A Hypervalent Cyclic Dibenzoiodolium Salt as a Halogen‐Bond‐Donor Catalyst for the [4+2] Cycloaddition of 2‐Alkenylindoles

A stable, hypervalent cyclic dibenzoiodolium salt acted as a strong halogen bonding (XB)‐donor catalyst for [4+2] cycloaddition of 2‐alkenylindoles, and not as an oxidizing agent. The cross‐[4+2] cycloaddition of 2‐vinylindoles with 2‐alkenylindoles was catalyzed smoothly by the hypervalent cyclic dibenzoiodolium triflate catalyst to give the tetrahydrocarbazoles in up to 99 % yield with 17 : 1 diastereoselectivity. The hypervalent cyclic dibenzoiodolium salt was also applicable to the Povarov reaction of 2‐vinylindole with N‐p‐methoxyphenyl (PMP) imine to give the indolyl‐tetrahydroquinoline in 83 % yield. Cat not ox: A trivalent cyclic dibenzoiodolium salt that does not act as an oxidizing agent becomes a halogen bonding (XB)‐donor catalyst. Using the I(III)‐XB catalyst (see scheme), a cross‐[4+2] cycloaddition of 2‐vinylindoles with 2‐alkenylindoles was furnished to give 3‐indolyl‐substituted tetrahydrocarbazoles in highly diastereoselective manner.