Propargylic substitution reactions with various nucleophilic compounds using efficient and recyclable mesoporous silica spheres embedded with FeCo/graphitic shell nanocrystals
© Jang et al.; licensee Springer. 2015
Received: 18 November 2014
Accepted: 9 December 2014
Published: 23 January 2015
Phosphomolybdic acid (PMA, H3PMo12O40) functioned as a catalyst for reactions of secondary propargylic alcohols and nucleophiles. Highly stable and magnetically recyclable mesoporous silica spheres (MMS) embedded with FeCo-graphitic carbon shell nanocrystals (FeCo/GC@MSS) were fabricated by a modified Stöber process and chemical vapor deposition (CVD) method. The FeCo/GC@MSS were loaded with phosphomolybdic acid (PMA@FeCo/GC@MSS), and their catalytic activity was investigated. Propargylic reactions of 1,3-diphenyl-2-propyn-1-ol with a wide range of nucleophiles bearing activating substituents were catalyzed under mild conditions. It was found that the MMS possess mesoporosities and have enough inner space to load FeCo and phosphomolybdic acid. The FeCo/GC@MSS were found to be chemically stable against acid etching and oxidation. This suggests that the nanocrystals can be used as a support for an acid catalyst. Moreover, the magnetic property of the nanocrystals enabled the facile separation of catalysts from the products.
KeywordsRecyclable Magnetic FeCo/GC Propargylic substitution Phosphomolybdic acid
Electrophilic attack on aromatic carbons is a useful method for functionalizing aromatic compounds [1–3]. Electrophilic aromatic substitution is an organic reaction, in which an electrophile replaces an atom (usually hydrogen) appended to an aromatic system. Among these reactions, the most important are the nitration, halogenation, sulfonation, and acylation reactions of aromatic compounds. Propargylic substitution reactions have been intensively studied in recent years. In these reactions, activated and inactivated propargyl alcohols, propargyl acetates, and/or propargyl esters react with alcohols, thiols, amines, and other molecules that have C-nucleophiles and heteroatom-centered nucleophiles [4, 5].
Heteropoly acids have been the focus of extensive research in organic synthesis due to their high catalytic activity, ease of control, and low cost . Among the various heteropoly acids, phosphomolybdic acid (PMA, H3PMo12O40) is one of the least expensive commercially available solid acids [7–11]. PMA not only enhances the activity of selected catalysts but also shows self-catalytic activity in various organic reactions [11–15]. However, the recovery and reuse of PMA still remains a challenge. Our efforts toward green chemistry have led to the development of new synthetic methodologies.
Iron(III) nitrate nonahydrate (Fe(NO3)3 · 9H2O, 99.99%), cobalt(II) nitrate hexahydrate (Co(NO3)2 · 6H2O, 99.999%), and phosphomolybdic acid (PMA) hydrate (H3PMo12O40 · 24H2O, 99.99%) were purchased from Sigma-Aldrich, St. Louis, MO, USA. Tetraethoxysilane (TEOS, 98%, Sigma-Aldrich, St. Louis, MO, USA) and n-octadecyltrimethoxysilane (C18TMS, 85%) were purchased from TCI, Tokyo Japan. All chemicals were used as received without further purification.
Synthesis of FeCo/GC@MSS, FeCo/GC, and FeCo@MSS nanocrystals
Mesoporous silica spheres composed of mesoporous shell and solid core (approximately 400 nm) were prepared by modifying the Stöber process . We added 1.00 g of MSS with 0.22 g (0.52 mmol) of Fe(NO3)3 · 9H2O and 0.12 g (0.38 mmol) of Co(NO3)2 · 6H2O in 50 mL of methanol and then sonicated it for 1 h. The samples were then dried at 80°C and placed in a tube furnace and heated under H2 flow at 800°C. The samples were then subjected to a methane flow of 500 cm3/min−1 for 5 min. After cooling, the samples were washed with ethanol and collected by centrifugation. To obtain the FeCo/GC nanocrystals, the samples were etched with 15% hydrogen fluoride (HF) in H2O (75%) and ethanol (10%) to dissolve the silica. The procedure for the synthesis of FeCo@MSS was similar to that of FeCo/GC@MSS, except that the methane flow at 800°C for 5 min was replaced with H2 flow at 800°C.
Synthesis of (PMA@FeCo/GC@MSS) nanocrystals
To prepare PMA@FeCo/GC@MSS, 0.82 g of FeCo/GC@MSS nanoparticles were added slowly to a solution of H3PMo12O40 · 24H2O (0.09 g, 0.05 mmol) in methanol (10 mL). The mixture was stirred at room temperature for 6 h, and the solvent was removed under reduced pressure to obtain 10 wt% PMA in SiO2 (a greenish-black powder).
