Rh(III)‐Catalyzed sequential ring‐retentive/‐opening [4 + 2] annulations of 2H‐imidazoles towards full‐color emissive imidazo[5,1‐a]isoquinolinium salts and AIE‐active non‐symmetric 1,1′‐biisoquinolines

Abstract

Rhodium-catalyzed C4aryl−H activation and ring-retentive annulation of 2H-imidazoles with internal alkynes to build imidazo[5,1-a]isoquinolinium salts with high yields and broad scope has been disclosed. These novel salts serve as new full-color emissive fluorophores (433−633 nm), just by simply modifying the substituents on C3 and C4 positions of isoquinoline ring. Furthermore, these salts can undergo ring-opening C5aryl−H activation/annulation with a different alkyne to form non-symmetric and AIE-active 1,1′-biisoquinolines, where NH4OAc plays an indispensable role that accounts for Hofmann elimination and imine formation, leading to an unprecedented imine dance: cyclic imine → N-alkenyl imine → NH imine. The 15N labelling experiments indicate that the 2nd annulation includes two pathways: N-exchange (major) and N-retention (minor).