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, CH3CN, 35 degrees C] afforded 4-trifluoromethyl pyrazoles in moderate to excellent yields with excellent regioselectivity.