Evidences of release and catch mechanism in the Heck reaction catalyzed by palladium immobilized on highly cross-linked-supported imidazolium salts

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Journal of Molecular Catalysis A: Chemical 387 (2014) 57–62

Contents lists available at ScienceDirect

Journal of Molecular Catalysis A: Chemical

jou rn al hom epage: www.elsev ier .com/ locate /molcata

vidences of release and catch mechanism in the Heck reactionatalyzed by palladium immobilized on highlyross-linked-supported imidazolium salts

inzia Paviaa, Francesco Giacalonea, Lucia Anna Bivonaa,b, Anna Maria Pia Salvoa, Chiaraetrucci c, Giacomo Strappavecciac, Luigi Vaccaroc, Carmela Aprileb, Michelangeloruttadauriaa,∗

Dipartimento di Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche (STEBICEF), Università di Palermo, Viale delle Scienze s/n, Ed. 17, I-90128alermo, ItalyLaboratory of Applied Material Chemistry (CMA), University of Namur, 61 rue de Bruxelles, 5000 Namur, BelgiumLaboratory of Green Synthetic Organic Chemistry, CEMIN – Dipartimento di Chimica, Università di Perugia, Via Elce di Sotto, 8, Perugia, Italy

r t i c l e i n f o

rticle history:eceived 31 October 2013eceived in revised form 22 February 2014ccepted 24 February 2014vailable online 4 March 2014

a b s t r a c t

Palladium (10 wt%) on a highly cross-linked imidazolium-based material was used as catalyst in 0.1 mol%in the Heck reaction between several alkenes and aryl iodides. Products were obtained from good tohigh yields. Deeper investigations showed a release of Pd species in solution and their capture by theimidazolium-based support. When a sixfold amount of support was employed the re-captured Pd species(0.5–0.6 wt%) were not anymore catalytically active. This result represents a new interesting aspect of

eywords:C cross-coupling

eterogeneous catalysisonic liquids

this work since the highly cross-linked imidazolium-based material can act also as Pd scavenger avoidingthe release of the metal in solution. Important differences between Heck and Suzuki reactions have beenevidenced when the reactions were carried out in the presence of the scavenging support.

© 2014 Elsevier B.V. All rights reserved.

eck reactionuzuki reaction

. Introduction

Heck coupling reaction has become a mainstay of modern syn-hetic organic chemistry for the formation of the carbon–carbononds [1–5]. The importance of coupling products from Suzuki,eck and other C C coupling reactions as components of manyinds of compounds, mainly pharmaceuticals and natural products6,7], as well as in the field of engineering materials, such as con-ucting polymers [8–10] has attracted enormous interest from thehemistry community. A plethora of palladium-based catalytic sys-ems have been developed with the aim of obtaining new catalystsisplaying increased activity and selectivity [11–16].

The Heck reaction has been one of the Pd-catalyzed processesxtensively investigated in ionic liquids (ILs). As reaction media

ave been used onium salts, pyridinium and imidazolium salts17–21] and more recently supported ILs and the so-called task-pecific ionic liquids, TSILs [22–25]. However, the reactions which

∗ Corresponding author. Tel.: +39 091 23897534; fax: +39 091596825.E-mail addresses: [email protected] (L. Vaccaro), [email protected] (C.

prile), [email protected] (M. Gruttadauria).

ttp://dx.doi.org/10.1016/j.molcata.2014.02.025381-1169/© 2014 Elsevier B.V. All rights reserved.

