Sustainable and recyclable heterogenous palladium catalysts from rice husk-derived biosilicates for Suzuki-Miyaura cross-couplings, aerobic oxidations and stereoselective cascade carbocyclizations

A new eco-friendly approach for the preparation of sustainable heterogeneous palladium catalysts from rice husk-derived biogenic silica (RH(P)-Si and RH(U)-Si). The designed heterogeneously supported palladium species (RH(P)-Si-NH(2)-Pd and RH(U)-Si-NH(2)-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 (H(2)O: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).