The Synthesis of Hydroquinolines from Nitroaldehydes and Ketones by Hydrogenation Sequences and Condensations

Catalytic reaction discovery or methodology development is preferably performed with homogeneous catalysts, but heterogeneous catalysts allow the design of complex multistep syntheses based on their reusability. We introduce here the catalytic synthesis of hydroquinolines starting from nitroaldehydes, ketones and hydrogen. The reaction is complex and proceeds via multiple selective hydrogenation and condensation steps. The nitroaldehyde is selectively hydrogenated forming an aminoaldehyde, followed by a base-catalyzed Friedländer synthesis and selective quinoline hydrogenation. The starting materials are inexpensive, simple regarding their structure and diversely available, and the hydroquinoline motif is part of numerous biologically active compounds. A nanostructured earth-abundant metal catalyst mediates our reaction most efficiently.