Solvent- and Transition Metal Catalyst-Dependent Regioselectivity in the [3+2] Cyclocondensation of Trifluoromethyl-α,β-ynones with Hydrazines: Switchable Access to 3- and 5-Trifluoromethylpyrazoles

The regioselectivity of the [3+2] cyclocondensation of trifluoromethyl‐α,β‐ynones with hydrazines can be readily tuned to preferentially afford either 3‐ or 5‐trifluoromethylpyrazoles through variation of the reaction conditions. Under catalysis with copper(II) acetate (2.0 mol%), cyclocondensation proceeded smoothly to yield 3‐trifluoromethylpyrazoles with high regioselectivity. In contrast, when the reaction was conducted in dimethyl sulfoxide under catalyst‐free conditions, the formation of 5‐trifluoromethylpyrazoles was predominantly observed.