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 (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).

. (a) Catalytic applications of the versatile Pd heterogeneous catalyst into various green chemical transformation. (b) Novel, low-cost and energy-efficient environmentally friendly approach for the preparation of rice husk (RH) based silica (RH U -Si and RH P -Si) through a simple process involving grinding, microwave assisted extraction, washing and calcination. (c) Synthetic strategy for further modification of the RH-derived silica (RH U -Si and RH P -Si) into a palladium based heterogenous catalyst ((RH P -Si-NH 2 -Pd(II), RH U -Si-NH 2 -Pd(II), RH P -Si-NH 2 -Pd(0) and RH U -Si-NH 2 -Pd(0)).

Results and Discussion
Initially, RH derived silica was obtained through a previously reported facile and novel approach (Fig. 1b) 31 . RH was grinded, underwent microwave-assisted extraction and then further purification (washing and calcination) to afford the silica products. Two type of silica materials were prepared, one unrefined biosilicate (RH U -Si, 90% purity, grey color) and the second one, pure biosilicate (RH P -Si, >98% purity, white color). Subsequently, the homogenous palladium catalyst was incorporated onto the RH U -Si and RH P -Si via conjugation with amino groups through a silylation step (RH U -Si-NH 2 and RH p -Si-NH 2 ), followed by treatment with the palladium precursor (Li 2 PdCl 4 ) providing the heterogenous palladium catalysts (RH U -Si-NH 2 -Pd(II) and RH p -Si-NH 2 -Pd(II)). These two heterogenous palladium (II) catalysts were simply converted to the corresponding RH U -Si-NH 2 -Pd(0) and RH p -Si-NH 2 -Pd(0) by a reduction step (Fig. 1c). The fabricated materials were thoroughly characterized (Fig. 2). Firstly, the porosity and pore size of the RH based materials (RH P -Si, RH U -Si-NH 2 -Pd(II)/Pd(0) and RH P -Si-NH 2 -Pd(II)/Pd(0)) were determined by nitrogen physisorption experiments (Figs. 2a and S3−10). Unmodified RH P -Si displayed a Brunauer-Emmet-Teller (BET) with a surface area of ca. 352 m 2 /g −1 and with mesoporous characteristics (8.0 nm) and pore volume of 0.56 cm 3 /g −1 (Fig. 2b). However, both surface area and pore volume decreased after palladium incorporation, whilst the pore size distribution changed to macropores (e.g. RH U -Si-NH 2 -Pd(II) = 163 m 2 /g −1 , 89.4 nm and 0.36 cm 3 /g −1 ). Such is a normal behaviour observed in our previous reports, indicating the binding and incorporation of palladium into the material 23,27,32 . The elemental analysis confirmed the palladium content of the various heterogenous catalysts as 20.30 wt% for RH U -Si-NH 2 -Pd(II), RH P -Si-NH 2 -Pd(II) = 19.11 wt%, RH U -Si-NH 2 -Pd(0) = 19.05 wt% and RH P -Si-NH 2 -Pd(0) = 16.90 wt%, respectively.
Moreover, the surface area decreased and pore size increased when the Pd(II) was reduced to Pd(0) (e.g. RH P -Si-NH 2 -Pd(II) = 153 m 2 /g −1 and 63.6 nm; RH P -Si-NH 2 -Pd(0) = 144 m 2 /g −1 and 70.5 nm). X-ray diffraction (XRD) patterns of the RH P -Si displayed a strong broad peak at about 22° 2θ angle indicating its amorphous structure (Fig. 2c). This is also consistent with previous reports on amorphous silica derived from rice husk 33 . The amorphous characteristics of the RH P -Si could also be confirmed by Scanning electron microscopy (SEM) micrographs ( Figure S11). Additionally, Transmission electron microscopy (TEM) of the RH P -Si at different magnification are presented in Fig. 2d, which further demonstrated the amorphous and porous structure. After palladium incorporation, a clear difference could be observed in the TEM, where well dispersed and spherical palladium nanoparticles could be visualized (Fig. 2e-h).
