Reversal in enantioselectivity of tert-butyl versus phenyl-substituted bis(oxazoline)copper(II) catalyzed hetero Diels-Alder and ene reactions.: Crystallographic and mechanistic studies

X-Ray crystal structures of the Cu [(S,S)-tert-Bu-bis(oxazoline)](H2O)(2)(SbF6)(2) (6), Cu[(S,S)-Ph-bis(oxazoline)](H2O)(2)(SbF6)(2) (7), and Cu [(S,S)-iso-Pr-bis(oxazoline)](H2O)(2)(SbF6)(2) (8) complexes are presented. Implications of the structural details are considered in the context of the opposite enantiofacial biases conferred by complexes Cu[(S,S)-tert-Bu-bis(oxazoline)](X)(2) (1, X = OTf or Sbf(6)) and Cu[(S,S)-Ph-bis(oxazoline)](X)(2) (2) in hetero Diels-Alder and glyoxylate ene reactions. Structural and mechanistic studies suggest that a change in geometry at the metal center is not necessarily responsible for the reversal in enantioselectivity. (C) 1999 Elsevier Science Ltd. All rights reserved.