Modular Access to Tetrasubstituted Imidazolium Salts through Acid-Catalyzed Addition of Isocyanides to Propargylamines

Propargylamines, prepared through A3‐coupling of primary amines, aldehydes and terminal alkynes, react with isocyanides in an HCl‐catalyzed process to yield tetrasubstituted imidazolium salts. The key step of the mechanism involves the generation of an amidine intermediate, from the isocyanide insertion into the N–H bond of the propargylamine, which in situ evolves by cyclization upon the alkyne moiety. The scope of the process is analyzed and only shows restrictions for aliphatic amines, whereas it is quite general regarding the aldehyde, alkyne and isocyanide inputs. The protocol allows the preparation of a wide array of adducts, tandem one‐pot processes being also feasible. Mechanistic studies using selected substrates have been used to determine the profile of the reaction and some substitution and functional group limitations. Some post‐synthetic transformations of the imidazolium salts have been performed as well. Propargylamines, synthesized through A3‐coupling of primary amines, aldehydes and terminal alkynes, react with isocyanides in an HCl‐catalyzed process to yield tetrasubstituted imidazolium salts. Tandem one‐pot protocols allow direct access to these adducts from the commercially available inputs.