When Hydrazonoyl Chlorides Meet Allenes: a Site- and Regio-Selective Copper(I)-Catalysed Approach to 5-Substituted Pyrazoles

The reaction between hydrazonoyl chlorides and monosubstituted allenes in the presence of catalytic amounts of copper(I) chloride gives 1,3,5-substituted pyrazoles under mild conditions and very short reaction times. This site- and regioselective process involves first the complexation of copper(I) on the external double bond of the allene moiety, followed by nucleophilic attack on the central carbon atom of the so-formed copper(I)-complexed allene by the terminal nitrogen of the hydrazonoyl chloride. Subsequent ring closure to the target pyrazole ring is possible in the presence of electron-attracting groups on the allene moiety. A catalytic cycle has been proposed on the basis of the experimental results.