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Immobilized palladium nanoparticles on a silica–starch substrate(PNP–SSS): as an efficient heterogeneous catalyst for Heck andcopper-free Sonogashira reactions in water
Ali Khalafi-Nezhad* and Farhad Panahi*
Received 4th April 2011, Accepted 15th June 2011DOI: 10.1039/c1gc15360a
This paper reports an efficient heterogeneous catalyst system based on the immobilization of Pdnanoparticles on a silica–starch substrate (PNP–SSS). The PNP–SSS is found as a highly effectiverecyclable catalyst in Heck and copper-free Sonogashira reactions. The silica–starch substrate(SSS) can stabilize the Pd nanoparticles effectively so that it can provide a platform and preventthe aggregation of nanoparticles and their separation from the substrate surface. It also providessuitable catalytic sites for reactions in aqueous media. The Heck and copper-free Sonogashirareactions are performed in the presence of a small amount of this catalyst in water as a greensolvent. The catalyst can be reused more than six times with almost consistent efficiency and canbe recovered by simple filtration.
Introduction
Carbon–carbon bond formation using Pd-catalyzed cross-coupling reactions has received great attention in recent years.1
The Heck reaction has been used as a synthetic tool for C–C bond formation from aryl halides and alkenes.2 There aremany chemicals and pharmaceutical intermediates with alkenestructural units, which are synthesized using the Heck reactionin the presence of palladium catalysts. Thus, the syntheticimportance of this reaction is obvious.3 This reaction hasbeen studied with different homogeneous and heterogeneouspalladium catalytic systems. However, there are widespreadstudies to improve the synthetic methods for this reaction andits applications to prepare new materials.4 High performancePd catalysts allow the use of aryl chlorides, which are lessexpensive than aryl bromides or iodides, for the Heck reaction.In the most recent works, phosphine-free methods, ligand-freepalladium catalysts and non-conventional reaction media arehighly regarded as environmentally friendly methods for theHeck reaction.5–7
Also, compounds which contain carbon–carbon triple bonds,especially aryl alkynes and conjugated enynes, are frequentlyused in materials science and medicinal chemistry due totheir potential applications in these flieds.8 Sonogashira cross-coupling reaction of terminal alkynes with aryl halides ortriflates, especially using palladium catalysts, is one of the most
Department of Chemistry, College of Sciences, Shiraz University, Shiraz,71454, Iran. E-mail: [email protected], [email protected];Fax: +98-711-2280926; Tel: +98-711-2282380
powerful and versatile strategies to obtain these compounds.9
An impressive variety of methodology modifications for thisreaction have been developed,10 but these modified methodsoften suffer from some drawbacks. The most important limita-tions associated with the application of these modified methodsare, using only reactive substrates, consuming high amountsof palladium catalyst and ligands as the major cost factors,using copper(I) iodide as the co-catalyst, which makes the systemmore complicated, using toxic solvents and producing alkyne-homocoupling products.
Hence, preparation of high performance Pd catalysts and theuse of green reaction conditions are two important strategiesfor the preparation of new Pd catalyst systems for palladiumcatalyzed Heck and Sonogashira reactions. The large fractionof metal atoms at the surface (high surface-to-volume ratio) andstrong synergistic interaction between noble metal nanoparticlesand the support leads to these materials being considered asefficient catalysts.11 Furthermore, these systems often possessthe capability for the recovery, which is an important factorfor the development of green catalysts.12 There is great interestin supported nano catalysts on metal oxides, especially oxidesof Si such as silica.13 Since silica is abundantly available andhas high stability, in many cases it is used as a solid surfacefor heterogeneous catalysts. On the other hand, organic groupscan be linked to the silica surface with strong bonding togenerate catalytic sites. The silica surface is highly porous andcan trap and stabilize the Pd nanoparticles, hence it has beenused as a substrate for the immobilization of Pd nanoparticles.14
The main problem with this catalytic system is that the Pdnanoparticles are easily separated from the substrate surface
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and released into the reaction media. Even when using it forthe first time in a reaction, the performance of this catalystcan be significantly reduced. Along the line of these studiesand in the course of our investigations into the developmentof an applicable new heterogeneous catalyst in green reactionconditions, herein, we wish to disclose the fact that when starchis covalently connected with silica, it can provide a platform andprevent the removal of nanoparticles from the silica surface.This point was observed in the increased catalyst recoverytimes. Also, the silica–starch substrate generates very suitablecatalytic sites for reaction in water media so it provides thepossibility of reaction in an aqueous environment. Consideringthe above reports, we decided to check the Heck and copper-free Sonogashira reactions using this new catalyst system in anaqueous environment and the details of this work are providedbelow.
