Palladium-Catalyzed Intramolecular Fujiwara-Hydroarylation: Synthesis of Benzo[a]phenazines Derivatives

An atom-economical Pd-catalyzed approach for the synthesis of benzophenazine derivatives using substituted 2-aryl-3-(aryl/alkylethynyl) quinoxaline in the presence of trifluoroacetic acid at 65 °C has been described. The chemistry involves in situ generation of cationic Pd­(II) species, which functionalized the aromatic C–H bonds via electrophilic metalation followed by concomitant intramolecular trans-insertion of C–C triple bond to aryl-Pd complex. The results were supported by various control experiments including with electron–deficient arenes and deuterium labeling studies. The deuterium labeling studies supports electrophilic palladation of aromatic C–H over activation of C–C triple bond of alkyne. The structure of synthesized compounds was further confirmed by X-ray crystallography studies. This catalytic protocol has been efficiently applied for novel synthesis of highly functionalized benzo fused phenazines.