Chiral Tertiary Sulfonium Salts as Effective Catalysts for Asymmetric Base‐Free Neutral Phase‐Transfer Reactions

Although chiral quaternary ammonium and phosphonium salts are commonly used for asymmetric organocatalysis, the catalytic ability of chiral tertiary sulfonium salts has yet to be demonstrated in asymmetric synthesis. Herein, we show that chiral bifunctional trialkylsulfonium salts catalyze highly enantioselective conjugate additions of 3‐substituted oxindoles to maleimides under base‐free neutral phase‐transfer conditions. Sulfonium catalyst: Whereas chiral quaternary ammonium and phosphonium salts are commonly used for asymmetric organocatalysis, chiral tertiary sulfonium salts have not been employed for such purposes. It is now shown that chiral bifunctional trialkylsulfonium salts catalyze the enantioselective conjugate addition of 3‐substituted oxindoles to maleimides under base‐free neutral phase‐transfer conditions.