Results and discussion
Preparation and characterization of palladium nanoparticles ona silica–starch substrate (PNP–SSS)
As you can see in Scheme 1, the PNP–SSS catalyst wasprepared in three steps. First, activated silica15 was convertedto silica chloride using a similar method to that which hasbeen reported in the literature.16 Second, it was reacted withstarch in chloroform and triethylamine to obtain the silica–starch substrate. Finally, the palladium acetate was reduced toPd nanoparticles using the method of alcohol reduction on SSS.
Scheme 1 Synthetic routes for the preparation of the PNP–SSScatalyst.
The PNP–SSS catalyst was characterized by the followingtechniques: Transmission Electron Microscopy (TEM) (Fig. 1);Powder X-ray Diffraction (XRD) (Fig. 2); Energy DispersiveX-ray spectroscopy (EDX) (Fig. 4); and X-Ray photoelectronspectroscopy (XPS) (Fig. 7). FT-IR spectroscopy was used forthe characterization of SSS (Fig. 5). Thermal behavior of the SSSwas also investigated by a thermogravimetric (TG) analyzer.
Fig. 1 A TEM image of the PNP–SSS catalyst that shows themorphology of Pd nanoparticles on the silica starch support in twodifferent positions on the surface (a and b). The TEM image of reusedPNP–SSS catalyst after being reused six times (c).
The TEM image of the PNP–SSS catalyst (Fig. 1) showsthat the Pd nanoparticles with near spherical morphology areassembled onto the SSS with relatively good monodispersity.
The histogram revealing the size distributions of Pd-nanoparticles is shown in Fig. 2 and is proposed according tothe data, which is obtained from the TEM image with an averagesize of 8 nm.
Fig. 2 A histogram representing the size distribution of Pd nanoparti-cles on the silica–starch substrate.
The XRD pattern of the PNP–SSS catalyst (Fig. 3) also showsthat we have palladium nanoparticles on the SSS. The strongestpeaks of the XRD pattern correspond to SiO2 and other peaksare indexed as the (111), (200), (220) and (311) planes of thepalladium nanoparticles.17
According to Fig. 3, a sharp peak of palladium is observed at2q = 40◦, which indicates the crystalline nature of the palladiumspecies. The size of the palladium nanoparticles was alsodetermined from X-ray line broading using the Debye–Scherrerformula18 given as D = 0.9L/bcos(q), where D is the averagecrystalline size, L is the X-ray wavelength used, b the angularline width at half maximum intensity and q is the Bragg’s angle.For q = 20◦, L = 1.06 A, and b = 0.68 mm, the average size ofthe Pd nanoparticles on silica starch substrate is estimated to be
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Fig. 3 The XRD spectrum of the PNP–SSS catalyst.
about 15 nm, this value is in good agreement with data obtainedfrom the TEM image. As clearly observed, according to theEDX spectrum (Fig. 4), the presence of palladium is indicatedon the SSS in the PNP–SSS catalyst. The EDX spectrum alsoshows other elements, including C, O and Si, which are presentin the silica–starch substrate.
Also, to confirm the Pd content, the supported catalyst wastreated with concentrated HCl and HNO3 to digest the Pdspecies and then analyzed by ICP analysis. The Pd content wasdetermined to be 23.8 ppm (23.8 mg L-1), which is equal to 2.38%w/w. The morphology of the catalyst was studied using FT-IRspectroscopy. The FT-IR spectra of pure silica, starch and SSSare shown in Fig. 5. This comparison shows the existence ofstarch on silica and confirms that silica–starch substrate wasproduced successfully.
