alpha-Trifluoromethylstyrenes with a formimidoyl, an N-hydroxyformimidoyl, or a tosylamidomethyl group at the ortho position undergo intramolecular addition or S(N)2'-type substitution at the trifluoromethylvinyl moiety, leading to a variety of isoquinoline derivatives bearing 4-trifluoromethyl, 4-difluoromethyl, and 4-difluoromethylene groups.