Upon characterization, the catalytic performance of the designed heterogenous palladium catalysts were further evaluated. The Suzuki cross-coupling reaction was selected as model reaction. The initial reaction between iodobenzene 1a and phenylboronic acid 2a, in the presence of potassium carbonate (K 2 CO 3 ) 29 and catalytic amounts of RH U -Si-NH 2 -Pd(II) (5 mol%) in water (H 2 O) provided the corresponding diphenyl product 3a in 54% isolated yield after 3 h at 100 °C (Table 1, entry 1). Further screening of the solvents showed that dimethylformamide (DMF) displayed the best efficiency among the investigated (toluene (85%) and ethanol (EtOH) (95%)) www.nature.com/scientificreports www.nature.com/scientificreports/ to afford 3a in 98% yield (Table 1, entries 2-4). However, enduring our vision in designing an eco-friendly process, we decided to mix H 2 O and ethanol (EtOH) (1:1) as reaction medium, and to our delight, the reaction provided the product 3a in 99% yield (Table 1, entry 5). This improvement could be due to the improved solubility of the substrates with the addition of ethanol than having solely H 2 O as solvent. It is well-known that silicate is not soluble or show very low solubility in water 34 . No differences in catalytic activity between the various palladium catalysts were observed (Table 1, entries 5-8), and moreover, a decrease in catalyst amount (from 5.0 to 0.25 mol%) did not negatively impact the reaction efficiency ( Table 1, entries 8-11). However, when the amount of the Pd-catalyst was decreased to 0.10 mol% the efficiency was decreased and provided the product 3a with 81% yield (Table 1, entry 12). Enduring the fine-tuning of the reaction, a decrease in reaction temperature and time were further investigated, where 1 h reaction time at 70 °C worked well (Table 1, entries 13-15). However, further decrease of the temperature to 50 °C decreased the efficiency of the reaction (72% yield, Table 1, entry 16). Delighted by these findings, the substrate scope of the reaction was further explored, which showed that the reaction tolerated a wide range of functionalities with both aryl iodide 1a and aryl bromide 1b and various boronic acids 2 (Table 2). Nevertheless, a slight decrease in yields was observed for the reaction between bromobenzene 1b and phenylboronic acid 2a and between the iodobenzene 1a and 4-(trifluoromethyl) phenylboronic acid 2e providing products 3a and 3 f (85 and 88% yields, Table 2, entries 2 and 8). Overall, the coupling reaction showed high efficiency and provided the coupling products in high yields (up to 99%). Since the recyclability of a heterogenous catalyst is an eminence feature both in the economic and environmental aspects, the recyclability and reusability of the devised palladium heterogenous catalysts were further studied. The heterogenous systems could be recycled for 6 consecutive cycles without losing any efficiency (Fig. 3a). Notable all the four heterogenous palladium catalysts (RH U -Si-NH 2 -Pd(II), RH P -Si-NH 2 -Pd(II only), RH U -Si-NH 2 -Pd(0), RH P -Si-NH 2 -Pd(0) were recycled at least one cycle. However RH P -Si-NH 2 -Pd(0) catalyst was selected for further recycling study. Moreover, no leaching was observed as determined by elemental analysis performed on the filtrate after the hot filtration and after the completion of the reaction.
To further broaden the application of the synthesized Pd catalyst, the selected optimum system (RH P -Si-NH 2 -Pd(0) was employed in the aerobic oxidation of cinnamyl alcohol 4 to the corresponding aldehyde as model reaction 32 . The reaction proceeded efficiently to afford cinnamic aldehyde 5 (>99%) as only product after 48 h, in toluene at 70 °C, employing 5 mol% of the catalyst in the presence of oxygen gas (Fig. 3b). In addition to the cross-coupling and oxidation reactions, the reaction portfolio was further expanded for the application of the heterogenous palladium catalyst in amine/palladium co-catalyzed carbocyclization reactions (Fig. 3c). The reaction was conducted between cinnamic aldehyde 5 and propargylcyanomalonate 6 in the presence of catalytic amount of palladium heterogenous catalyst (RH U -Si-NH 2 -Pd(II) and RH P -Si-NH 2 -Pd(II) (5 mol%)) and the chiral amine catalyst 7 (20 mol%). The chemical transformation proceeds via formation of the enaminyne I intermediate 35 , and subsequent stereoselectivity nucleophilic enamine addition provided the carbocycle 8, in high yields (up to 82%) and diastereoselectivities (up to 27:1 dr determined through 1 H-NMR analysis and the e.r. where not determined. However, the obtained optical rotation ([α] D 25 = −6.31 (c = 1.0 CHCl 3 )) resembles to the previously reported ee of >97.5:2.5 er) (Fig. 3c) 23  www.nature.com/scientificreports www.nature.com/scientificreports/ conclusion A highly efficient Pd-heterogeneous catalyst from the biomass-based Rice husk waste was synthesized. The novel preparation approach integrating the valorization of renewable starting materials represents a green, facile and simple eco-friendly method for catalyst design. The devised heterogenous palladium catalysts were proved to be highly versatile, being successfully employed in Suzuki-Miyaura cross-couplings (high product yields, up to 99%, wide range of functionalities), the aerobic oxidation of cinnamoyl alcohol and the amine catalyzed stereoselective www.nature.com/scientificreports www.nature.com/scientificreports/ carbocylization reaction (cyclopentene derivatives obtained in high yields and stereoselectivities, up to 82%, 27:2 dr). Additionally, the Pd system was highly recyclable and could be reused after simple centrifugation in 6 cycles without any loss of efficiency. The disclosed protocol represents a green and sustainable chemical approach that may find relevant and suitable future applications in additional chemical transformations.

