Abstract
A new eco-friendly approach for the preparation of sustainable heterogeneous palladium catalysts from rice husk-derived biogenic silica (RHP-Si and RHU-Si). The designed heterogeneously supported palladium species (RHP-Si-NH2-Pd and RHU-Si-NH2-Pd) were fully characterized and successfully employed as catalysts for various chemical transformations (C–C bond-forming reactions, aerobic oxidations and carbocyclizations). Suzuki-Miyaura transformations were highly efficient in a green solvent system (H2O:EtOH (1:1) with excellent recyclability, providing the cross-coupling products with a wide range of functionalities in high isolated yields (up to 99%). Palladium species (Pd(0)-nanoparticles or Pd(II)) were also efficient catalysts in the green aerobic oxidation of an allylic alcohol and a co-catalytic stereoselective cascade carbocyclization transformation. In the latter case, a quaternary stereocenter was formed with excellent stereoselectivity (up to 27:1 dr).
Highlights
A new eco-friendly approach for the preparation of sustainable heterogeneous palladium catalysts from rice husk-derived biogenic silica (RHP-Si and RHU-Si)
Based on the above challenges and our previous experience in developing palladium based heterogenous catalysts for various green chemical transformations[23,24,25,26,27,28,29], we designed an eco-friendly approach for the preparation of heterogeneous and multifunctional palladium catalysts suitable for a wide spectrum of chemical transformations
The activity and versatility of the designed catalysts was demonstrated in the Suzuki-Miyaura cross-coupling reaction, aerobic oxidation approach of allylic alcohols and stereoselective cascade carbocyclization reactions (Fig. 1a)[30]
Summary
A new eco-friendly approach for the preparation of sustainable heterogeneous palladium catalysts from rice husk-derived biogenic silica (RHP-Si and RHU-Si). Silica is a very important component for various industrial and biomedical applications[3], being high surface area silicates highly desirable and good candidates as catalyst support[4] For these reasons the development of efficient methods for the preparation of silica and silica based materials are of high interest. Esmaeilpour et al disclosed the preparation of dendrimer-encapsulated Pd(0) nanoparticles immobilized on nanosilica (nSiO2-dendrimer-Pd(0)) and their application in the Sonogashira-Hagihara reactions in the absence of any copper and phosphorous ligand in water under aerobic conditions[21] Despite these notable advances, there is a need to find novel, facile and green approaches to obtain high purity and surface area silica rice husk (RH-Si) for further catalyst design into multifunctional, recyclable and highly efficient heterogenous catalytic systems. The activity and versatility of the designed catalysts was demonstrated in the Suzuki-Miyaura cross-coupling reaction, aerobic oxidation approach of allylic alcohols and stereoselective cascade carbocyclization reactions (Fig. 1a)[30]
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