Toward Safer Processes for C−C Biaryl Bond Construction: Catalytic Direct C−H Arylation and Tin-Free Radical Coupling in the Synthesis of Pyrazolophenanthridines

Abstract

A series of pyrazolo[1,5-f]phenanthridine derivatives has been efficiently synthesized by a short, straightforward sequence. A tandem amine-exchange/heterocyclization of enaminones was successfully applied to the regioselective preparation of 1,5-diarylpyrazole intermediates with structure resemblance to relevant nonsteroidal anti-inflammatory drugs such as celecoxib or tepoxalin. The final key step, cyclization by intramolecular biaryl bond formation, was accomplished by two alternative methodologies: radical coupling and catalytic direct arylation via C-H activation. The scope and limitations of the two methodologies have been explored and their complementariness has been established. In addition, polymer-supported heterogeneous catalysts have been compared with homogeneous analogues. In the radical process, toxic tin derivatives have been avoided in order to employ environmentally safer protocols.