Study and Applications of Tetrasubstituted Hypervalent Selenium–Halogen Species in Catalytic Electrophilic Halogenations

Electrophilic halogenation reactions are highly useful in various areas. N-Haloamides are commonly used as halogen sources because of high stability and commercial availability. In order to activate N-haloamides, Lewis basic chalcogens are commonly used as catalysts to site-isolate the strongly coordinating amide moieties. However, the corresponding trisubstituted chalcogenonium–halogen cationic intermediate is sensitive to moisture and nucleophiles, leading to poor compatibility in some reactions. Herein, we report an efficient catalytic halogenation protocol using phenyl selenium with ortho-substituted carboxylic acid as the catalyst. Mechanistic study suggests that a tetrasubstituted neutral hypervalent Se–halogen species is responsible for the high reactivity. This active intermediate was found to be moisture-stable, and the catalytic system is applicable to a wide range of electrophilic functionalization reactions including haloamidation, intermolecular haloesterification, halocycloetherification, halolactonization, aromatic halogenation, halopolyene cyclization, and selenylation reactions.