Comparison between the FT-IR spectra of pure silica, starchand the silica–starch substrate, reveals some absorption bands
Fig. 4 The EDX spectrum of the PNP–SSS catalyst.
in the SSS that are present in both silica and starch. The peaks ataround 2077, 1659, 1451, 1360, 958, 837, 798, 576, 463 cm-1 arerelated to the bonds in SSS. The observed shift in the absorptionsof these peaks is related to the chemical bond formed between thestarch and silica because the formation of bond between silicaand starch probably resulted a change in the electron densityaround the chemical bonds. The FT-IR transmission spectrumof the silica starch substrate shows three bands at around 1659,798 and 463 cm-1, which are presumably due to asymmetricstretching (nas), symmetric stretching (ns), and bending modesof Si–O–Si, respectively.15 The weak band at 2077 cm-1 is due tothe stretching vibration of C–H and C–C bonds. Also, the bandat 1360 cm-1 is associated with the stretching vibration of theC–O bond.19
Fig. 6 depicts a typical TGA curve of the PNP–SSS catalystand it shows three main weight losses. The first one occurs at~120 ◦C and was assigned to adsorbed water. Another one,
Fig. 5 A comparison between the FT-IR spectra of pure silica, starch and silica–starch substrate.
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Fig. 6 A typical TGA curve of the PNP–SSS catalyst.
above ~240 ◦C (and continuing to ~580 ◦C) is related to thedecomposition of starch from the silica substrate. This part of thethermogram reveals the amounts of grafted starch on silica sup-port. This was estimated to ~22% (w/w). Finally, the decreasein the weight percentage of the catalyst at temperatures up to~850 ◦C is related to the decomposition of the silica support. So,the elevated temperature for starch removal indicates the highthermal stability for the silica–starch substrate, because starchis covalently bonded to silica.
To evaluate the chemical oxidation state of the Pd in PNP–SSScatalyst, it was analyzed using XPS. The XPS spectrum (Fig. 7)reveals that we have only Pd(0) on the silica–starch substrate.20 Itseems that the silica–starch substrate efficiently stabilized the Pdnanoparticles, probably with its abundant hydroxyl groups andprevents from their oxidation during the preparation process.
Fig. 7 XPS spectra of PNP–SSS catalyst. a) fresh catalyst b) after 6time of reusability.
Heck reaction catalyzed by PNP–SSS catalyst
The PNP–SSS catalyzed Heck reaction between bromobenzeneand ethyl acrylate was chosen as a model reaction to evaluate theeffects of type of solvent, type of base, temperature and quantityof catalyst. Optimization conditions are shown in Table 1.
In our initial selection we used dimethylformamide (DMF)as solvent and K2CO3 as base for the reaction between bro-mobenzene and ethyl acrylate in the presence of the PNP–SSScatalyst at 120 ◦C, where ethyl cinnamate was obtained in 83%isolated yield after 6 h. It was observed that the yield of productwas enhanced to 93% when the ratio of DMF : H2O (5 : 1) wasused as solvent. Is noteworthy that the increased ratio of waterto the DMF (1 : 5) did not reduce the product yield (Table 1,
Table 1 PNP–SSS catalyzed Heck reaction between bromobenzeneand ethyl acrylatea
Entry Solvent Base T/◦C Time/h Yield (%)b
1 DMF K2CO3 120 6 832 DMF:H2O (5 : 1) K2CO3 100 6 933 DMF:H2O (1 : 5) K2CO3 100 6 914 H2O K2CO3 reflux 6 905 H2O K2CO3 reflux 12 75c
24 78c
48 80c
6 H2O K2CO3 reflux 48 81d
7 H2O K2CO3 reflux 48 83e
8 H2O K2CO3 reflux 12 92f
9 H2O K2CO3 80 12 6310 H2O K2CO3 r.t. 24 NR11 H2O NaOH reflux 12 7112 H2O Et3N reflux 12 5413 toluene Cs2CO3 reflux 24 5114 CH3CN K2CO3 reflux 24 37
a Reaction conditions: bromobenzene (1 mmol), ethyl acrylate(1.2 mmol), base (2.0 mmol), amount of catalyst (0.05 g, 1.2 mol%)and solvent (5 mL). b Isolated yield. c Amount of catalyst used: 0.025 g(0.6 mol%) d Amount of catalyst used: 0.033 g (0.8 mol%) e Amount ofcatalyst used: 0.042 g (1.0 mol%) f Amount of catalyst used: 0.075 g (1.8mol%)
entry 3) and regarding this point, aqueous solvent was selectedfor this reaction as a green medium (Table 1, entry 4). As youcan see, with increasing the amount of catalyst (1.8 mol %),the yield of desired product does not change much (Table 1,entry 6), but reducing quantity of the catalyst (0.6, 0.8 and 1.0mol %) decreased the yield of product (Table 1, entries 5, 6, 7and 8). With increasing the time of the reaction at low loadingof the catalyst, the yield of the product was not significantlyimproved, even after 48 h. Therefore the low catalyst loadingwith prolonged reaction time is not a suitable strategy for thispalladium catalyst. As the type of base was changed the resultsrevealed that in the presence of K2CO3 a promising yield wasobtained (Table 1, entries 4, 11 and 12). During our optimizationstudies, various temperatures were examined and it was foundthat the temperature plays a significant role in terms of reactionrate and isolated yield. As you can see in Table 1, by decreasingthe temperature, the yield of reaction significantly decreases(entry 9). At the end of the optimization studies, we can saythat the Heck reaction can be done under aerobic conditionsin refluxing water in the presence of the PNP–SSS catalyst (1.2mol %), using K2CO3 as the base, without the addition of freeligands or any promoting additives.
