Transition-metal aqua complexes of 4,6-dibenzofurandiyl-2,2′-bis(4-phenyloxazoline).: Effective catalysis in Diels-Alder reactions showing excellent enantioselectivity, extreme chiral amplification, and high tolerance to water, alcohols, amines, and acids

Cationic aqua complexes are prepared from a trans-chelating tridentate ligand, (R,R)-4,6-dibenzofurandiyl-2,2'-bis(4-phenyloxazoline) (DBFOX/Ph), and various transition-metal(II) perchlorates. These complexes are effective catalysts in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazolidinone dienophiles and 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. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. The absolute configuration induced in the reaction can be readily predicted on the basis of the C-2-symmetric structure of the complexes as well as the simple structure of the substrate complex. The aqua complex prepared from Ni(II) or Zn(II) perchlorate results in highly effective chiral amplification in the Diels-Alder reaction. Use of the DBFOX/Ph ligand of a low enantio purity of 20% ee leads to a 96% ee for the endo cycloadduct. Two mechanisms for amplification are involved in this remarkable chiral amplification: (1) precipitation of an 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.