In situ generation and conversion of a half-zirconocene catalyst for the synthesis of N-acylpyrazoles

Pyrazoles are an important class of five-membered nitrogen heterocyclic compounds that have been widely used in agriculture and medicine. Exploring their synthesis methods under mild conditions has always been a hot research topic. Herein, a new strategy was developed to enhance the activity of a zirconium metal centre for the synthesis of N-acylpyrazole derivatives using Cp2ZrCl2 as a pre-catalyst. Cp2ZrCl2 was activated in situ within the catalytic system through cyclopentadienyl ring dissociation, leading to the formation of an activated species, [CpZrCl(acac)2]. The new approach demonstrates broad substrate scope under mild reaction conditions, leading to formation of (3,5-dimethyl-1H-pyrazol-1-yl)(phenyl)methanones with yields of up to 97%.