Metal-free synthesis of quinolines catalyzed by carbon aerogels: Influence of the porous texture and surface chemistry

We report herein an experimental and theoretical study of the Friedlander reaction, from 2-amino-5-chlorobenzaldehyde and ethyl acetoacetate, catalyzed by free-metal nanocatalysts based on carbon aerogels, to afford quinoline 3a. The developed methodology implies the combined use of carbon aerogels with solvent-free technologies under MW irradiation yielding the corresponding quinoline with moderated yield (66%) in only 5 min of reaction time. Our results demonstrated that the reactivity of the samples upon MW irradiation is strongly dependent on the porosity and surface chemistry of the carbon aerogels, the most active catalytic species being the most acidic oxygenated functional groups,-CO2H groups originated by oxidant treatment, or even in situ by hydrolysis of-CO-O-CO-, over the carbon surface. The theoretical investigation of the reaction mechanism, by using computational methods, demonstrated that the synthesis of quinoline 3a in the absence or in the presence of carbon aerogels takes place by aldolization, subsequent heterocyclization and finally double dehydration. Relatively strong pi-pi stacking interactions between carbon support and reagents could be behind of the observed catalytic performance also extended for the oxygenated models. Furthermore, the concentration of-CO2H groups over the carbon surface is a key factor favoring each step of the reaction but acting as individual catalytic sites. (C) 2016 Elsevier B.V. All rights reserved.