Remarkably Stable Chiral Zirconium Complexes for Asymmetric Mannich-Type Reactions

Isolable, air-stable, storable, and highly selective chiral zirconium catalysts for asymmetric
Mannich-type reactions have been developed. The reactions of imines with silicon enolates proceeded
smoothly using 1−10 mol % of the powdered zirconium catalyst to afford the corresponding adducts in
high yields with high stereoselectivities. The catalyst could be recovered and reused without significant
loss of activity. On the other hand, zirconium single crystals for X-ray analysis were obtained, and the
crystals also showed high performance in the asymmetric Mannich-type reactions. Although NMR analyses
of these zirconium catalysts showed different structures in dichloromethane, the formation of the same
key intermediate from the different catalysts was indicated.