Construction of Pyrazol‐5‐Ones From N ‐(α‐Bromoacyl) Aldehyde Hydrazones via Ground‐State Palladium(0)‐Catalyzed Intramolecular Radical Cyclization

N ‐(α‐Bromoacyl) aldehyde hydrazones were designed as new skeletons for the construction of pyrazol‐5‐ones at room temperature. In this transformation, the ground‐state palladium(0) was used as the catalyst and circumvented external light irradiation. The hybrid alkyl Pd(I)‐radical species were generated through the halogen atom transfer (XAT) from alkyl halides to ground‐state Pd(0) to give the pyrazol‐5‐ones after intramolecular radical cyclization at room temperature. This reaction has broad substrate scope, variously substituted 4,4‐dimethyl‐1 H ‐pyrazol‐5(4 H )‐ones were obtained in good yields without the requirement of external radical precursors. Moreover, the late‐stage modification of biologically active molecules was also achieved using this protocol.