Bis (oxazoline) copper (II) complexes are highly enantioselective catalysts in Diels-Alder reactions involving bidentate dienophiles. Cationic [Cu ((S, S)-t-Bu-box)] X2 complexes with different counterions have been used as catalysts, revealing a profound influence of the counterion on the rate and stereoselectivity of the catalyst. A square-planar catalyst- substrate complex is proposed to account for the high diastereo-and enantioselectivities ...