Mild and Regioselective Synthesis of 3‐CF3‐Pyrazoles by the AgOTf‐Catalysed Reaction of CF3‐Ynones with Hydrazines

Gold‐ and silver‐catalysed reactions of trifluoromethylated ynones with aryl (alkyl) hydrazines were investigated. The use of (THD‐Dipp)AuOTf and AgOTf resulted in quick heterocyclization reactions to selectively give 3‐CF3‐pyrazoles. AgOTf was found to be the catalyst of choice, and various 3‐CF3‐pyrazoles were formed in up to 99 % isolated yield with high regioselectivity. The reaction has a broad scope: 3‐CF3‐pyrazoles with alkyl and aryl substituents as well as different functional groups can be prepared by this approach. The known pyrazole drugs Celebrex® and SC‐560 were efficiently prepared to demonstrate the utility of the method. Mechanistic investigations revealed that the reaction involves the formation of a hemiaminal as a key intermediate.