Synthesis of Helical Carbazoles Incorporating an Imidazole Ring

The carbazole core is a fundamental type of heterocycle due to the widespread occurrence of this ring system in bioactive compounds, advanced materials, and natural products (Org. Biomol. Chem. 2023, 21, 8956). On the other hand, helicenes are polycyclic aromatic compounds containing angularly fused rings that display enhanced chiroptical properties arising from their deviation from planarity (Chem. Rev., 2021, 121, 2373; Org. Biomol. Chem. 2025, 23, 7501). Consequently, the development of an efficient protocol for the synthesis of compounds incorporating both the carbazole-based helicene and imidazole frameworks is highly desirable for the pursuit of blue emitters and pharmacologically active heterocycles. In our research group we have described the metal-catalyzed synthesis of iodocarbazole derivatives from an indolic butynol. (ACS Catal., 2015, 5, 3417; Adv. Synth. Catal., 2021, 363, 1449). Based on this outcome, we envisioned a strategy to construct an imidazole scaffold embedded within a carbazole-based azahelicene. To our delight, the reaction of the indolic terminal alkyne with 3-iodo-2-formylindoles under conventional Sonogashira conditions did not stop at the internal alkyne stage, instead yielding chromatographically separable iodocarbazole intermediates. These fused tricycles are functionalized with a formylindole moiety, which should be amenable to further transformations. Remarkably, after several trials, we found that the copper-catalyzed treatment of the iodocarbazoles in the presence of 1,2-diamines afforded the target imidazole-containing helical carbazoles.