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Journal of Molecular Catalysis A: Chemical 306 (2009) 97–101

Contents lists available at ScienceDirect

Journal of Molecular Catalysis A: Chemical

journa l homepage: www.e lsev ier .com/ locate /molcata

luorous silica gel-supported perfluoro-tagged palladium nanoparticlesatalyze Suzuki cross-coupling reaction in water

iang Wang, Chun Cai ∗

hemical Engineering College, Nanjing University of Science & Technology, Nanjing 210094, PR China

r t i c l e i n f o

rticle history:eceived 8 January 2009eceived in revised form 18 February 2009

a b s t r a c t

The Suzuki cross-coupling reactions using perfluoro-tagged palladium nanoparticles on fluorous silicagel (FSG) as catalyst, K2CO3 as base and TBAB as additive in H2O affording the corresponding biphenylsin moderate to high yields have been described. The catalyst can be recovered by simple filtration and

ccepted 19 February 2009vailable online 4 March 2009

eywords:luorous silica gelalladium nanoparticles

reused several times with slight decrease in activity.© 2009 Elsevier B.V. All rights reserved.

olid-supported catalystuzuki cross-coupling

. Introduction

Palladium-catalyzed Suzuki cross-coupling reaction hasmerged as one of the most powerful, attractive, and widely uti-ized method for the construction of carbon–carbon bonds [1–4]. Inecent years, there has been considerable interest in the prepara-ion of new and highly active palladium catalysts to facilitate suchransformation [5–11]. Palladium catalysts with phosphines ligand12], carbenes ligand [13], palladacycle [14] and other coordinates15] have shown high activity and have improved the stability ofhe reactions with water or under air. However, problems suchs expensive poisonous phosphine ligands and unrecyclability ofhe catalyst, which impacts cost and palladium contaminationn the product, extremely limited industrial applications. Thus,he development of efficient and recyclable catalysts that utilizenexpensive and phosphine-free ligands is a topic of enormousmportance.

Recently, the application of metal nanoparticles in catalysis has

ecome an important frontier of research [16–20]. Among other

nteresting properties, their high surface area and the density ofhe unsaturated surface coordination sites render them attractive inatalysis [16,18,21,22]. Since metal nanoparticles are unstable with

Abbreviations: FSG, fluorous silica gel; ICP, inductively coupled plasma; TEM,ransmission electron microscope; TBAB, tetra-n-butylammonium bromide; DIPEA,,N-diisopropylethylamine; EtOAc, ethyl acetate; DMF, dimethyl formamide; SDS,odium dodecylsulphate.∗ Corresponding author. Tel.: +86 25 84315514; fax: +86 25 84315030.

E-mail address: c.cai@mail.njust.edu.cn (C. Cai).

381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.molcata.2009.02.030

respect to aggregation and precipitation to the bulk metal, stabilizersuch as surfactants [23], organic ligands [24], polymers [25], den-drimers [26], ionic liquids [27] and aerogels [28] are generally usedduring their preparation to prevent agglomeration and to controlthe particle size. The surface properties of these metal nanoparti-cles and their catalytic activity are crucially controlled by the natureof these stabilizers.

The stabilization of nanoparticles by fluorinated compounds[29] is surprising due to their well-known phobic character. It iswell known that a consequence of the low polarizability of perfluo-rocarbons is very weak intermolecular dispersion interactions andan extremely low surface tension as there are very small attrac-tive interactions among themselves and other materials. Despitethese properties, some groups have reported that some heav-ily fluorinated compounds can indeed stabilize transition-metalnanoparticles [30–34]. As an extension to design new constituentsof protecting shields for nanoparticles, recently, Vallribera and co-workers [35] have prepared new star-shaped heavily fluorinatedcompounds which possess different features that could enhancethe stabilizing effects such as heavily perfluorinated, sterical effect,functional group with high affinity for metals. Also, perfluoro-tagged palladium nanoparticles could be supported on fluoroussilica gel (FSG) and such catalyst showed high activity in Heckreaction and could be reused for 15 runs [36]. However, it is stillsubjected to the use of organic solvent.

