Gold- and Silver-Catalyzed Tandem Amination/Ring Expansion of Cyclopropyl Methanols with Sulfonamides as an Expedient Route to Pyrrolidines

An efficient synthetic route to pyrrolidines that relies on AuCl/AgOTf- catalyzed tandem amination/ring expansion of substituted cyclopropyl methanols with sulfonamides is reported herein. The reactions proceed rapidly at 100 degrees C with catalyst loadings as low as 2 mol % and produce the pyrrolidine products in yields of 30-95%. The method was shown to be applicable to a broad range of cyclopropyl methanols, including unactivated ones, and sulfonamide substrates containing electron-withdrawing, electron-donating, and sterically-demanding substituents. The mechanism is suggested to involve activation of the alcohol substrate by the AuCl/AgOTf catalyst, followed by ionization of the starting material, which causes ring opening of the cyclopropane moiety and trapping by the sulfonamide nucleophile. The resultant aminated acyclic intermediate undergoes subsequent intramolecular hydroamination to give the pyrrolidine.