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

Abstract

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). Rice husk (RH) is a major waste product from the rice industry with high content of silica. This abundant material is a sustainable and cheap raw material and therefore within the context of sustainability 1. De facto, rice husk ash (RHA) is known to contain 94% silica 2. Here, 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. In general, the production of silica from RH is energy intensive i.e. obtained by burning RH in a muffle furnace at high temperature (500, 600 or 700 °C) 5,6. Nevertheless, even though there are some reports on simple and energy-efficient method for the generation of silica from RH, drawbacks such as risk for mineral contamination 7 , several acid and based extraction steps or tedious approaches 8 are encountered 9. In this context, novel methods for the extraction of silica in an eco-friendly, cost-efficient, scalable and facile approach can be highly attractive. This report discloses the preparation of palladium based multifunctional heterogenous catalysts from RH-derived silica as support. There are reports on the immobilization of various metals on RH and their use as heterogeneous catalysts for chemical syntheses 10-16. In this context, Chang and co-workers

A tertiary phosphine oxide ligand-based recyclable system for the Suzuki–Miyaura and Negishi reactions: evidence for pseudo-homogeneous catalysis

“…A report by Afewerki et al described the use of a Pd heterogeneous catalyst derived from rice husk waste which could be reused in up to 6 cycles in Suzuki-Miyaura chemistry. 17 Ye and co-workers reported the recyclability of a Pd-based electride material Y 3 Pd 2 ; which was recycled 20 times. The catalyst recyclability is excellent but 40 mol% of catalyst was required!…”

Palladium and Copper: Advantageous Nanocatalysts for Multi-Step Transformations

“…Córdova and co-workers studied a stereoselective cascade carbocyclization transformation [ 96 ]. PdNPs supported on amine-functionalized rice-husk-derived silica were prepared, giving well-dispersed particles and showing high activity for Suzuki–Miyaura coupling, oxidation of alcohols and carbocyclization reaction with excellent stereoselectivity (27:1) in the presence of a chiral amine ( Scheme 21 ).…”

Silica‐Supported 1st Row Transition Metal (Nano)Catalysts: Synthetic and Catalytic Insight

“…Metal‐catalyzed reactions are critical to achieve value‐added products and specifically heterogeneous catalysts, including supported metal nanoparticles and immobilized catalysts, have shown important performance [1–2] . Immobilized catalysts, in liquid or solid phases, offer several advantages over conventional homogeneous catalysts, including: i) increased thermal and chemical stability: they are less likely to be affected by environmental changes such as temperature, pH, and pressure; ii) increased selectivity: they may be able to catalyze specific reactions more efficiently, in particular those where regio‐ and stereoselectivity should be controlled; iii) increased cost‐effectiveness: there can be used for longer times, so they are more cost‐effective in the long run [3–10] …”