Water as solvent in transition-metal catalysis has many advan-tages for the recycling of catalyst and product recovery and alsoconcerning safety and environmental aspects [37]. The beneficialeffects of using water as solvent especially in Suzuki reactions arewell documented [38,39]. Quite recently, our experiments showed

9 r Catal

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8 L. Wang, C. Cai / Journal of Molecula

hat FSG-supported perfluoro-tagged Pd nanoparticles could cat-lyze Suzuki reaction efficiently in water with the additive TBABr SDS. Here we describe the catalytic performance and recyclingfficiency of the catalyst.

. Experimental

.1. General remarks

All of the reagents and solvents are commercially availablend were used without further purification. Melting points wereetermined with a WRS-1B apparatus and were uncorrected. IRpectra were recorded in KBr disks with a Bomem MB154S FT-IRpectrometer. 1H NMR and 13C NMR were recorded on a BrukerdvanceRX300 analyzer. MS and GC analyses were performed on aaturn 2000GC/MS instrument. Palladium content was measuredy inductively coupled plasma (ICP) on a Varian AA240 analyzer.ransmission electron microscope (TEM) images TEM images wereollected on a JEOL-2100 transmission electron microscopy at00 kV and the images were recorded digitally with a Gatan 794harge-coupled device (CCD) camera. All the products are knownompounds and were identified by comparing of their physical andpectra data with those reported in the literature.

.2. Preparation of heavily perfluorinated compounds 1

A round-bottom flask was charged with 2,4,6-chloro-1,3,5-riazine (1.110 g, 1 mmol) and �,�,�-trifluoromethyltoluene (15 ml).

solution of 1H,1H,2H,2H-perfluorodecylthiol (5.405 g, 3 mmol)n �,�,�-trifluoromethyltoluene (15 ml) was added, followed by,N-diisopropylethylamine (1.60 ml, 3.2 mmol). The solution was

efluxed for 12 h, cooled and filtered. The solvent was removed byotary evaporation and the solid residue was washed with waternd acetone to afford 2,4,6-tris(1H,1H,2H,2H-perfluorodecylthio)-,3,5-triazine, 1 (1.26 g, 83%). mp 112–114 ◦C (113–115 ◦C) [35]; IRKBr): � 1470, 1242, 1196, 1142 cm−1; 1H NMR (CDCl3, 300 MHz): ı.63 (m, 6H), 3.38 (t, J = 8.9 Hz, 6H).

.3. Preparation of perfluoro-tagged palladium nanoparticlesd-1

A mixture of PdCl2 (0.060 g, 0.34 mmol), NaCl (0.022 g,.38 mmol) and 2 ml of MeOH was stirred at room temperature

or 24 h. The mixture was filtered through a glass wool plug.dditional MeOH (28 ml) was added to the filtrate. The solutionas heated at 60 ◦C under stirring and 2,4,6-tris(1H,1H,2H,2H-erfluorodecylthio)-1,3,5-triazine (0.250 g, 0.16 mmol) was added.hen, the mixture was heated under stirring during 24 h. AcONa

Scheme 1. Preparation of fluorous nanoparticle stabili

ysis A: Chemical 306 (2009) 97–101

(0.190 g, 2.32 mmol) was added and stirring was maintained atroom temperature for 1 h. The formed black solid was filtered,washed with MeOH, H2O and Me2CO; it was then dried to afford0.266 g of Pd-1 as a black solid. Pd analysis (ICP): 13.0%.

2.4. Preparation of FSG-supported palladium nanoparticlesPd-1/FSG

0.020 g of Pd-1 was added to 10 ml of perfluorooctane andthe mixture was heated at 100 ◦C for 14 h. Then, 1 g of FSG (C8;35–70 �m) was added and the mixture was stirred at the same tem-perature for 1 h. After this time, the solvent was evaporated undervacuum to obtain the desired catalyst.

2.5. Typical procedure for Suzuki reaction and recycling ofcatalyst

A mixture of bromobenzene (1 mmol), catalyst Pd-1/FSG (40 mg,0.1 mol%), phenylboronic acid (1.5 mmol), TBAB (0.5 mmol), K2CO3(2 mmol) and H2O (3 ml) was stirred at 100 ◦C for a indicate time(monitored by GC) under air atmosphere. After cooling to roomtemperature, water (5 ml) and ether (10 ml) were added and thecatalyst was filtered off, washed with water and ether, and driedfor next cycle. The organic layer was separated, washed with water,dried over Na2SO4, and evaporated. The crude product was sub-jected to column chromatography on silica gel with petroleumether/EtOAc as eluent.

