Synthesis of Indazole Fused 2-Benzazepines with Polarity-Dependent Fluorescence Based on Formal [4 + 3] Annulation of 3-Aryl-1H-indazoles with Cyclopropenones.

The effective assembly of benzazepine skeletons in a sustainable and atom-economical fashion remains a challenging goal in modern organic synthesis. Presented herein is a novel synthesis of indazole fused 2-benzazepine derivatives based on a formal [4 + 3] annulation of 3-aryl-1H-indazoles with cyclopropenones. The formation of products proceeds through Ir(III)-catalyzed aryl C-H bond metalation and cyclopropenone ring-opening leading to aryl acylation, followed by an intramolecular N-nucleophilic conjugated addition. By using this method, a number of valuable benzazepine derivatives were effectively generated. This protocol addresses the challenges in constructing medium-sized rings through cascade C-H/C-C bond activation and C-C/C-N bond formation. Moreover, the photophysical properties of the products thus obtained were also evaluated. It turned out that all compounds tested showed solvent polarity-dependent fluorescence features, which could be potentially applied for revealing the polarity of their immediate environments.