Chiral Bimetallic Catalysts Derived from Chiral Metal Phosphates: Enantioselective Three-Component Asymmetric Aza-Diels–Alder Reactions of Cyclic Ketones

A new
type of chiral bimetallic catalyst is disclosed. These chiral
bimetallic catalysts are easily formed through mixing a metal Lewis
acid and a metal binaphthyl phosphate (MLA/M­[P]3) in solution. 1H and 31P NMR spectroscopy, electron paramagnetic
resonance (EPR) spectroscopy, matrix-assisted laser desorption ionization
time-of-flight (MALDI-TOF) mass spectrometry, and X-ray crystallographic
analysis reveal a bimetallic structure of the Y­(Yb)III/Y­[P]3 complexes with bridging binaphthyl phosphate ligands. The
Lewis acidity of these chiral bimetallic catalysts is readily tuned
by changing either the metal Lewis acid or the chiral metal phosphate.
Through cooperative metal Lewis acid–enamine catalysis, asymmetric
three-component aza-Diels–Alder reactions of 5-, 6-, and 7-membered
cyclic ketones, unsaturated ketoesters, and arylamines were successfully
achieved to afford fused bicyclic dihydropyridines in high yields
(up to 94%) with high enantioselectivity (up to 99% enantiomeric excess)
and excellent chemoselectivity.