CAN A CHIRAL CATALYST CONTAINING THE SAME LIGAND METAL COMPONENTS PROMOTE THE FORMATION OF BOTH ENANTIOMERS ENANTIOSELECTIVELY - THE BIS(OXAZOLINE)-MAGNESIUM PERCHLORATE-CATALYZED ASYMMETRIC DIELS-ALDER-REACTION/

The Diels-Alder (DA) reaction between cyclopentadiene (1) and 3-acrylo yl-1,3-oxazolidin-2-one (2), to give the endo adduct 5 as the main pro duct, has been found to occur enantioselectively in the presence of ca talytic amounts of magnesium perchlorate (MP) and 2,2-bis{2-[(4R)-phen yl-, [(4R,5S)- and [(4R,5R)-diphenyl-1,3-oxazolinyl]}propane (3a-c res pectively). The intermediates are the tetrahedral complexes 4a-c, with 2 and 3 coordinated around Mg(II), and their structures were investig ated by NMR. Whereas 3a and 3b gave ee of (S)-5 in the range 28-73%, 3 c was an excellent ligand and 97% ee of (S)-5 was obtained. When two e quivalents of hydroxylic ligands (water or alcohols) are added, all th ese complexes can expand the coordination number of Mg(II) from 4 to 6 . The octahedral complexes 6a-c are again good catalysts of the DA rea ction but, with 3a and 3b, the enantioselectivity is reversed and (R)- 5 is obtained with ee up to 89%. For the first time, achiral auxiliary ligands can be used to invert the enantioselectivity induced by a chi ral bis(oxazoline), so that both enantiomers can be synthetized with t he same metal cation and chiral ligand. The catalytic enantioselective processes are discussed and a rationalization of the experimental res ults is proposed. (C) 1997 Elsevier Science Ltd.