C(sp³)−H Arylation Promoted by a Heterogeneous Palladium-N-Heterocyclic Carbene Complex in Batch and Continuous Flow

A heterogeneous reusable palladium(II)-bis(N-heterocyclic carbene) catalyst was prepared and shown to catalyze the intramolecular C(sp3)-H activation/cyclization of N-alkyl-2-bromoanilines furnishing indolines. This new catalytic system was based on a bis-imidazolium ligand immobilized on a spaced cross-linked polystyrene support. The iodide ligands on the catalyst played a central role in the efficiency of the process occurring through a "release and catch" mechanism. The heterogeneous nature of the catalyst was further exploited in the design of a continuous-flow protocol that allowed a more efficient recovery and reuse of the catalyst, as well as a very fast and safe procedure.