Heck reaction of various substrates catalyzed by the PNP–SSScatalyst
After optimizing the reaction conditions using bromobenzeneand ethyl acrylate and in view of the fact that PNP–SSS was anefficient and reusable catalyst for the Heck reaction in aqueousmedia, the catalyst was used with other reactants.
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Table 2 The Heck reaction between different aryl halides and acrylateor styrene in the presence of PNP–SSS catalyst
Entry R X Y Time/h Yield (%)a
1 H Cl CO2Et 12 612 H Br CO2Et 6 903 H I CO2Et 2 924 H Cl Ph 8 645 H Br Ph 5 926 H I Ph 1.5 957 Br I Ph 2 918 Cl Br Ph 4 889 OMe Cl CO(CH2)3CH3 12 5510 SO2Me Br 4-Br–Ph 6 7711 NO2 Cl CO2Et 2 8712 H Cl Ph 12 8913 OMe Br Ph 12 86
a Isolated yield.
Table 2 clearly demonstrates that the PNP–SSS catalyst iseffective for the Heck reaction. As clearly shown in Table2, the catalytic activity of PNP–SSS depended on the typeof halide and all aryl iodides were rapidly converted to thecorresponding products with excellent yields (Table 2, entries3, 6 and 7). Also, the electron-withdrawing groups on the arylring increased the reaction rate (Table 2, entries 10, 11 and 12).The Heck reaction of both electron-rich and electron-deficientaryl bromides also proceeded smoothly to furnish the desiredproducts with excellent yields (Table 2, entries 10 and 13). Asyou can see in Table 2, moderate yields of products were achievedusing aryl chlorides in the presence of the PNP–SSS catalyst.
Copper-free Sonogashira reaction catalyzed by the PNP–SSScatalyst
Now in this part, we report another important application ofPNP–SSS as a catalyst for the efficient Sonogashira couplingreaction between terminal alkynes and aryl halides in neat waterunder copper-free conditions. Catalytic efficiency of the PNP–SSS for coupling between bromobenzene and phenylacetylene asa simple model substrate was first evaluated. According to dataobtained from the Heck reaction, water was used as reactionmedium in this protocol as well, potassium carbonate as thebase and PNP–SSS (1.2 mol%) as the catalyst.
According to Scheme 2, under the mentioned conditions thePNP–SSS catalyst is effective in the environmentally benignsystem and 1,2-diphenylethyne compound was obtained withsatisfactory 91% isolated yield after 8 h. We also checked theeffect of amount of catalyst and we found that with increasingthe amount of catalyst (1.8 mol%; 92% isolated yield), the yield
Scheme 2
of the product did not change much and decreasing the amountof catalyst (0.6 mol%; 68% isolated yield) leads to a decreasein the yield of product. The copper-free Sonogashira reactionis not accomplished at room temperature using the PNP–SSScatalyst in water. It seems that temperature plays an importantrole in the progress of the Sonogashira reaction under optimizedconditions, because the yield of product is reduced when thetemperature is decreased (for example: at 80 ◦C, 73% isolatedyield was obtained). In this way, we have selected optimumconditions for the copper-free Sonogashira reaction as follows:PNP–SSS catalyst (0.05 g, 1.2 mol%), K2CO3 as base (2.0 mmolper 1 mmol of aryl halide) and refluxing water.
