C-H Activation Reactions Mediated by Pd Based Heterogeneous Catalysts : Regioselective Formation of C-O and C-X Bonds

Abstract: New classes of N-heterocyclic carbene-palladium(II) complexes containing either a vinyl group in different positions or an anthracene tag in the backbone of the N-heterocycle were synthesized and fully characterized. The vinyl based complexes were copolymerized with divinylbenzene to fabricate robust polymer-supported NHC-Pd(II) systems, while the anthracene based complexes were π-stacked onto the surface of reduced graphene oxide (rGO) producing immobilized catalysts with interesting features. Advanced solid-state characterization techniques focused on the structure and oxidation state of the Pd(II) center of the all the supported complexes and showed the presence of Pd(II) species in the native catalysts without any trace of Pd(0) nanoparticles, confirming the success of the preparation pathways. The supported catalysts were applied as heterogeneous catalysts in C-H activation of arenes using different methodologies. In the undirected C-H acetoxylation of simple, methylated, and poly arenes, the supported catalysts showed a similar or superior performance compared to their homogeneous analogues. In addition to that, the polymer based catalysts demonstrated a high catalytic activity and an excellent regioselectivity, up to 100% in many cases, in directed C-H halogenation of arylpyridines. Further advantages of these heterogeneous systems are their high stability and activity, their easy separation and recovery, meaning that they are recyclable for several runs with no leaching of palladium and no loss of activity. Furthermore, the best polymeric catalyst disclosed an exceptional activity and 100% selectivity towards the mono-halogenated product under continuous flow conditions for up to 6 days, during which the original oxidation state of Pd(II) was maintained.