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.