Acid-Promoted Bicyclization of Diaryl Alkynes: Synthesis of 2H‑Indazoles with in Situ Generated Diazonium Salt as Nitrogen Source

An unprecedented transition-metal-free tandem bicyclization of diaryl alkynes has been disclosed, which provides a streamlined access to a range of polycyclic 2H-indazoles in high to excellent yields. The salient features of this reaction include readily available starting materials, good functional group compatibility, mild reaction conditions, no column chromatography, high bond-formation efficiency, and ease in further transformations. Notably, this is the first example for the synthesis of 2H-indazoles with in situ generated diazonium salt as the nitrogen source, and a mechanistic rationale involving an acid-promoted tandem diazonium salt formation/bicyclization process is discussed.