The morphology and structure of the samples were investigated by transmission electron microscopy (TEM) (JEOL JEM-2100 F, Akishima-shi, Japan operated at 200 KV) with selected area electron diffraction patterns and energy dispersive analyses of X-ray emission. The samples for the TEM analyses were prepared by adding the diluted sample to ethanol drop-wise on a 300-mesh carbon support copper grid (Ted Pella, Inc., Redding, CA, USA). Powdered X-ray diffraction (XRD) patterns were collected on a Rigaku Miniflex II (4.5 KW) diffractometer (Rigaku Corporation, Shibuya-ku, Tokyo) using Cu-Kα radiation at 30 kV and 15 mA. The magnetic measurements were carried out on a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS SQUID-VSM, Quantum Design, San Diego, USA). The Brunauer-Emmett-Teller (BET)-specific surface areas and porosity of the samples were evaluated on the basis of nitrogen adsorption isotherms using a BELSORP-max instrument (BELSORP-max, Nippon Bell, Japan).
Propargylic substitution reactions of PMA@MSS@FeCo/GC nanocatalysts
Propargylic substitution reactions of 1,3-diphenyl-2-propyn-1-ol were carried out in a 10-mL glass vial. PMA@MSS@FeCo/GC nanocatalysts (0.05 mol%), 1,3-diphenyl-2-propyn-1-ol (0.19 ml, 1.0 mmol), phenol (0.113 g, 1.2 mmol), and acetonitrile (5.0 mL) were added, and the mixture was stirred for 30 min at 323 K. Following the reaction, the nanoparticles were separated from the solution with a magnet. The reaction products were analyzed using a 1H NMR Varian Mercury Plus spectrometer (300 MHz) (Varian, Inc., Palo Alto, CA, USA). Chemical shift values were recorded in parts per million relative to tetramethylsilane as an internal standard unless otherwise indicated, and the coupling constants were reported in Hertz.
Results and discussion
Synthesis and structural characterization
The major steps involved in the synthesis of PMA@FeCo/GC@MSS are highlighted in Figure 1. We prepared FeCo/GC@MSS as a light gray powder by modifying the Stöber process  and CVD method. The MSS were then used as templates for loading FeCo/GC and PMA. A 0.9 mmol of metal precursors, Fe(NO3)3 · 9H2O and Co(NO3)2 · 6H2O, at a 58:42 molar ratio were loaded onto 1.0 g of the MSS by impregnation in methanol solutions, followed by solvent removal under reduced pressure. To deposit carbon on to the FeCo nanocrystals formed in the MSS, the metal-loaded MSS was heated to 800°C under H2 and then subjected to methane CVD. Once the MSS were cooled to room temperature, any metal impurities were removed by washing with a 10% aqueous HCl. When loading PMA on FeCo/GC@MSS (10 wt% of PMA in SiO2), FeCo/GC@MSS was added to PMA dissolved in methanol and then sonicated for 5 min. This was followed by stirring for 6 h at room temperature and solvent removal under reduced pressure to afford PMA@FeCo/GC@MSS as a light greenish powder.
Propargylic substitution reactions of 1,3-diphenyl-2-propyn-1-ol with PMA@FeCo/GC@MSS
Propargylic substitution reactions of 1,3-diphenyl-2-propyn-1-ol with phenol using PMA@MSS@FeCo/GC a
Catalyst (PMA mol%)
0.05 mol% PMA
Recovered from number 5
Recovered from number 11
Recovered from number 12
Recovered from number 13
In summary, we have successfully prepared MSS embedded with FeCo/GC nanocrystals through a simple one-step CVD process. This superparamagnetic FeCo/GC@MSS showed high saturation magnetization and superior chemical stability against acid etching and oxidation. PMA-loaded FeCo/GC@MSS worked as a green catalyst for propargylic substitution reactions of various aromatic compounds with 1,3-diphenylprop-2-yn-1-ol. The catalyst can be easily separated and reused at least five times without any appreciable loss in its catalytic efficiency, thereby showing great potential for large-scale applications. The results indicate that such materials can be used as catalysts in organic reactions.
This work was supported by a 2-year Research Grant of Pusan National University and the Ministry of Education (MOE) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (No. 2012H1B8A2026225). KHP thank to the TJ Park Junior Faculty Fellowship and LG Yonam Foundation.
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