Copper-free Sonogashira reaction of various substrates catalyzedby the PNP–SSS catalyst
According to optimum condition, PNP–SSS is also bestcandidate catalyst for copper-free Sonogashira reaction. Todetermine the scope of the this reaction for preparation of ethynederivatives, a number of commercially available terminal alkyneshave coupled with some aryl halides under optimized reactionconditions, and the results are depicted in Table 3.
Considering Table 3, it is clear that the PNP–SSS catalystis also effective for the copper-free Sonogashira reaction.Although the catalytic activity of PNP–SSS is dependent onthe type of halide, all aryl halides (chloride, bromide and iodide)were converted to the corresponding products with excellentyields. Electron-withdrawing groups on the aryl ring increasedthe reaction rate and also isolated yield, for example 1-chloro-4-nitrobenzene after 8 h give 85% isolated yield of desired product.The Sonogashira reactions of both aliphatic and aromaticalkynes were carried out with excellent yields in the presence ofthe PNP–SSS catalyst (Table 3, for example: entries 3, 6 and 9).Moderate yields of products were achieved using 2-methylbut-3-yn-2-ol as a hydroxy-functionalized alkyne in the presence ofthe PNP–SSS catalyst (entries 4, 5 and 6).
Table 3 The copper-free Sonogashira reaction between different arylhalides and alkynes in the presence of the PNP–SSS catalyst
Entry G X Y Time/h Yield (%)a
1 H Cl Ph 12 812 H Br Ph 8 913 H I Ph 3 954 H Cl C(CH3)2OH 12 845 H Br C(CH3)2OH 8 906 H I C(CH3)2OH 4 927 H Cl CH3CH2CH2CH2 12 918 H Br CH3CH2CH2CH2 8 939 H I CH3CH2CH2CH2 3 9710 4-CN Cl Ph 8 9111 4-OMe Br Ph 8 8512 4-NO2 Cl Ph 8 8513 4-Me Br Ph 8 90
a Isolated yield.
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Along the line of this study and in the course of ourinvestigations on the PNP–SSS catalyst and its application inthe Heck and copper-free Sonogashira reactions, we observedthat the experimental conditions (K2CO3, refluxing water) donot have any destructive influence on behavior of the graftedstarch on the silica during the Heck and copper-free Sonogashirareactions. Although phenomena (such as dehydration) mayhappen for the silica starch substrate during the reaction, thesechanges do not have any adverse effect on the progress ofthe Heck and copper-free Sonogashira reactions. It should benoted that the PNP–SSS catalyst system in some cases is moreefficient for Heck21 and copper-free Sonogashira22 reactions incomparison with other heterogeneous catalysts, especially foraryl chlorides.
Recycling of the PNP–SSS catalyst for the Heck andcopper-free Sonogashira reactions
For practical applications of heterogeneous catalysts, the levelof reusability is a very important factor. The possibility ofrecycling the catalyst for the Heck reaction was examinedusing the reaction of bromobenzene with ethyl acrylate underoptimized reaction conditions. When the reaction was complete,the reaction mixture was filtered and the solid was washed withwater (2 ¥ 10 mL) and dichloromethane (2 ¥ 10 mL), and therecycled catalyst was dried in an oven (at 120 ◦C for 2 h) to usein the next operation. The recycled catalyst could be reused fivetimes and no observation of any appreciable loss in the catalyticactivity of the PNP–SSS catalyst was observed (Table 4).
The catalytic reusability of the PNP–SSS catalyst for thecopper-free Sonogashira reaction was tested using the reactionbetween bromobenzene and phenylacetylene under optimizedconditions. The reaction mixture was filtered (after finishing thereaction) and then the filtered sediment was washed with water(2 ¥ 10 mL) and dichloromethane (2 ¥ 10 mL). The recoveredcatalyst was heated in an oven at 120 ◦C for 2 h and then the driedcatalyst was applied for the next operation. The recycled catalystcould be reused five times without any treatment (Table 5).
The ICP analysis of the aqueous phases of reaction mixtureafter five cycle of reusability (for both Heck and sonogashirareactions) showed that only a very small amount of the Pd metalis removed from the silica starch substrate (Table 6).
