Access to 1-Aryl-pyrazolin-5-ones via Photocatalytic Chemoselective Tri/Difluoromethylation and Cyclization of N-Methacryloyl Aldehyde Hydrazones

A photocatalytic trifluoromethylation and 5-endo-trig cyclization of (E)-N′-arylidene-N-phenylmethacrylohydrazides was developed for the construction of trifluoromethylated pyrazolin-5-one derivatives using easily available trifluoromethyl thianthrenium triflate (TT-CF₃⁺OTf^–) as the trifluoromethyl source. This protocol exhibits a broad substrate scope and excellent chemoselectivity. The utility of the current strategy is further highlighted by its application in the late-stage modification of drug molecules. Furthermore, difluoromethylative cyclization was realized using PhI(O₂CCHF₂)₂ under similar conditions.