Article pubs.acs.org/IECR
Magnetic Silica-Supported Palladium Catalyst: Synthesis of Allyl Aryl Ethers in Water R. B. Nasir Baig and Rajender S. Varma* Sustainable Technology Division, National Risk Management Research Laboratory, U.S. Environmental Protection Agency, MS 443, Cincinnati, Ohio 45268, United States S Supporting Information *
ABSTRACT: A simple and benign procedure for the synthesis of aryl allyl ethers has been developed using phenols, allyl acetates, and magnetically recyclable silica-supported palladium catalyst in water; the performance of the reaction in air and easy separation of the catalyst using an external magnet are sustainable attributes of the reaction.
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INTRODUCTION Allylic ethers are important precursors for a wide range of organic reactions and have found use in polymerization reactions, 1,3-hydrogen shift or [3,3]-sigmatropic rearrangement, and an integral part of popular protecting group strategy for alcohols.1−4 Williamson-type allyl ether synthesis is a conventional method, which involves the use of strongly basic metal alkoxide anions and highly active allyl halides or their equivalents.5 The addition of oxygen nucleophiles to η3allylmetal complexes of transition metals provides a milder alternative approach for their synthesis using much less reactive allyl alcohols, allyl carbonates, or allyl esters.6−13 However, most of the existing methods for transition-metal-catalyzed allylation via η3-allylmetal intermediates are accomplished using a nonrecyclable homogeneous catalyst in toxic organic solvents often requiring an inert atmosphere.6−13 The use of heterogeneous catalysts in organic transformations has become an interesting area of research that facilitates the recyclability of catalysts under the umbrella of green chemistry.14 Magnetic nanoparticles have emerged as a robust, highsurface-area heterogeneous catalyst support.15−18 Magnetic recoverability, which eliminates the necessity of catalyst filtration after completion of the reaction, is an additional attribute of these materials compared to most of the heterogeneous catalysts deployed.19−29 These catalysts work well but have an inherent drawback requiring an elaborate and tedious procedure for their synthesis, which involves three steps: (i) synthesis of nanoferrite as a core, (ii) post-synthetic modification via anchoring of ligands which may be toxic, and (iii) immobilization of Pd metal on the surface and high percentage of Pd loading.30 In continuation of our effort toward the development of sustainable methods for organic synthesis,31−38 and to overcome aforementioned drawbacks, a onestep procedure for the synthesis of magnetically retrievable silica supported palladium catalyst has been developed and its application in the O-allylation of phenol in aqueous media has been demonstrated.
the sequential addition of reagents in one-pot (Scheme 1). The magnetic nanoferrite (Fe3O4) was generated in situ via a Scheme 1. One Pot Synthesis of Fe3O4@SiO2Pd catalyst
hydrolysis method by stirring the solution of FeSO4·7H2O and Fe2(SO4)3 in water at pH 10 (adjusted using (25%) ammonium hydroxide solution) followed by heating in a water bath at 50 °C for 1 h. The reaction mixture was cooled to room temperature, tetraethyl orthosilicate (TEOS) was added, and vigorous stirring continued for 24 h under ambient conditions. To this solution, PdCl2 was added and the pH of the solution was adjusted to ∼10 using NH3 (25%) and stirring continued for another 12 h (Scheme 1). The ensuing magnetic silica-supported Pd nanoparticles were separated using an external magnet, washed with water, followed by acetone, and dried under vacuum at 50 °C for 8 h. Catalyst characterization by X-ray diffraction (XRD) and transmission electron microscopy (TEM) (Figure 1) confirmed Special Issue: Ganapati D. Yadav Festschrift
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Received: March 14, 2014 Revised: April 21, 2014 Accepted: April 28, 2014
RESULTS AND DISCUSSION The first step in the accomplishment of this goal was the easy synthesis of a magnetic silica-supported palladium catalyst via © XXXX American Chemical Society
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dx.doi.org/10.1021/ie501081q | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Table 1. Reaction Optimization for O-Allylation of Phenol
entry 1 2 3 4
Figure 1. TEM image of Fe3O4@SiO2Pd catalyst.
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the formation of single-phase silica-coated Fe3O4 nanoparticles (nano-Fe3O4@SiO2Pd) with spherical morphology and a size range of 20−35 nm. The signals pertaining to Pd metal and Si were not detected in XRD (see Figure S1 in the Electronic Supporting Information (ESI)), indicating that the Pd and Si species are highly dispersed on ferrites. The weight percentage of Pd and Si was found to be 2.96% and 6.73%, respectively, by inductively coupled plasma−atomic emission spectroscopy (ICP-AES) analysis. The Fe3O4@SiO2Pd catalyst was examined in the Oallylation of phenol with allylic acetates in the presence of K2CO3 as the base. In our first attempt, a mixture of m-cresol, cinnamyl acetate, K2CO3, and Fe3O4@SiO2Pd catalyst in water was heated at 100 °C for 6 h. The desired product was formed in 73% of yield. In order to optimize the reaction conditions, a series of experiments were performed (Table 1). Among the different conditions explored to optimize the reaction, heating at 100 °C in water for 6 h using NHCO3 as a base gave the best result (Table 1, entry 9). The active role of palladium in the catalytic cycle of the reaction was established by control experiments; no product formation occurred in the absence of a catalyst (Table 1, entry 11) or base (Table 1, entry 10). Allyl ether was not formed either when the same reaction was carried out using Fe3O4@SiO2 (without Pd; Table 1, entry 12). However, catalyst (Fe3O4@SiO2Pd) gave good yields thus confirming the essential role of palladium as the catalyst. Under these optimized conditions, the scope of reaction was studied for various substrates with different phenols and allylic acetates (see Table 2). A series of substituted cinnamyl acetates underwent coupling with a variety of substituted phenols to afford the corresponding allyl aryl ethers (Table 2). The variation in the structure of allyl acetates has not shown any peculiar effect on yield and product outcome of the reaction. In general, the reactions have been clean and products were obtained in high yields. Several functional groups such as Me, OMe, NO2, Br, and Cl do not alter the result of the reaction. Branched allylic acetate (Table 2, entry 10) could be used with equal efficiency to provide linear allyl aryl ether, which gives clear evidence that the reaction proceeds through the η3-allyl-Pd complex intermediate (Scheme 2). The reaction of p-cresol with simplest allyl acetate gave only a trace amount of product in water, possibly due to loss of allyl acetates under refluxing conditions. However, in DMF at 80 °C, the reaction afforded 89% of the desired aryl ethers (entry 8, Table 2). The turnover number (TON) is calculated by carrying the reactions (Table 2, entries 1−5) at 20 mmol scale, using 50 mg
6 7 8 9 10 11 12
catalyst Fe3O4@ SiO2Pd Fe3O4@ SiO2Pd Fe3O4@ SiO2Pd Fe3O4@ SiO2Pd Fe3O4@ SiO2Pd Fe3O4@ SiO2Pd Fe3O4@ SiO2Pd Fe3O4@ SiO2Pd Fe3O4@ SiO2Pd Fe3O4@ SiO2Pd Fe3O4@ SiO2
solvent
temp (°C)
time (h)
yield (%)a,b
K2CO3
DMF
110
12
71
K2CO3
THF
66
12
45
K2CO3
CH3CN
82
12
49
K2CO3
toluene
110
12
