Palladium-Catalyzed Cascade Reactions for Annulative π -Extension of Indoles to Carbazoles through C–H Bond Activation

The annulative π-extension (APEX) reactions through C-H bond activation has tremendous potential to access fused aromatic systems from relatively simple aromatic compounds in a single step. This state-of-the-art technique has the ability to streamline the synthesis of functionalized materials useful in material science, biomedical research, agroand pharmaceutical industries. Furthermore, C-H activation strategy does not require prefunctionalization steps, which allows for the late-stage modification of the functional molecule with requisite molecular properties. Owing to their unique photophysical properties, carbazoles are widely used in photovoltaic cells, biomedical imaging, fluorescent polymer, etc. It is also ubiquitously found in many natural products, agrochemicals and privileged medicinal scaffolds. Hence, direct conversion of easily accessible indole to carbazole remains an active research area. In the last decades, significant advancement has been made to access carbazole moiety directly from indole through cascade C-H activation. The underlying mechanism behind this cascade π-extension strategy is the facile electrophilic metalation at the C-3 position of the indole moiety, 1,2- migration and electro cyclization. In this review, we will discuss recent literature reports for the palladium-catalyzed π-extension of indole to carbazole moiety through C-H bond activation.