Catalysis Based on C-I⋅⋅⋅π Halogen Bonds: Electrophilic Activation of 2-Alkenylindoles by Cationic Halogen-Bond Donors for [4+2] Cycloadditions.

Homo- and cross-[4+2] cycloadditions of 2-alkenylindoles, catalyzed by cationic halogen-bond donors, were developed. Under mild reaction conditions, 3-indolyl-substituted tetrahydrocarbazole derivatives were obtained in good to excellent yields. Experimental and quantum calculation studies revealed that the electrophilic activation of 2-alkenylindoles was achieved by C-I⋅⋅⋅π halogen bonds.