Relay Catalysis of Bismuth Trichloride and Byproduct Hydrogen Bromide Enables the Synthesis of Carbazole and Benzo[α]carbazoles from Indoles and α-Bromoacetaldehyde Acetals

Article abstract of DOI:10.1002/adsc.201800669

Benzo[α]carbazoles were synthesized from 2-phenylindoles and α-bromoacetaldehyde using bismuth trichloride as a catalyst. The reaction was triggered by a bismuth trichloride-catalyzed Friedel-Crafts alkylation of these two precursors, which provided a tryptaldehyde intermediate that underwent intramolecular olefination to form the final product. Interestingly, the HBr byproduct generated in the upstream step of the reaction catalyzed the following downstream reaction steps, thus creating a byproduct-participated relay catalytic process. Motivated by this mechanism, we developed a three-component reaction of indole, α-bromoacetaldehyde acetal, and 1,3-dicarbonyl compounds. This study offers a straightforward method for synthesizing substituted carbazoles. (Figure presented.).