Development of efficient and chemoselective quasi-heterogeneous catalytic systems for C-C cross-coupling reactions mediated by Pd/Ce based solidsolution catalysts.

Abstract

Construction of carbon-carbon bonds is of pivotal importance in chemistry and their careful assembly can allow complex molecules such as peptides to be created. As a result, the development of carbon-carbon bond forming reactions has turned into an area appreciated by Chemists both in academia and industry. In addition, numerous Nobel prizes in chemistry have been awarded for this area of research. The C-C forming reactions have also facilitated the synthesis of natural products, pharmaceuticals, agrochemicals, conjugated polymers and nanomaterials. In this study, the intention is to synthesize heterogeneous palladium based catalysts that can efficiently catalyze Heck-Mizoroki, Sonogashira and Suzuki- Miyaura C-C cross-coupling reactions. The heterogenisation of these coupling reactions improves their overall efficiency, since it allows for easy work-up and the reusability of the expensive palladium catalysts. In this direction, we used palladium and ceria based solid-solution oxides, PdxCe1-xO2-δ, as heterogeneous nanocrystalline catalysts for the three C-C cross-coupling reactions. The PdxCe1- xO2-δ based solid solution oxides were synthesized in one-step using a ureaassisted solution combustion method. These PdxCe1-xO2-δ solid solution oxides were fully characterized by XRD, ICP-OES, BET, XPS, SEM, EDX, TEM, TGA and Raman spectroscopy. All characterization techniques strongly suggested that Pd2+ was successfully incorporated into the lattice structure of ceria. The effect of reaction conditions on the catalytic properties of the PdxCe1-xO2-δ solid-solution oxide catalysts was studied in detail with the model Heck- Mizoroki, Sonogashira and Suzuki-Miyaura cross-coupling reactions to obtain the optimum reaction conditions for each transformation. Then, a wide range of aryl halides was efficiently coupled to various alkenes or alkynes or boric acids. All the PdxCe1-xO2-δ based solid solution oxide catalysts exhibited high activity and afforded the desired products in good to excellent yields. A careful investigation through a series of suitable tests clearly showed that the C–C crosscoupling is accomplished via a quasi-homogeneous mechanism by leached palladium(0) species. Characterization of the used catalyst suggests that Pd2+ in PdxCe1-xO2-δ is reduced in situ to Pd0 when employed in the cross-coupling reactions. However, the PdxCe1-xO2-δ catalysts were found to be both recoverable and recyclable.