3. Results and discussion

Previous protocol for the synthesis of fluorinated compound1 was via SNAr reaction of 2,4,6-trichloro-1,3,5-triazine with1H,1H,2H,2H-perfluorodecylthiol in THF in the presence of Cs2CO3and substoichiometric amounts of Bu4NCl. However, long time(7 day) was required while moderate yield (68%) was obtained[35]. An alternative method involving condensation of 2,4,6-trichloro-1,3,5-triazine with 1H,1H,2H,2H-perfluorodecylthiol inrefluxing �,�,�-trifluoromethyltoluene in the presence of N,N-diisopropylethylamine (DIPEA), as shown in Scheme 1, was chosenas the optimal condition (83% of yield, 12 h) (Scheme 1).

The perfluoro-tagged palladium nanoparticles Pd-1 and theimmobilized catalyst Pd-1/FSG were prepared according to previ-ous literature [36]. The TEM images were shown in Fig. 1. It was

obvious that the spherical palladium nanoparticles were success-fully dispersed in the silica matrix with an average size of 2–3 nm(Fig. 1b).

With these good results at hand, we next examined variousparameters to optimize reaction conditions. To begin our study, we

zer and fluorous silica gel-supported Pd catalyst.

L. Wang, C. Cai / Journal of Molecular Catalysis A: Chemical 306 (2009) 97–101 99

1. (b)

paRp(tw1aa9clbgt

tnvwT

Fig. 1. TEM images (Pd-1 and Pd-1/FSG): (a) TEM image of Pd-

erform the reaction between bromobenzene and phenylboroniccid with 0.1 mol% of catalyst in systems that widely reported.esults are summarized in Table 1. It was found that reactionsroceeded smoothly to afford biphenyl in moderate to high yieldsTable 1, entries 1–4). Adding water to the system did not facili-ate the reaction except for DMF/H2O system. Only moderate yieldas obtained when choosing water as solvent after refluxing for

2 h using K2CO3 as base (entry 9). The reaction was remarkablyccelerated when sodium dodecylsulphate (SDS) or tetra-n-butylmmonium bromide (TBAB) was added and good yields, 89% and7%, respectively, were obtained. It is believed that SDS or TBABan act as a surfactant or phase-transfer catalyst and also can stabi-ize palladium nanoparticles avoiding aggregation [38,40]. Variousases such as K2CO3, Na2CO3, NaOAc, KF, K3PO4 were also investi-ated for the reaction, however, only K2CO3 and K3PO4 were foundo be efficient (entries 11–15).

Next, different catalyst loadings between 0.01 and 1 mol% were

ested for the reaction. For the higher catalyst loadings (entry 16),early complete conversion was observed. On the contrary, the con-ersion was quite low in the first run with 0.01 mol% of catalyst,hile the conversion was increased significantly in the second run.

his might be due to insufficient wetting of the support during the

TEM image of Pd-1/FSG. (c) TEM image of recovered Pd-1/FSG.

first run [41]. Besides, it was worth noting that relatively lower yieldwas obtained when using Pd-1 as catalyst under the optimal con-ditions (entry 19), which was maybe caused by the aggregation ofpalladium nanoparticles.

Under the optimized conditions, we then evaluated the effi-ciency of Pd-1/FSG with different substrates (Table 2). As shownfrom Table 2, aryl bromides bearing either electron-donating orelectron-withdrawing substituents in the ortho and para posi-tions, afforded the corresponding biphenyls in good to excellentyields. Aryl trifluoromethanesulfonate and aryl perfluorooctane-sulfonate were more active than bromobenzene in terms ofyield as well as the time (Table 2, entries 7 and 8). However,chlorobenzene was not active for the reaction and only moder-ate yield was obtained even if the catalyst was increased up to1 mol% (entry 11). When the activated aryl chloride was used,relatively higher yield was obtained while the yield is still unsat-isfactory (entry 12). Recycling studies were also investigated

which showed that the supported catalyst can be reused sev-eral times with slight decrease in its activity (entries 1, 7 and8) and the Pd leaching was less than 10 ppm. Meanwhile, therecovered catalyst was also examined by TEM and it was obvi-ous that the size of palladium nanoparticles increased to about

100 L. Wang, C. Cai / Journal of Molecular Catalysis A: Chemical 306 (2009) 97–101

Table 1Screening of reaction parameters for Suzuki reaction.