involve ionic liquids as solvents suffer from severe problems relatedwith high cost as well as high viscosity of ionic liquids. To over-come these drawbacks, adsorbed ionic liquids or covalently linkedionic liquids to several supports have been developed. These mate-rials have found interesting applications for both metal catalyzedand organocatalyzed reactions [26]. Imidazolium-based networksserve as the reaction phase in which the homogeneous catalyst isdissolved. This class of advanced materials shares the properties oftrue ionic liquids, behaving as bulk ionic liquids and the advantagesof a solid support. In several cases the Heck reaction, takes placethrough the formation of soluble complexes/Pd clusters formedfrom supported PdNPs containing ILs moieties either deposited ona solid support as a thin film [27] or covalently attached to the solid(organic and inorganic) [28,29]. In most instances, the release of sol-uble species has been observed [30]. The most important questionis if such soluble species are recaptured at the end of the reaction[31–33]. Although the recapture of Pd-based catalytic species hasalready been described it is important to get further evidences of

such “release and catch” mechanism [34] in order to fully under-stand the general applicability of this phenomenon.

Recently, we have prepared several supported imidazoliumsalts on silica gel by using a new approach, i.e. the thiol-ene

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58 C. Pavia et al. / Journal of Molecular Catalysis A: Chemical 387 (2014) 57–62

and 1-(bromomethyl)-4-iodobenzene catalyzed by 1a.

rvtiopbabiwwntaptfa[bftc

2

2

eCaaa

2

(Dhadpv

2

f(Ttar

entry 9). In this case the use of 0.1 mol% of catalyst gave the alkenein 39% yield and 92/8 2/3 ratio. When the Heck reaction was carriedout using 1-(bromomethyl)-4-iodobenzene we directly isolated thecorresponding quaternary ammonium salt 2q in excellent yield

Table 1Effect of the solvent and base on the Heck reaction between styrene andiodobenzene.a

Entry Solvent Base Yield (%)b 2a/3a ratio

1 DMF:H2O (4:1) NEt3 89 >99/12 DMF NEt3 71 95/53 DMF:H2O (4:1) K2CO3 21 96/44 DMF:H O (4:1) K CO /NEt c 27 95/5

Scheme 1. Heck reaction between 4-vinyltoluene

eaction between a mercaptopropyl-modified silica gel and bis-inylimidazolium salts. These materials have been employed forhe non-covalent immobilization of organic molecules, such as aonic liquid-tagged proline [35] or TEMPO catalysts [36]. On thether hand, the same supports have been successfully employed foralladium nanoparticles immobilization. The latter materials haveeen used in the Suzuki–Miyaura reaction between arylboroniccids and arylhalides to give biaryls in high yields both underatch [37,38] and continuous-flow conditions [39]. The interest-

ng aspect of these materials is the high imidazolium salt contenthich allows a high Pd content (10 wt%). Since these catalystsere used in only 0.1 mol%, only 1 mg of catalytic material waseeded per mol of substrate. Moreover, the Suzuki–Miyaura reac-ion under flow conditions was carried out by placing catalystnd solid base (K2CO3) in two separate columns. Here we reportreliminary observation about the use of our catalytic materialsoward the Heck reaction under batch conditions as a first stepor a study under flow conditions, exploring scope and limitationsnd trying to get evidences of a “release and catch” mechanism34]. Moreover, the ability of highly cross-linked imidazolium-ased material to act as Pd scavenger from its solutions orrom Suzuki reactions [39,40] could be extended to Heck reac-ions allowing the obtainment of products with a minimal metalontamination.

. Experimental

.1. General

The NMR spectra were recorded on a Bruker 300 MHz spectrom-ter. Mass spectra were recorded on a Shimadzu GCMS-QP2010.arbon and nitrogen contents were determined by combustionnalysis in a Fisons EA 1108 elemental analyzer. EDX values aren average of several measures, at least 10 per sample. XPS valuesre the average of three different points.

.2. General procedure for the Heck reaction

To a round-bottom flask catalyst 1a (0.1 mol%, 1 mg), alkene1.5 mmol), aryliodide (1 mmol), triethylamine (2 mmol) andMF/H2O (4 mL + 1 mL) were added. The reaction mixture waseated at 90 ◦C for 21 h. Then, the reaction mixture was cooledt room temperature, diluted with water and extracted withichloromethane. The organic phase is evaporated under reducedressure and the residue purified with a short plug of silica underacuum using petroleum ether/ethyl acetate as eluent.

