Ru(II)-Catalyzed Aldehyde-Directed Cascade Annulation of Aromatic Aldehydes With Acrylates to Access Indenes

The metal-catalyzed directing-group-assisted cascade annulation via C─H bond activation has provided a highly efficient and atom-economical strategy to access cyclic compounds. In this context, a ruthenium-catalyzed [3 + 2] cascade annulation of aromatic aldehydes with acrylates has been developed for the construction of indene frame in one pot under aerobic conditions. This efficient transformation proceeds through weakly coordinating aldehyde-assisted C─H bond activation and is featured by being free of nitrogen-bearing transient directing groups, operational simplicity, low cost, and commercial availability of starting materials. The protocol involves a sequence reaction of aldehyde-directed C─H activation, coordination between acrylates and ruthenium, migratory insertion, protonation, and intramolecular Aldol condensation.