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-acryloyl-1,3-oxazolidin-2-one (2), to give the endo adduct 5 as the main product, has been found to occur enantioselectively in the presence of catalytic amounts of magnesium perchlorate (MP) and 2,2-bis{2-[(4R)-phenyl-, [(4R,5S)- and [(4R,5R)-diphenyl-1,3-oxazolinyl]}propane (3a-c respectively). The intermediates are the tetrahedral complexes 4a-c, with 2 and 3 coordinated around Mg(II), and their structures were investigated by NMR. Whereas 3a and 3b gave ee of (S)-5 in the range 28-73%, 3c was an excellent ligand and 97% ee of (S)-5 was obtained. When two equivalents of hydroxylic ligands (water or alcohols) are added, all these 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 reaction 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 chiral bis(oxazoline), so that both enantiomers can be synthetized with the same metal cation and chiral ligand. The catalytic enantioselective processes are discussed and a rationalization of the experimental results is proposed. (C) 1997 Elsevier Science Ltd.