Atroposelective synthesis of 2-arylindoles via chiral phosphoric acid-catalyzed direct amination of indoles

Indole-based atropisomers are a very important class of axially chiral compounds. However, the atroposelective synthesis of axially chiral 2-arylindole remains largely unexplored. In this study, we report the successful synthesis of atropisomeric 2-arylindoles using direct amination of indoles with p-quinonediimines in the presence of chiral phosphoric acid as a catalyst. Quinonediimine acts as an aminating reagent through formal polarity inversion of imine. The malonate group on the 2-aryl of 2-indoles was found to be essential for high enantioselectivity of the products. This could be due to the additional interaction between the ester group and the catalyst, as well as the intramolecular hydrogen bonding. Our findings provide a new strategy for the asymmetric construction of 2-arylindole atropisomers.