Copper-catalyzed chemoselective synthesis of 4-trifluoromethyl pyrazoles.

A series of 4-trifluoromethyl pyrazoles have been prepared via the copper-catalyzed cycloaddition of 2-bromo-3,3,3-trifluoropropene with a variety of N -arylsydnone derivatives under mild conditions. This new protocol under optimized reaction conditions [Cu(OTf) 2 /phen, DBU, CH 3 CN, 35 °C] afforded 4-trifluoromethyl pyrazoles in moderate to excellent yields with excellent regioselectivity.