Copper-Catalyzed\nAlkynylation of <i>In Situ</i>-Generated Azoalkenes: An\nUmpolung Approach to Pyrazoles

A straightforward umpolung approach to regioselectively\nN-protected\npolysubstituted pyrazoles starting from aromatic α-halohydrazones\nand terminal alkynes has been developed. In this process, azoalkenes\ngenerated <i>in situ</i> from α-halohydrazones are\ninvolved in the Cu(I)-catalyzed Michael addition with alkynes to give\nα-alkynyl-substituted hydrazones that cyclize to give the target\npyrazoles in 37–85% yield. The method employs readily available\nstarting materials and features good functional group compatibility\n(nitro, sulfonyl, cyano, trimethylsilyl, and primary bromoalkyl groups,\nesters, and alcohols are tolerated) and scalability.