Br2- or HBr-catalyzed synthesis of asymmetric 3,3-di(indolyl)indolin-2-ones

Under the catalysis of 1 mol% of Br (2) or HBr at room temperature, indoles undergo a rapid reaction with 3-hydroxy-3-(indol-3-yl) indolin-2-ones to give asymmetric 3,3-di(indol-3-yl) indolin-2-ones with high efficiency and wide substrate scope. This is a rare example of Br (2) acting as a Lewis acid catalyst. Theoretical calculations suggest that both the catalytic activity of the catalysts and the stability of reaction intermediates are responsible for the high efficiency of this reaction.