Investigation on the mechanism of water-assisted palladium-catalyzed benzylic C-H amination by N-fluorobenzenesulfonimide.

A combination of computational and experimental methods was carried out to elucidate the mechanism of palladium-catalyzed water-assisted benzylic C-H amination with N-fluorobenzenesulfonimide (NFSI), which involved the oxidative addition of Pd(II) to Pd(IV)-species as a rate-limiting step, followed by water-assisted concerted metalation-deprotonation (CMD) of the Pd(IV) complex and water-assisted reductive elimination (RE) processes, and then a nucleophilic addition process to generate the final product and complete the catalytic cycle. The stability of the Pd(IV) complex could be ascribed to the suitable ligands with strong σ-donors and resistance to decomposition, as well as being sufficiently bulky because the water-clusters assembled the ligands through hydrogen bonds to act as one multidentate ligand. Calculation results suggested that water also plays a crucial role as a proton transferring bridge in water-assisted CMD and RE processes. The corresponding experimental findings substantiate the expectation. Additionally, NFSI was found to act as both the oxidant and the nitrogen source to facilitate the reaction, while the steric effect of the bulky -N(SO2Ph)2 group contributed to circumventing the o-C-H amination. In this reaction, we investigated a novel spiro-cyclopalladation intermediate, formed by the reaction of the Pd(IV) centre with pristine-carbon instead of ortho-carbon, which might be valuable for our understanding and further development of transition metal catalyzed C-H functionalization.