Cascade Dearomative Hydride Transfer/Enantioselective Semipinacol Rearrangement of Quinolines by a Chiral Brønsted Acid

Herein, we report a chiral Brønsted-acid-catalyzed
dearomatization
of quinolines. The reaction proceeds via dearomative hydride transfer
and subsequent enantioselective semipinacol rearrangement. The key
to this reaction sequence is to guide the imine intermediate to a
rearrangement pathway other than over-reduction. Utilizing a bulky
chiral imidodiphosphorimidate catalyst guarantees the desired reactivity.
A series of chiral spiro tetrahydroquinoline products is afforded
in good yields (up to 93%) and enantioselectivity (up to 93% ee).
Detailed mechanistic insights are obtained based on DFT calculations.