Methods
General and materials. Chemicals and solvents were either purchased puriss p. a. from commercial suppliers or were purified by standard techniques. Commercial reagents were used as purchased without any further purification. Aluminum sheet silica gel plates (Fluka 60 F254) were used for thin-layer chromatography (TLC), and the compounds were visualized by irradiation with UV light (254 nm) or by treatment with a solution of phosphomolybdic acid (25 g), Ce(SO 4 ) 2 ·H 2 O (10 g), conc. H 2 SO 4 (60 mL), and H 2 O (940 mL), followed by heating. 1 H NMR spectra were recorded on a Bruker Avance (500 MHz or 400 MHz) spectrometer. Chemical shifts are reported in ppm from tetramethylsilane with the solvent resonance resulting from incomplete deuterium incorporation as the internal standard (CDCl 3 : δ 7.26 ppm). Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, q = quartet, br = broad, m = multiplet), and coupling constants (Hz), integration. 13 C NMR spectra were recorded on a Bruker Avance (125.8 MHz or 100 MHz) spectrometer with complete proton decoupling. Chemical shifts are reported in ppm from tetramethylsilane with the solvent resonance as the internal standard (CDCl 3 : δ 77.16 ppm). Gas sorption measurements were carried out on a Micrometrics ASAP2020 analyzer and recorded at 77 K. N 2 adsorption measurements on the RH-Si were performed at 77 K by using a Micromeritics ASAP 2000 volumetric adsorption analyzer. The samples were degassed for 24 h at 130 °C under vacuum (P o < 10 −2 Pa) and subsequently analyzed. Surface area of the RH-Si was calculated according to the Brunauer-Emmet-Teller (BET) equation. Mean pore size diameter and pore volumes were obtained from porosimetry data by using Barret-Joyner-Halenda (BJH) method. Wide-angle X-ray diffraction experiments were recorded on a Pan-Analytic/Philips X'pert MRD diffractometer (40 kV, 30 mA) with CuK α (λ = 0.15418 nm) radiation. Scans were performed over a 2θ = 10-80 °C at step size of 0.0188 with a counting time per step of 5 s. TEM image of the RH-Si was obtained on JEM 2010F (JEOL) and Phillips Analytical FEI Tecnai 30 microscopes. All other TEM experiments were carried out on a 200 kV JEOL JEM-2100F field-emission electron microscope equipped with an ultra-high-resolution pole piece. A Gatan ultra-high tilt tomography holder was used. TEM samples were prepared by crushing, and the tomography data was acquired between −60° and +60° with 1° increments. Each image per tilt angle was recorded with a Gatan Ultrascan 1000 camera. The data acquisition was assisted by a commercial tomography packed, TEMography (version 2.15.07) developed by JEOL System Technology Co. Ltd. SEM micrographs of the RH-Si was recorded in a JEOL-SEM JSM-6610 LV scanning electron microscope in backscattered electron mode at 3/15 kV. Elemental analyses were carried out by Medac LTD Analytical and chemical consultancy services (United Kingdom) by ICP-OES.
preparation of the RH p -Si-pd-heterogeneous catalysts Rice husk silica preparation (RH p -Si). The particle size of the rice husk (RH) was reduced by grinding in a Retsch-PM-100 planetary ball mill using a 125 mL reaction chamber end eighteen stainless steel balls (10 nm diameter, 5 g weight). Milling was conducted at 350 rpm for 10 min. The obtained silica from RH was treated to microwave assisted extraction in ETHOS-ONE. The RH was then refluxed in a 0.29 M HCl solution at 300 W for 30 min. The silica solution was www.nature.com/scientificreports www.nature.com/scientificreports/ cooled to room temperature, filtered and washed with distilled water and then dried in oven at 100 °C for 24 h. The resulting solid was calcined in a furnace at 550 °C for 4 h to obtain pure silica (RH P -Si).
