A π–Cu(II)−π Complex as an Extremely Active Catalyst for Enantioselective α-Halogenation of <i>N</i>-Acyl-3,5-dimethylpyrazoles
Kazuki Nishimura, Yanzhao Wang, Yoshihiro Ogura, Jun Kumagai, Kazuaki Ishihara
Abstract
Novel chiral π–copper(II)−π complex catalyzed enantioselective α-chlorination and -bromination of N-acyl-3,5-dimethylpyrazoles are described. The π–copper(II)−π complexation of Cu(OTf)2 with 3-(2-naphthyl)-l-alanine-derived amides greatly increases the Lewis acidity and triggers the in situ generation of enolate species without an external base, which has a suppressing effect for α-chlorination and -bromination due to undesired halogen bonding. This strategy provides facile access to α-halogenated compounds in high yield with excellent enantioselectivity. X-ray crystallographic and ESR analyses of the catalyst complexes suggest that the release of two counteranions (2TfO–) from the copper(II) center might be crucial for the efficient activation of N-acyl-3,5-dimethylpyrazoles.