Fused Imidazo‐Pyrrolo‐Isoquinolinedione Architectures: Emerging Through [4 + 2] Annulation Sequence

Cobalt(II)‐catalyzed sequential C(sp ² )─H and N(sp ^3–2 )─H activations have been inforced to carry out [4 + 2] annulation sequence between imidazole derivatives with maleimides, enabling streamlined access to fused pyrrolo‐isoquinolinedione architectures. This approach exhibits broad functional group tolerance involving cost‐effective and earth‐abundant metal catalyst and offers high atom economy. This strategy further allows late‐stage functionalization of biologically relevant fused heterocyclic frameworks, including ibuprofen and nicotinic acid, while accommodating a range of post‐synthetic modifications such as chemoselective reductions and oxidative transformations. Notably, a phenanthroimidazole‐embedded derivative ( 6j ) exhibits a pronounced fluorescence “turn‐off” response toward Fe ²⁺ ions, displaying high sensitivity and selectivity. Comprehensive mechanistic studies corroborated through control experiments provide detailed insight into the underlying reaction pathway and catalyst behavior.