An effect of the ionic radii of lanthanide(III) ions on the structure and catalytic properties of chiral Schiff base-lanthanide(III) complexes

Tetradentate chiral Schiff base-Ln(III) complexes (where Ln represents a lanthanide series element) have been prepared from the reaction of N,N-bis(2-pyridylmethylidene)-1,2-(R,R)-cyclohexanediamine with Ln(NO3)(3)(.)nH(2)O (Ln = La, Ce, Pr, Nd, Sm, Eu. (for n = 6), Gd (for n = 5), and Yb (for n = 3)), LnCI(3)(.)nH(2)O (Ln = Ce, Pr (for n = 7), Nd, Er, and Yb (for n = 6)), and Yb(CF3SO3)(3). The structures of the complexes were determined using X-ray crystallography, and an effect of the ionic radius on the structure of the complexes was studied. The chiral Schiff base-Ln(III) complexes prepared in situ in acetonitrile displayed catalytic activity for the asymmetric Diels-Alder reaction of 3-(2-propenoyl)-2-oxazolidinone with cyclopentadiene. The (R)-endo isomer of the cyclo adduct was obtained with high diastereoselectivity and moderate enantioselectivity. The enantioselectivity was sensitive to the ionic radius of the Ln(Ill) ion, and the enantiomeric excess values increased monotonically in the order: La < Pr < Nd < Eu < Dy < Er < Yb approximate to Lu. The enantioselectivity increased via close contact of 3-(2-propenoyl)-2-oxazolidinone and the chiral Schiff base ligand coordinated to the small Ln(Ill) ion. (C) 2003 Elsevier Science B.V. All rights reserved.