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, )-4,6-dibenzofurandiyl-2,2'-bis(4-phenyloxazoline) (DBFOX/Ph) , and various transition-metal(II) perchlorates. These complexes are e ffective catalysts in the Diels-Alder reactions of cyclopentadiene wit h 3-alkenoyl-2-oxazolidinone dienophiles and show excellent enantiosel ectivities. The active catalyst complex prepared from nickel(II) perch lorate hexahydrate has an octahedral structure with three aqua ligands , and it can be isolated and stored for months without loss of catalyt ic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. The absolute configuration induced in the reacti on can be readily predicted on the basis of the C-2-symmetric structur e of the complexes as well as the simple structure of the substrate co mplex. The aqua complex prepared from Ni(II) or Zn(II) perchlorate res ults in highly effective chiral amplification in the Diels-Alder react ion. Use of the DBFOX/Ph ligand of a low enantio purity of 20% ee lead s to a 96% ee for the endo cycloadduct. Two mechanisms for amplificati on are involved in this remarkable chiral amplification: (1) precipita tion 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 ol igomers.