A π–Cu(II)−π Complex as an Extremely Active Catalyst for Enantioselective α-Halogenation of <i>N</i>-Acyl-3,5-dimethylpyrazoles

Novel chiral π–copper(II)−π complex catalyzed enantioselective α-chlorination and -bromination of N-acyl-3,5-dimethylpyrazoles are described. The complexation Cu(OTf)2 with 3-(2-naphthyl)-l-alanine-derived amides greatly increases the Lewis acidity triggers in situ generation enolate species without an external base, which has a suppressing effect for due to undesired halogen bonding. This strategy provides facile access α-halogenated compounds high yield excellent enantioselectivity. X-ray crystallographic ESR analyses catalyst complexes suggest that release two counteranions (2TfO–) from copper(II) center might be crucial efficient activation N-acyl-3,5-dimethylpyrazoles.