Selective N-cycle hydrogenation of quinolines with sodium borohydride in aqueous media catalyzed by hectorite-supported ruthenium nanoparticles

Abstract

A new catalyst containing metallic ruthenium nanoparticles intercalated in hectorite (nanoRu′@hectorite) was found to catalyze the reduction of quinoline and quinoline derivatives by NaBH4 in aqueous solution to give selectively the corresponding 1,2,3,4-tetrahydroquinolines (N-cycle hydrogenation). In most cases the reaction can be done under mild conditions (25–60 °C) without pressure equipment, conversion and selectivity being superior to 99%. In the case of sterically hindered derivatives, the reaction can be done in a pressure vessel under self-generated pressure (up to 9 bar). Isoquinoline and quinoxalines also undergo selective N-cycle hydrogenation, but 2-phenyl-quinoline is hydrogenated to give 2-phenyl-5,6,7,8-tetrahydroquinoline (C-cycle hydrogenation). Isotope labeling experiments combined with semi-empirical calculations of the electrostatic potentials support a heterolytic hydrogenation mechanism involving a hydride from NaBH4 and a proton from H2O. The catalyst nanoRu′@hectorite can be recycled and reused.