Halogen bond catalysis of the [4+2] cycloaddition reaction of 2-alkenylindoles: catalytic modes and stereoselectivity

I⋯π halogen bond catalysis on the [4+2] cycloaddition reaction was investigated. There are two modes either on the ethenyl or the five-membered ring of 2-alkenylindole. Moreover, the stereoselectivity of the endo and exo pathways was compared.