Asymmetric cycloaddition reactions catalyzed by transition metal complexes - New guidelines for structural design of chiral catalyst

Cationic aqua complexes of a trans-chelating tridentate ligand, (R,R)-4,6-dibenzofurandiyl-2,2'-bis (4-phenyloxazoline), and transition metal(II) perchlorates are effective catalysts in the cyclopentadiene Diels-Alder reactions, nitrone cycloadditions, and diazomethane cycloadditions with 3-alkenoyl-2-oxazolidinones to show excellent enantioselectivities. The active catalyst complex prepared from nickel( II) perchlorate hexahydrate has an octahedral structure with three aqua ligands, and it can be isolated and stored for months without loss of catalytic activity. The aqua complex prepared from Ni (II) or Zn(II) perchlorate results in highly effective chiral amplification in the Diels-Alder reaction. Two mechanisms for amplification are involved for this remarkable chiral amplification:(1) precipitation of S-4-symmetric meso-2:1 complex between DBFOX/Ph and Ni(II) ion, and (2) associative formation of 1:1 heterochiral complexes by the aid of hydrogen bonds based on aqua ligands to produce stable meso oligomers.