An Efficient Reusable Mesoporous Solid-Based Pd Catalyst for Selective C2 Arylation of Indoles in Water

The completely C2-selective arylation of N-methylindole was achieved by using a reusable solid Pd catalyst and water as a solvent without any other ligands or additives or exclusion of air. The catalysts possess well-dispersed Pd nanoparticles (similar to 1.5 nm), N-containing functional groups (similar to 11 wt %), and uniform mesopores (similar to 5 nm). The turnover frequency (TOF) is calculated to be 82 h(-1). Arylations of both N-H indoles and N-protected indoles with diaryliodonium salts were achieved to give the desired products in high to excellent isolated yields in water. Mercapto-functionalized silica as a selective trapping agent, together with hot filtration, confirms the undetected leaching of Pd. The catalyst samples can be reused at least eight times without any additional activation treatment. The catalytic performance of the Pd catalyst is also compared with the performance of other mesoporous Pd catalysts on silica, carbon, and resins. The catalytic activity and stability of the present catalyst may be related to the well dispersed Pd nanoparticles, the electron-rich environment stabilized by the N-containing functional groups, and the large, uniform mesopores.