Copper-Catalyzed Isomerization and Cyclization of E/Z-o-Haloaryl N-Sulfonylhydrazones: Convenient Access to 1H-Indazoles

The isomerization of the C=N double bond in hydrazones has drawn considerable attention because of its potential for application in a broad range of chemical transformations. Generally, this isomerization can be realized either photochemically or thermally. A new isomerization approach is presented, namely a copper-catalyzed C=N double bond isomerization of hydrazones, which is followed by an efficient intramolecular C=N coupling reaction, providing an unprecedented catalytic approach for the synthesis of 1H-indazoles from readily accessible Z/E mixture of o-haloaryl N-sulfonylhydrazones.