To confirm this point that the activity originated fromthe supported palladium nanoparticles on the silica starch–substrate and not from leached Pd, we did the following test.When the reaction between bromobenzene with ethyl acrylate
Table 4 Reusability of the PNP–SSS Catalyst in the Heck Reactiona
Entry Yield of product (%) Recovery of PNP–SSS (%)
fresh 90 > 991 90 992 89 983 89 984 87 985 88 97
a Reaction conditions: PNP–SSS (0.1 g), K2CO3 (4.0 mmol), H2O (4 mL),bromobenzene (2.0 mmol) and ethyl acrylate (2.4 mmol)
Table 5 Reusability of the PNP–SSS catalyst in the copper-free Sono-gashira reactiona
Entry Yield of product (%) Recovery of PNP–SSS (%)
fresh 91 991 91 992 91 983 89 984 89 975 87 97
a Reaction conditions: PNP–SSS (0.1 g), K2CO3 (4.0 mmol), H2O (4 mL),bromobenzene (2.0 mmol) and phenylacetylene (2.2 mmol).
Table 6 The ICP analysis results of fresh and reused catalyst after 5time of reusability
Entry PNP–SSS catalyst condition ppm of Pd % (w/w)
1 fresh 23.8 2.382 reused from Heck reactiona 22.4 2.243 reused from Sonogashira reactionb 22.1 2.21
a The Heck reaction was accomplished between bromobenzene and ethylacrylate. b The Sonogashira reaction was done between bromobenzeneand phenylacetylene.
was complete and the hot filtration had been accomplished, theobtained aqueous solution from the filtrate was applied for thenext reaction between bromobenzene with ethyl acrylate underoptimized condition. After a similar purification processes, theproducts from the Heck reaction were obtained in less than 5%isolated yield. The ICP analysis of the aqueous solution that wasobtained from filtration is in good agreement with these results,due to only 1.1 ppm of Pd being observed in this solution. It maybe caused by the redeposit ion process23 for leached Pd metal onthe support surface. To resolve this issue, the recovered catalystswere characterized using TEM images and ICP analysis toinvestigate any changes in Pd particle morphology/size and Pdcontent on the catalyst surface in comparison with fresh catalyst.The TEM image of the catalyst showed that the morphologyand size of the catalyst after recycling five times does not changesignificantly (Fig. 1c).
Also, the obtained ICP data from the reused catalyst (afterrecycling six times) show that only a low amount of Pdnanoparticles (1.4 ppm) are lost from the silica–starch substrateduring reaction process. So, these results confirmed that thePNP–SSS catalyst provides the high catalytic activity, and notany leached palladium.
Conclusions
In conclusion, we report herein the preparation, characterizationand utilization of a new and high performance catalyst systemfor the Heck and copper-free Sonogashira reactions. The silica–starch substrate was prepared from abundant, commerciallyavailable, and relatively cheap starting materials. The palladiumnanoparticles with near spherical morphology and average sizeof 8 nm were immobilized on the silica–starch substrate usingthe alcohol reduction method. By use of this catalyst system, wesynthesized some of the alkene and alkyne derivatives with highyield in a green environment. Reusability and easy workup were
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two other advantages of this catalyst system. Also, this catalystprovides great promise toward further useful applications inother palladium transformations in the future.
Experimental section
Chemical, instrumentation and analysis
Chemicals were purchased from Fluka, Merck and AldrichChemical Companies. For recorded 1HNMR and 13C NMRspectra we used Brucker (500 and 250 MHZ) Avance DRX inpure deuterated DMSO-d6 and CDCl3 solvents with tetram-ethylsilane (TMS) as internal standards. Mass spectra wererecorded on a FINNIGAN-MAT 8430 mass spectrometeroperating at 70 eV. FT-IR spectroscopy (Shimadzu FT-IR8300 spectrophotometer), was employed for characterizationof the compounds and PNP–SSS catalyst. The transmissionelectron microscopy (TEM) was obtained using TEM apparatus(CM-10-philips, 100 kV) for characterization of the PNP–SSS catalyst. The thermogravimetry analysis (TGA) of thesamples was analyzed using a lab-made TGA instrument.The X-ray diffraction (XRD, D8, Advance, Bruker, axs) wasemployed for characterization of the PNP-SSS catalyst. Meltingpoints were determined in open capillary tubes in a BarnsteadElectrothermal 9100 BZ circulating oil melting point apparatus.The reaction monitoring was accomplished by TLC on silica gelPolyGram SILG/UV254 plates. Column chromatography wascarried out on columns of silica gel 60 (70–230 mesh).
