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.