.

Entry Catalyst loading (mol%) Solvent Base Additive T (◦C) t (h) Yield (%)a,b

1 0.1 DMF K2CO3 – 100 12 922 0.1 Toluene Cs2CO3 – 110 12 813 0.1 Dioxane Cs2CO3 – 100 12 784 0.1 THF KF – 65 12 655 0.1 DMF–H2O (1:1) K2CO3 – 100 8 956 0.1 DMF–H2O (1:1) K2CO3 – 50 12 377 0.1 C2H5OH–H2O (1:1) K2CO3 – 50 12 128 0.1 CH3CN–H2O (1:1) K2CO3 – 80 12 469 0.1 H2O K2CO3 – 100 12 59

10c 0.1 H2O K2CO3 SDS 100 8 8911 0.1 H2O K2CO3 TBAB 100 8 9712 0.1 H2O Na2CO3 TBAB 100 8 8613 0.1 H2O NaOAc TBAB 100 8 4614 0.1 H2O K3PO4 TBAB 100 8 9515 0.1 H2O KF TBAB 100 8 7516 1.0 H2O K2CO3 TBAB 100 5 9917 0.01 H2O K2CO3 TBAB 100 8 11 (71)d

18 0.001 H2O K2CO3 TBAB 100 8 6 (49)d

19e 0.1 H2O K2CO3 TBAB 100 8 79

a Reaction conditions: bromobenzene, 1 mmol; phenylboronic acid, 1.5 mmol; solvent, 3 ml; base, 2 mmol; Pd catalyst, 0.1 mol%, TBAB, 0.5 mmol.b

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E

Isolated yield.c 75 mg SDS was used.d Yield of second run.e Pd-1 as catalyst.

–10 nm, which proved the decrease in its activity after several runsFig. 1c).

Hot filtration was then performed to investigate whether theeaction proceeded in a heterogeneous or a homogeneous fash-on. The catalyst was removed after 10 min while the filtrate kepttirring in refluxing water for 12 h. It was found that the reactionontinues after removing the catalyst and 52% yield of productas obtained. This might be explained by a “release and recap-

ure” mechanism as described by previous literatures [14,40–43].

t is believed that the support acts as a reservoir, and only a smallraction of active catalyst is released into the solution. That verymall catalyst amounts can lead to high conversion was alreadyhown above (Table 1, entry 18). Thus, the reactions were spec-

able 2d-1/FSG-catalyzed Suzuki reactions.

.

ntry X R1 R2 Time (h) Yield (%)a

1 Br H H 12 97 (98,95,90,86)b

2 Br 4-NO2 H 8 993 Br 4-CH3CO H 8 954 Br 4-CH3 H 12 905 Br 4-CH3O H 12 866 Br 2-CH3O H 12 837 OTf H H 8 96 (96,90,84,80)b

8 OPf H H 8 98 (95,90,87)b

9 Br H 4-CH3 12 9110 Br H 4-Cl 12 8711 Cl H H 12 26, 69c

12 Cl 4-CH3CO H 12 41

a Isolated yield.b Catalyst was reused.c 1 mol% of catalyst was used.

[

ulated to proceed in solution rather than on heterogeneous solidsupport.

4. Conclusion

In summary, the perfluoro-tagged palladium nanoparticles weresuccessfully immobilized on fluorous silica gel and the solid-supported catalyst could efficiently catalyze the Suzuki reactionin water with TBAB as additive. This protocol is an environmen-tally friendly process and can be used to generate a diverse rangeof biphenyls in moderate to excellent yields. The simple procedurefor catalyst preparation, easy recovery and reusability of the cata-lyst is expected to contribute to its utilization for the developmentof benign chemical process and products.

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