Towards selective synthesis of quinoxalines by using transition metals-doped carbon aerogels

Abstract

Transition metal (TM)-carbon aerogels, where TM = Mo, Fe, Co or Cu, were found to be active and selective catalysts for the synthesis of quinoxalines 1, from o-phenylenediamine 2 and α-hydroxy ketones 3, becoming an efficient and sustainable alternative to other carbon-based catalysts or even MOF.Doping metal phase consisting of the corresponding metal oxides but also as zero-valent metals depending on the metal and carbonization temperature, and metal loading at the surface of carbon aerogel are key factors conditioning both reactivity and selectivity. Although metal oxides are probably the predominant active catalytic species, zero-valent metals nanoparticles (Cu0 or Co0) could be implied in the last dehydrogenation step of the reaction. Moreover, the additional functionalization with oxygenated surface groups (Co-1000PO catalyst) resulted on an enhanced reactivity probably due to the cooperation between both functions. Remarkably, Mo-500 catalyst was the most efficient sample selectively leading to the quinoxaline 1a in high conversion.Finally, our results strongly suggest different operative pathways when using TM-doped carbon aerogels depending on the metallic phase at the carbon surface. While TM-doped carbon aerogels (where TM = Fe, Co, or Cu) probably act catalyzing a sequential acid-base steps and subsequent aromatization leading to the corresponding quinoxaline 1a, Mo-doped catalyst would work via the initial oxidation of α-hydroxy ketones followed by condensation-dehydration reactions.