Rhodium‐Catalyzed [4+2] Annulation of N‐Aryl Pyrazolones with Diazo Compounds To Access Pyrazolone‐Fused Cinnolines.

An efficient synthesis of novel dinitrogen‐fused heterocycles such as pyrazolo[1,2‐a]cinnoline derivatives have been accomplished by the rhodium(III)‐catalyzed reaction of N‐arylpyrazol‐5‐ones with α‐diazo compounds. This reaction proceeds through a cascade C−H activation/intramolecular cyclization with a broad substrate scope. Furthermore, this protocol is successfully extended to the unusual phosphorus‐containing α‐diazo compounds and cyclic diazo compounds as the cross‐coupling partners to deliver the two new kinds of pyrazolo[1,2‐a]cinnolinones. The control experiments were performed to reveal insight into the mechanism of this reaction, involving reversible C−H activation, migratory insertion of the diazo compound, and cascade cyclization as the key steps of the transformation. Moreover, gram‐scale synthesis and further transformation of the target product demonstrate the synthetic utility of the present protocol. [ABSTRACT FROM AUTHOR]