.3. General procedure for the Suzuki reaction

To a round-bottom flask catalyst 1a (0.1 mol%, 1 mg), 4-ormylphenyl boronic acid (1.13 mmol), 4-bromobenzaldehyde1 mmol), K2CO3 (1.2 mmol) and EtOH/H2O (5 mL, 1:1) were added.

he reaction mixture was heated at 50 ◦C for 19 h. Then, the reac-ion mixture was cooled at room temperature, diluted with waternd extracted with dichloromethane. The organic phase is evapo-ated under reduced pressure and the residue purified with a short

Fig. 1. Structure of catalysts 1a and b.

plug of silica under vacuum using petroleum ether/ethyl acetate aseluent.

3. Results and discussion

As catalyst we have employed material 1a (Fig. 1) which hasbeen prepared as previously reported [37]. As a first approach, wehave investigated the reaction between styrene and iodobenzene(Table 1). Two different bases, namely triethylamine and potassiumcarbonate, and several solvents were preliminarily screened. Thebest reaction condition was found when a 4:1 DMF/water solventmixture in the presence of triethylamine as base was employed.The presence of water increased the yield (Table 1, entries 1–2). Noproduct was isolated when other solvents (isopropanol/water 4:1,dioxane, ethanol, water) were used under the same conditions. Alsothe use of potassium carbonate as base had a detrimental effecton the yield (entries 3–4). In each case the reaction afforded theexpected trans-alkene with small amount of the gem-alkene.

Then we employed the optimized conditions for a set ofreactions between styrene or styrene derivatives and several arylio-dides (Table 2).

Alkenes were obtained in high yields and high selectivity, beingthe expected trans isomer the major product. Only in one case thereaction was carried out using a larger amount of catalyst (0.4 mol%,

2 2 3 3

a Reaction conditions: styrene (1.5 mmol), iodobenzene (1 mmol), base (2 mmol),1a (0.1 mol%, 1 mg), solvent (5 mL), 90 ◦C, 21 h.

b Isolated product.c 1:1 ratio.

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C. Pavia et al. / Journal of Molecular Catalysis A: Chemical 387 (2014) 57–62 59

Table 2Heck reaction between styrenes and aryliodides.a

Entry 2 or 3 Compound Yield (%)b 2/3 ratio

R1 R2

1 A H H 89 >99/12 B H 4-OCH3 88 92/83 C H 3-OCH3 73 92/84 D H 4-Br 96 99/15 E H 4-COCH3 97 98/26 F H 4-COOCH3 96 99/17 G CH3 4-COOCH3 91 99/18 H CH3 4-COCH3 82 97/39c I CH3 4-OCH3 85 86/14

10 J Cl 4-COOCH3 89 98/211 K Cl 4-OCH3 85 98/212 L Cl 3-OCH3 75 98.5/1.513 M Cl H 93 97/314 N Cl 4-COCH3 97 99/115 O CHO 4-OCH3 72 >99/116 P CHO 4-Br 62 >99/1

a Reaction conditions: styrene derivative (1.5 mmol), aryliodide (1 mmol), TEA(

(b

oag

cW

Sb

TH

(

Table 4Filtration tests on Heck reaction between methyl acrylate and 4′-iodoacetophenone.a

Entry 1st step 2nd step

t (h) Yield (%)b t (h) Yield (%)b

1 0.5 5c – –2 0.5 – 21 993 1.0 69 – –4 1.0 – 21 995d 0.5 – 21 5[c]

6d 1.0 – 21 757e 0.5 – 21 998f – – 21 999g – – 21 99

a Reaction conditions (1st step): methyl acrylate (1.5 mmol), 4-iodo-acetophenone (1 mmol), TEA (2 mmol), catalyst 1a (0.1 mol% 1 mg), DMF/H2O(5 mL, 4:1) 90 ◦C; (2◦ step): 21 h, 90 ◦C.

b Isolated yield.c determined by 1H NMR.d 2nd step: after filtration, support 6 has been added (6 mg).e

2 mmol), 1a (0.1 mol%, 1 mg), DMF/H2O (5 mL, 4:1), 90 ◦C, 21 h.b Isolated product.c catalyst 1 (0.4 mol%).

