Enhancing stability by trapping palladium inside N-heterocyclic carbene-functionalized hypercrosslinked polymers for heterogeneous C-C bond formations

Abstract

Catalyst deactivation caused by the aggregation of active metal species in the reaction process poses great challenges for practical applications of supported metal catalysts in solid-liquid catalysis. Herein, we develop a hypercrosslinked polymer integrated with N-heterocyclic carbene (NHC) as bifunctional support to stabilize palladium in heterogeneous C-C bond formations. This polymer supported palladium catalyst exhibits excellent stability in the one-pot fluorocarbonylation of indoles to four kinds of valuable indole-derived carbonyl compounds in cascade or sequential manner, as well as the representative Suzuki-Miyaura coupling reaction. Investigations on stabilizing effect disclose that this catalyst displays a molecular fence effect in which the coordination of NHC sites and confinement of polymer skeleton contribute together to stabilize the active palladium species in the reaction process. This work provides new insight into the development of supported metal catalysts with high stability and will also boost their efficient applications in advanced synthesis.

Fig. 1. Schematic illustration of catalyst evolution.

The “Cocktail” type nature and evolution of the catalytic species in C–C bond formation reactions catalyzed by supported Pd catalysts.

Fig. 2. Structure and characteristics of KAP-Pd-PEPPSI.

a Structures of KAP-Pd-PEPPSI with different degrees of crosslinking. b Simulation model for hypercrosslinked KAP-Pd-PEPPSI-x (x = 2, 3). c SEM image of KAP-Pd-PEPPSI-2. d SEM image of KAP-Pd-PEPPSI-2 after swelling in DMF for 24 h at 80 °C. e STEM image of KAP-Pd-PEPPSI-2. f, g EDS mapping images for N and Pd in KAP-Pd-PEPPSI-2.

Fig. 3. Recyclability of the selected catalyst.

a Recyclability of the supported Pd complex precatalysts in fluorocarbonylation of indole. Reaction conditions: 1a (0.25 mmol), CsF (1 mmol), catalyst (5 mol%), I₂ (0.5 mmol), base (0.5 mmol), DMF (2 mL), 80 °C, 24 h, 2.0 MPa. b Pd XPS spectra and c TEM images of different catalysts after being used for five times. d Structures of the catalysts used here.

Fig. 4. Stabilizing effect of KAP.

a Control experiments to determine the stabilizing effect of NHC under the optimized reaction conditions. b Pd XPS spectra and c TEM images of recycled KAP-2/PdCl₂ and KAP-4/PdCl₂. d Control experiments to determine the stabilizing effect of catalyst structure. Reaction conditions: 1a (0.25 mmol), CsF (1 mmol), Pd catalyst (2.5 mol%), I₂ (0.3 mmol), base (0.5 mmol), DMF (2 mL), 80 °C, 16 h, 2.0 MPa. e TEM images of different KAP-Pd-PEPPSI-x after the 6th cycle.

Fig. 5. Reaction process investigation.

a Kinetic investigation. b Hot filtration test. Under the optimized reaction conditions, the reaction mixture was quickly divided into two parts by centrifugation after reaction for 4 h, and the upper clear liquid proceeded to react for another 20 h in the presence of CsF. c Stabilizing effect of the hypercrosslinked polymer on active Pd species in KAP-Pd-PEPPSI catalyzed fluorocarbonylation of indole.

Fig. 6. Synthetic applications.

The synthesis of representative bioactive molecules via KAP-Pd-PEPPSI catalyzed carbonylations of indoles.