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.