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.
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