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

Abstract 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.