Asymmetric α-Amination of 4-Substituted Pyrazolones Catalyzed by a Chiral Gd(OTf)₃/<i>N,N′</i>-Dioxide Complex: Highly Enantioselective Synthesis of 4-Amino-5-pyrazolone Derivatives

The asymmetric α-amination of 4-substituted pyrazolones with azodicarboxylates was investigated for the first time, employing an N,N′-dioxide gadolinium(III) complex as the catalyst. The novel transformations exhibited high yield, and 4-amino-5-pyrazolone derivatives bearing a chiral quaternary center were obtained in excellent yields (up to 99%) and enantioselectivities (90%−97% ee) for a broad scope of 5-pyrazolones by using 1 mol % or only 0.05 mol % of catalyst.