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Short Paper | Regular issue | Vol 102, No. 10, 2021, pp.1995-2006
Published online, 19th July, 2021
DOI: 10.3987/COM-21-14506
When Hydrazonoyl Chlorides Meet Allenes: a Site- and Regio-Selective Copper(I)-Catalysed Approach to 5-Substituted Pyrazoles

Giorgio Molteni,* Stefano Baroni, Marco Manenti, and Alessandra Silvani

*Department of Chemistry, University of Milano, Via Golgi 19, 20133 Milano, Italy

Abstract

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 thetarget pyrazole ring is possible in the presence of electron-attracting groups on theallene moiety. A catalytic cycle has been proposed on the basis of the experimental results.