Visible-Light-Promoted Bromocyclization of N-Methacryloyl Hydrazones for the Synthesis of Brominated Pyrazolones

A mild and efficient photoinduced radical cascade cyclization strategy that enables the introduction of a Br atom and the construction of the pyrazolone scaffold from N-methacryloyl hydrazones with tetrabutylammonium bromide (TBAB) has been developed. This transformation employs cheap and readily available TBAB as the bromine source through in situ generation of key radical species, followed by a 5-endo-trig cyclization. This protocol features excellent functional group tolerance and a broad substrate scope and could be successfully applied to the late-stage modification of drug molecules, highlighting its practical utility.