General preparation of the RH p -Si-nH 2 . The preparation of the RH P -Si-NH 2 -Pd-catalyst started with the amino functionalization of the RH P -Si. Dry toluene (20 mL) was added to the RH P -Si (1.38 g, 22.2 mmol mg, 1.0 equiv.), followed by addition of a solution of 3-aminopropyltrimethoxy silane (7.8 mL, 44.4 mmol, 2.0 equiv.) in toluene (10 mL). The mixture was stirred under nitrogen for 10 minutes, and then refluxed for 24 h. The mixture was allowed to cool to room temperature and the solid was collected by filtration and washed several times with toluene, ethanol, acetone and dichloromethane to remove any unreacted precursor. The material was further dried under vacuum giving RH P -Si-NH 2 (1.405 g).
General preparation of the RH p -Si-nH 2 -pd(ii). The amino-functionalized RH P -Si-NH 2 (1.0 g) was suspended in deionized water (15 mL) and the solution was pH-adjusted to pH 9, by the use of 0.1 N LiOH. In parallel Li 2 PdCl 4 (600 mg) was diluted in deionized water (10 mL) and the solution was pH-adjusted to pH 9, by the use of 0.1 N LiOH. This solution was transferred to the flask containing RH P -Si-NH 2 solution. The reaction was stirred at room temperature for 24 h. Subsequently, the suspension was then centrifuged, and the solid material was further washed with water (3 × 40 mL) and acetone (3 × 40 mL) and dried overnight under vacuum to afford RH P -Si-NH 2 -Pd(II) (1.408 g). Elemental analysis on the Pd content were 20.30 wt.% for RH U -Si-NH 2 -Pd(II) and 19.11 wt.% for RH P -Si-NH 2 -Pd(II).
General preparation of the RH p -Si-nH 2 -Pd(0). RH P -Si-NH 2 -Pd(II) (500 mg) was suspended in deionized water (15 mL), followed by slow addition of a solution of NaBH 4 (310.2 mg, 8.2 mmol) in water (5 mL) at room temperature. The reaction was stirred at room temperature for 30 minutes. Afterwards the solution was centrifuged and the solid diluted with water (3×40 mL), acetone (3×40 mL) and centrifuged. The material was then dried overnight under vacuum providing RH P -Si-NH 2 -Pd(0). Elemental analysis on Pd content were 19.05 wt.% for the RH U -Si-NH 2 -Pd(0) and 16.90 wt.% for RH P -Si-NH 2 -Pd(0), respectively.
General procedure for Pd-catalyst catalyzed Suzuki-Miyaura reaction (Table 1). A microwave vial equipped with a magnetic stir bar was charged with the Pd-catalyst (mol%), phenylboronic acid 2a (146.4 mg, 1.2 mmol, 1.2 equiv.), K 2 CO 3 (414.6 mg, 3.0 mmol, 3.0 equiv.), followed by addition of solvent (3.0 mL). Subsequently, iodobenzene 1a (204.0 mg, 1.0 mmol, 1.0 equiv.) was added and the reaction mixture heated and run for the temperature and time stated at Table 1. Next, the reaction mixture was either centrifuged and the solid diluted with acetone (3×10 mL) and centrifuged and then concentrated before purification or directly subjected to flash chromatography on silica (petroleum ether/EtOAc 100−90%) affording the pure product 3a.
Typical procedure for the hot-filtration test. A microwave vial equipped with a magnetic stir bar was charged with pure RH P -Si-NH 2 -Pd(0) catalyst (1.6 mg, 0.25 mol%), phenylboronic acid 2a (146.4 mg, 1.2 mmol, 1.2 equiv.), K 2 CO 3 (414.6 mg, 3.0 mmol, 3.0 equiv.), followed by addition of solvent (3.0 mL). Subsequently, iodobenzene 1a (204.0 mg, 1.0 mmol, 1.0 equiv.) was added and the reaction mixture heated to 70 °C. RH P -Si-NH 2 -Pd(0) catalyst was removed through centrifugation after 30% conversion was reached and the solid free filtrate was allowed to stir for 24 h reaction conditions. Analysis of the reaction mixture showed that no further conversion of the substrate had occurred. procedure for the catalytic aerobic oxidation. To a suspension of RH P -Si-NH 2 -Pd(0) (5 mol% Pd to 4, 7.6 mg) in toluene (0.5 mL) placed in an oven-dried microwave vial equipped with a magnetic stir bar was charged with cinnamyl alcohol 4 (32.3 mg, 0.24 mmol, 1.2 equiv.). The vial was capped, evacuated and an O 2 -balloon was