Scheme 1). In this case the triethylamine (2 equiv.) acted both asase and as nucleophile.

In place of styrene derivatives we also investigated the use ofther starting alkene compounds. Reaction between iodobenzenend methyl vinyl ketone gave (E)-4-phenylbut-3-en-2-one 4 in

ood yield using catalyst 1a in 0.1 mol% (Scheme 2).

Reaction between several aryliodides and methyl acrylate gaveompounds 5a–i in high yields and excellent selectivity (Table 3).

hen 1-bromo-4-iodobenzene or 1-bromo-3-iodobenzene were

cheme 2. Heck reaction between methyl vinyl ketone and iodobenzene catalyzedy 1a.

able 3eck reaction between methyl acrylate and aryliodides.a

Entry 5 Ar Yield (%)b

1 A Ph 742 B 4-NO2-Ph 923 C 4-COCH3-Ph 994 D 4-COOCH3-Ph 975 E 4-CH3O-Ph 946 F 3-CH3O-Ph 847c G 4-Br-Ph 928d H 3-Br-Ph 949 I 2-thienyl 80

a Reaction conditions: methyl acrylate (1.5 mmol), aryliodide (1 mmol), TEA2 mmol), catalyst 1a (0.1 mol% 1 mg), 90 ◦C, 16 h.

b Isolated product.c Disubstituted compound 4%.d Disubstituted compound 3%.

2nd step: after filtration, support 6 has been added (2 mg).f No filtration, support 6 has been added (6 mg) in the starting reaction mixture.g No filtration, support 6 has been added (12 mg) in the starting reaction mixture.

employed, the corresponding disubstituted compounds weredetected in only 4% and 3% respectively (entries 7–8). Also 2-iodo-thiophene gave the corresponding acrylate 5i in high yield (entry9).

It has been proposed that when high loading supported ionicliquid-like phase (SILLP) materials at high temperatures were used,the Heck reaction takes place through a leaching mechanism inwhich soluble Pd(II) species are the molecular catalysts [33]. Evi-dences of leached active Pd species have been reported in othercases [41–44]. Therefore we investigated if a leaching occurs dur-ing the Heck reaction between 4′-iodoacetophenone and methylacrylate catalyzed by material 1a (Table 4).

The model reaction was carried out in parallel in different flasks.Two reaction mixtures were quenched after 0.5 h and 1 h respec-tively (Table 4, entries 1 and 3). Other two reactions were filteredafter 0.5 h or after 1 h and the resulting filtrates were heated at 90 ◦Cfor further 21 h (Table 4, entries 2 and 4). After 0.5 h only a minorconversion was observed (ca. 5%, entry 1), however, after catalystremoval a complete conversion was obtained (entry 2). When thefiltration was carried out after 1 h a higher conversion was observed(69%, entry 3) and, again, complete conversion was reached after21 h (entry 4).

These observations are in agreement with the fact that theleached Pd species actively catalyzed the Heck reaction. At thisstage, we wondered if under the reaction conditions, the supportedimidazolium network used for the preparation of Pd-catalyst 1a isable to re-capture the leached Pd species as observed in the case ofSuzuki reaction under flow condition [39].

In order to verify this hypothesis, two runs of the model reac-tions were additionally performed. In these cases, after filtratingthe reaction mixtures after 0.5 h and 1 h respectively, imidazolium-modified silica 6 (Fig. 2) was added in sixfold the amount ofPd-catalyst 1a normally used to run the experiments. In both cases,the process was quenched (entries 5 and 6). Only a minor increasein conversion, from 69% to 75%, was observed in the case of the

reaction filtered after 1 h (entry 6). These findings allow us tohypothesize that the reaction is catalyzed by leached Pd speciesand that such soluble Pd species were re-captured on the support 6becoming not catalytically active. The latter result could be ascribed

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60 C. Pavia et al. / Journal of Molecular Catalysis A: Chemical 387 (2014) 57–62

Fig. 2. Structure of support 6.

