Anion-Controlled Regioselectivity in Halonium-Mediated Arylations: A Kinetic and Mechanistic Study

Hypervalent halonium salts are versatile arylating agents. While the reactivity of iodonium salts is well-studied, the reactivity of bromonium and chloronium salts remains underexplored. Here, we investigate the interplay between σ-hole coordination and aryl transfer in cyclic chloronium and bromonium salts, demonstrating how external anions modulate their reactivity and the regioselectivity of their transformations. Kinetic studies reveal that fluoride and chloride additives activate chloronium and bromonium salts. DFT calculations support a concerted nucleophilic substitution mechanism, where anion coordination influences transition-state energetics. Notably, fluoride anions completely invert selectivity in chloronium-mediated arylations, favoring meta-substitution via a proposed aryne intermediate. These findings expand the synthetic utility of halonium salts, offering new strategies for regiocontrolled functionalization.