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

Abstract

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 SbF6) and Cu[(S,S)-Ph- bis(oxazoline)](X)2 (2) in hetero Diels-Alder and glyoxytate 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.