Lewis Base-Catalyzed Amino-Acylation of Arylallenes via C-N Bond Cleavage: Reaction Development and Mechanistic Studies

Lewis base-catalyzed transformations of allenes have received much attention over the last decades. However, this type of reaction has so far been limited to activated allenes bearing an electron-withdrawing group. On the other hand, cleavage of an amide C-N bond to forge other chemical bonds has been widely reported but restricted to low atom economy due to the waste of the amine moiety of amides. We initiated a project of metal-catalyzed amino-acylation of allenes via cleavage of amide C-N bonds. Surprisingly, an amino-acylation of weakly activated aryl allenes was discovered via Lewis base catalysis, providing 2-methyl-3-aroylindole products, privileged structures in drug discovery. This is a unique example of Lewis base catalysis of weakly activated allenes, which was not reported yet. Extensive experimental and computational studies have been conducted to provide insight into the reaction mechanism. The nucleophilic addition of Lewis base catalyst to aryl allene is the rate-limiting step. A challenging [1,3]-proton transfer is realized by nitrogen anion intermediate assisted sequential [1,4]- and [1,6]-proton transfer in the reaction pathway.