Synthesis of Tetrahydroquinolines by Scandium-Catalyzed [3 + 3] Annulation of Anilines with Allenes and Dienes

This work reports a redox-neutral C–H/N–H annulation of anilines with allenes and dienes (1,3-dienes and 1,4-dienes) by using a scandium catalyst with a simple β-diketiminato ligand. In the reactions, both allene and diene coupling partners served as three-carbon building blocks, selectively producing a family of tetrahydroquinolines in high isolated yields with a broad substrate scope and high atom efficiency. The resultant products facilely underwent different kinds of derivatizations to form other important molecules. Mechanistic investigations including control experiments and kinetics revealed that the reaction probably proceeded via the sequential N–H allylation, Hofmann–Martius rearrangement, and hydroamination cyclization.