Polymer‐Supported Palladium(II) Carbene Complexes: Catalytic Activity, Recyclability, and Selectivity in C−H Acetoxylation of Arenes

Abstract

Heterogeneous catalysts for selective oxidation of C-H bonds were synthesized by co-polymerization of new N-heterocyclic carbene-palladium(II) (NHC-Pd ) monomers with divinylbenzene. The polymer-supported NHC-Pd -catalysed undirected C-H acetoxylation of simple and methylated arenes as well as polyarenes, with similar or superior efficiency compared to their homogeneous analogues. In particular, the regioselectivity has been improved in the acetoxylation of biphenyl and naphthalene compared to the best homogeneous catalysts. The new polymer-supported catalysts maintain the original oxidation state of Pd after repeated catalytic reactions, and exhibit no significant leaching of palladium. In addition, the new catalysts have been successfully recovered and reused without loss of activity over several cycles of reactions.

Confinement of Metal–N‐Heterocyclic Carbene Complexes to Control Reactivity in Catalytic Reactions

“…Hence, ac ovalentlyb uilt-in porous organic polymer based on NHC-Pd complexes was used in aC ÀHa cetoxylation of arenes. [25] In the reaction with naphthalene, the regioselectivity with the polymerw as found to be higher than that observed with PEPPSI-IPr-Pd. This regioselectivity could be enhanced when exchanging the bipyridine as ligand witha cetonitrile.…”

“…It is therefore logical that the effect of confinement on regioselectivity was studied. Hence, a covalently built‐in porous organic polymer based on NHC–Pd complexes was used in a C−H acetoxylation of arenes . In the reaction with naphthalene, the regioselectivity with the polymer was found to be higher than that observed with PEPPSI‐IPr‐Pd.…”

“…Similarly,c atalytic POPs having covalently built-in Pd II -NHCs were obtained through radicalco-polymerization of divinylbenzene and preformed Pd-NHC complexes having NHC ligands with polymerizablearms ( Figure 8). [25] b) Post-functionalization of azolium-based main-chain organic polymers…”

Palladium Supported on Mesoporous Silica/Graphene Nanohybrid as a Highly Efficient and Reusable Heterogeneous Catalyst for C−H Functionalization

“…Accordingly, using the heterogeneous catalysts own promising advantages such as simplifying the reaction workup, purification, recyclability, high stability and low cost . However, only a few studies have involved the application of supported palladium as catalysts in the heterogeneous C−H functionalization processes . Wendt and coworkers prepared the polymer‐supported NHC‐Pd II ‐which proved to be effective in catalyzing the undirected C−H acetoxylation of simple and methylated arenes as well as polyarenes .…”

“…However, only a few studies have involved the application of supported palladium as catalysts in the heterogeneous C−H functionalization processes . Wendt and coworkers prepared the polymer‐supported NHC‐Pd II ‐which proved to be effective in catalyzing the undirected C−H acetoxylation of simple and methylated arenes as well as polyarenes . Cao and coworkers applied highly dispersed Pd nanoparticles encapsulated in the mesoporous metal–organic framework MIL‐101 as a catalyst for the direct C‐2 arylation of various indoles with aryl boronic acids .…”