Photoredox-catalyzed trifluoromethylation of 2-indazoles using TT-CFOTf in ionic liquids.

A protocol for metal and oxidant free photoredox catalyzed trifluoromethylation of 2-indazoles was developed by using Eosin Y as the photocatalyst and recoverable ionic liquids as the solvents. A series of trifluoromethylated products were obtained in moderate to good yields in this protocol under mild conditions. The reaction proceeded a free-radical mechanism with a broad substrate range, excellent regioselectivity, and good functional group tolerance. Furthermore, the utility of this protocol was demonstrated by the synthesis of a highly selective ligand for estrogen receptor beta (ERβ) and the drug granisetron. The protocol provides a mild and environmentally friendly solution for trifluoromethylation reaction.