Sc(OTf)3-Catalyzed Dearomatization of Indoles for the Synthesis of 3,3′-Bisindoles

A new Sc(OTf)(3)-catalyzed dearomatization of indole-2-methanols is reported. By using the characteristics of umpolung of the preformed indole-2-methanols in the presence of acid catalysts, its nucleophilic center at 3-position of indole ring could be transformed into the electrophilic site, thereby realizing the coupling reaction with another molecule indoles, which led to the synthesis of a series of 3,3'-bisindoles with exocyclic double bond unit in moderate to excellent yields. Among them, the strong induction and large steric effects of N-sulfonyl group are the key to the dearomatization of indole ring from indole-2-methanols. Based on the experimental results and literature reports, the possible reaction mechanism is proposed, which involves the dehydroxylation and nucleophilic addition of indole-2-methanol derivatives. In addition, this protocol features high functional group compatibility, mild conditions and simple operation.