Catalyst Preparation
Silica chloride
In a three-necked flask (100 mL) that was equipped with adropping funnel containing thionyl chloride (SOCl2, 40 mL),condenser and a gas inlet tube for conducting HCl gas overan adsorbing solution (10% NaOH), 10 g of silica was charged.Thionyl chloride was added drop-wise over a period of 30 min atroom temperature. The HCl gas evolved from the reaction vesselimmediately. When the addition was completed, the mixture wasstirred for 24 h at the refluxing temperature of SOCl2. Then,the untreated thionyl chloride was removed by distillation. Thesilica chloride was dried in a vacuum at 90 ◦C and the resultinggrayish powder was stored in a desiccator under vacuum. Todetermine the amount of chlorosilyl groups on the silica surfacewe prepared a suspension containing 1 g of silica chloride in100 mL of sodium hydrogen carbonate (0.005 M). This solutionwas stirred for 1 h at room temperature. Then the releasedchloride ions were titrated with silver nitrate (AgNO3) solution(0.01 M, 88 mL) using potassium chromate solution as anindicator and in this way the amount of chlorosilyl groups onsilica surface was obtained ~0.9 mmol g-1.24
Silica–starch substrate (SSS)
To a magnetically stirred mixture of silica chloride (5 g) in CHCl3
(30 mL), potato starch (purchased from Aldrich company) (2 g)and triethyl amine (0.5 mL) were added and refluxed for 12 h.Then, the mixture was filtered and washed with chloroform (3 ¥10 mL) and water (3 ¥ 10 mL). It is noteworthy that washingthe SSS with water is necessary until all unreacted chlorosilyl
groups are eliminated. After drying in the oven at 120 ◦C for2h, the silica–starch substrate was obtained as a white powder(6.35 g).
Pd nanoparticles on the silica–starch substrate (PNP–SSS)
To a mixture of the silica–starch substrate (5 g) in absoluteethanol (30 mL), palladium acetate (0.3 g, 1.3 mmol) was addedand stirred 24 h at room temperature. Then, the mixture wasfiltered and washed with ethanol (3 ¥ 15 mL) and diethyl ether(2 ¥ 15 mL). After drying in a vacuum oven at 100 ◦C overnight,the PNP–SSS catalyst was obtained as a dark solid.
General procedure for the Heck reaction in the presence of thePNP–SSS catalyst
In a typical experiment, to a mixture of aryl halide (1 mmol),acrylate or styrene (1.2 mmol), and K2CO3 (2 mmol) in 2 mL wa-ter, PNP–SSS catalyst (0.05 g, 1.2 mol%) was added and heatedin an oil bath at 100 ◦C for the time specified in Table 2. Thereaction was followed by TLC. After completion of the reaction,the mixture was cooled down to room temperature and filteredand the remaining solid was washed with dichloromethane (3 ¥5 mL) in order to separate the catalyst. After the extraction ofdichloromethane from water, the organic extract was dried overNa2SO4. The products were purified by column chromatography(hexane/ethyl acetate) to obtain the desired purity.
General procedure for Sonogashira reaction catalyzed by thePNP–SSS catalyst
Into a conical flask (10 mL) a mixture of aryl halide (1 mmol),terminal alkyne (1.1 mmol), K2CO3 (2 mmol), water (2 mL)and PNP–SSS catalyst (0.05 g, 1.2%) were stirred at refluxingtemperature of water. The reactions were monitored by TLC.Stirring was continued until the consumption of the startingmaterials based on reaction time in Table 3. After completionof the reaction, the mixture was filtered and washed with water(2 ¥ 10 mL) dichloromethane (2 ¥ 10 mL) and the catalystwas separated by simple filtration. The solvent was removedunder reduced pressure and the product was purified by silicagel column chromatography employing n-hexane/ethyl acetateas the eluent, affording the pure corresponding product.
Acknowledgements
We acknowledge Shiraz University for partial support of thiswork. Also, we are thankful to Dr Reza Yousefi for their helpfulcomments.
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