Rhenium-Catalyzed Formation of Indene Frameworks via C−H Bond Activation:  3+2 Annulation of Aromatic Aldimines and Acetylenes

A rhenium complex, ReBr(CO)3(thf)2, catalyzes the reaction of an aromatic aldimine with an acetylene to give an indene derivative in a quantitative yield. The reaction proceeds via C-H bond activation, insertion of the acetylene, intramolecular nucleophilic cyclization, and reductive elimination. In contrast to ruthenium and rhodium catalysts, which are usually employed in this type of reaction, the rhenium catalyst promotes the intramolecular nucleophilic cyclization of the alkenylmetal species generated by insertion of the acetylene.