Table 5Pd content in materials 7 and 1b.

Entry Material Pd (mass%)a Pd(II) (%)b Pd(0) (%)b

1 7 0.5 100 –2c 7 0.6 100 –3 1b 1.3 88 12

a

ti6s2

liAffimtPPbl

iirr

4sIsaes

rtgl

Sp

Scheme 4. Heck reaction between methyl acrylate and aryl iodides catalyzed bymaterial 1a.

Table 6Suzuki reaction in the presence of variable amount of support 6.a

Entry ArX Cat.1a/Supp. 6 (wt./wt.) Conv. (%) Yield (%)b

1 1/0 >99 99

2 1/2 >99 91

3 1/3 27 20

4 1/4 4 4

5 1/6 0 0

6 1/6 0 0

a Reaction conditions: 4-formylphenyl boronic acid (1.13 mmol), 4-

Determined by EDX.b Determined by XPS.c Duplicated experiment.

o the very low Pd loading on the surface of the new material (videnfra). We repeated the reaction by using a lower amount of support

(twofold amount, entry 7). In the latter case, the lower amount ofupport 6 did not quench the reaction and it was completed after1 h.

We verified our hypothesis that support 6 is able to re-captureeached Pd species. We carried out the reaction between 4′-odoacetophenone and methyl acrylate on larger scale (5 mmol).fter 1 h catalyst 1a was removed by filtration and support 6 (six-

old, 30 mg) was added. After 21 h the support 6 was recovered byltration and characterized. This experiment was duplicated. EDXeasurements showed the presence of Pd on material 7 (Fig. 3),

hen, demonstrating that support 6 was able to capture the leachedd species. Moreover, XPS data indicated mainly the presence ofd(II) species, whereas Pd(0) species were not detected probablyecause of the not good resolution of XPS spectra due to the very

ow Pd content (Table 5, entries 1–2 and figures at pages S42–S43).In order to test if catalyst 1 having a low content of Pd is catalyt-

cally active, we prepared a new material 1b containing palladiumn 1.3 wt% (determined by EDX). We used such material in the Heckeaction between methyl acrylate and 4′-iodoacetophenone but noeaction occurred (Scheme 3).

We also carried out the reaction between methyl acrylate and′-iodoacetophenone by adding the sixfold or 12-fold amount ofupport 6 in the starting reaction mixture (Table 4, entries 8–9).n these cases, the reactions were complete. Since the leached Pdpecies are re-captured by the support (becoming not catalyticallyctive) the completion of the reaction could be explained consid-ring that a heterogeneous catalysis pathway promoted by thetarting material 1a could be also active.

In Scheme 4 is described the plausible reaction pathway. Mate-ial 1a releases Pd species in solution. The leached Pd catalyzes

he Heck reaction. Addition of support 6 captures Pd in solution toive material 7 that is not catalytically active because of the veryow Pd content. Only a minor amount of the starting Pd is leached

cheme 3. Heck reaction between methyl acrylate and 4′-iodoacetophenone in theresence of material 1b.

bromobenzaldehyde (1 mmol), K2CO3 (1.2 mmol), catalyst 1a (0.1 mol% 1 mg),EtOH/H2O (5 mL, 1:1) 50 ◦C.

b Isolated yield.

(see Table 5, entries 1–2) then, an active role of material 1a in thecatalytic process could be taken into account.

