On the Absolute Configurational Stability of Bromonium and Chloronium Ions

摘要

Electrophilic halofunctionalizations of olefins, in which electrophilicnhalonium ions are generated from olefins and opened by nucleophiles,1nare among the oldest and simplest organic transformations. Despitenthe versatility of these reactions,2 only a few examples of practicalnenantioselective variants have been reported, even fewer of whichnrequire substoichiometric amounts of chiral promoter.3 Several recentnsyntheses of chlorosulfolipids in racemic form,4 as well as the synthesisnof (-)-napyradiomycin A1, which featured a stoichiometric enantioselectivenchlorination,3c serve to highlight the need for practical,ncatalytic enantioselective methods for halofunctionalization of olefins.nThe paucity of such methods can be ascribed in part to a lack ofnunderstanding of the factors that influence the configurational stabilitynof the intermediate halonium ions.