AgBF₄-Induced Site-Selective Synthesis of 4-Sulfonylindoles in Au-Catalyzed Cyclization-Sulfonyl Migration Reactions

The reactions of ortho-alkynyl-N-arenesulfonylanilines in the presence of SPhosAuCl (2 mol %) and AgBF₄ (6 mol %) produced the corresponding 4-sulfonylindoles in good yields with satisfactory site-selectivity at the C4-position of the indole ring. This reaction proceeded via cyclization followed by sequential [1,2]-rearrangement of the sulfonyl group. The selective migration of the sulfonyl group to the C4-position was induced by using an excess amount of AgBF₄ relative to the gold catalyst to form a dinuclear Au─Ag complex to decelerate the migration to C3-position. Furthermore, the tetrafluoroborate anion with its suitable gold affinity and hydrogen bonding characteristics, plays a crucial role in enabling selective migration to the C4-position.