In order to get more insight on the behavior of material 7 and1b, we carried out EDX and XPS experiments. First, freshly preparedcatalyst 1b with low Pd content showed about 12% of Pd(0) (Table 5,entry 3). By contrast, a Pd catalyst having the same Pd(0)/Pd(II)ratio and a high Pd amount (10 wt%) was highly active [39]. In ouropinion, material 7 and 1b are not catalytically active because thePd species are strongly bound into the inner surface of the support.Probably, the high imidazolium loading and its multilayer nature,well stabilize the Pd species.

Since catalytic material 1a has been recently employed by usin the heterogeneous Suzuki–Miyaura reaction, we verified if the

addition of support 6 can change the course of such reaction(Table 6). Both mechanisms at the NPs surface [45–49] and involv-ing homogeneous molecular Pd(II) catalysts [32,50–54] have beensuggested for the Suzuki reaction. Increasing amount of support

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C. Pavia et al. / Journal of Molecular Catalysis A: Chemical 387 (2014) 57–62 61

pping

6b(eoeb6odtrcmtAhitmca[sa

4

cwaattew(

[

Fig. 3. EDX ma

in the starting reaction mixture quenched the Suzuki reactionetween 4-formylphenyl boronic acid and 4-bromobenzaldehydeTable 6, entries 1–5). When a sixfold amount of support 6 wasmployed no biaryl compound was obtained. The same result wasbserved when a different aryl halide was used (4-iodotoluene,ntry 6). These results are in sharp contrast with the Heck reactionetween methyl acrylate and 4′-iodoacetophenone (Table 4, entry). In the latter case, the addition of a sixfold or even 12-fold amountf support 6 did not quench the reaction, demonstrating that twoifferent mechanisms are operative. In the case of the Suzuki reac-ion, the reaction takes place with the leached Pd species and theeaction is quenched when a large amount of support is added. Theaptured Pd species gave a non-catalytically active supported Pdaterial. On the other hand, in the case of Heck reaction, the reac-

ion could be catalyzed by both leached and supported Pd species. similar hypothesis was given when a supported Pd catalyst onigh loading SILLP material was used in the Heck reaction between

odobenzene and methylacrylate [33]. The accepted mechanism ofhe Heck reaction involves the presence of soluble anionic inter-

ediate that can operate via a homeopathic catalytic Pd(0)–Pd(II)ycle [55,56]. ESI-MS studies confirmed the presence of [PhPdI2]−

nionic species in solution and also [PdI3]− and its dimer [Pd2I6]2−

33]. Such anionic species can be exchanged with bromide anion ofupport 6 and then efficiently removed, as confirmed by XPS, when

sixfold amount of support is added to the reaction mixture.

. Conclusions

We have demonstrated that palladium (10 wt%) on a highlyross-linked imidazolium-based material is an efficient catalysthen used in 0.1 mol%, for the Heck reaction between aryl iodides

nd alkenes (styrene derivatives, methyl vinylketone and methylcrylate) in DMF/H2O in the presence of TEA at 90 ◦C. Underhese conditions such catalyst released Pd species in solution and

he leached Pd species catalyzed the reaction, but in the pres-nce of a sixfold amount of the imidazolium-based support theyere very efficiently scavenged. The new supported Pd species

0.5–0.6 wt%) were not catalytically active as well as a low loading

[

[[[

of material 7.

Pd catalyst (1.3 wt%). The non-catalytic activity of low loadedPd material can be explained assuming that such Pd species arestrongly bound in the inner surface of the multilayered structureof the high loading silica gel-supported imidazolium salt. A sim-ilar behavior was observed in the case of the Suzuki reaction.These results constitute the basis for the development of an effi-cient flow approach for the Heck reaction in which the released Pdspecies could be recaptured giving a product with a minimal metalcontamination.

Acknowledgements

We gratefully acknowledge the Università degli Studi diPalermo, the Università degli Studi di Perugia, the University ofNamur for financial support and the “Centro Grandi Apparecchia-ture – UniNetLab – Università di Palermo funded by P.O.R. Sicilia2000–2006, Misura 3.15 Quota Regionale”.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.molcata.2014.02.025.

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