Chiral amplification in Diels-Alder and 1,3-dipolar cycloadditions catalyzed by bis(oxazoline)-Zn(II)-based chiral complexes

A Zn(II)-bis(oxazoline) chiral complex catalyzes the cycloaddition reactions of alkenoyl-oxazolidinones with both cyclopentadiene and diphenyl nitrone. The 1,3-dipolar cycloaddition becomes exo selective with an ee of up to 86%. While the Mg(II)-based catalysts give a linear relationship between the ligand's optical purity and the ee of the reaction product, a remarkable positive nonlinear effect was observed in both cycloadditions by using Zn(II) as the cationic core of the catalyst. The result may be rationalized by the formation of a stable, insoluble, and catalytically inactive heterochiral complex. (C) 1999 Elsevier Science Ltd. All rights reserved.