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.