitroquinolines to the corresponding aminoquinolines was achieved by visible-light photoreduction. The described method uses ascorbic acid as a sacrificial reductant and hydrogen source, which make it particularly useful. The method is green and has broad functional group tolerance.