Polymer-supported bis-1,2,4-triazolium ionic tag framework for an

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Polymer-supported bis-1,2,4-triazolium ionic tag framework for an efficient Pd(0) catalytic system in biomass derived #-valerolactone Federica Valentini, Hamed Mahmoudi, Lucia Anna Bivona, Oriana Piermatti, Mojtaba Bagherzadeh, Luca Fusaro, Carmela Aprile, Assunta Marrocchi, and Luigi Vaccaro ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06502 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 25, 2019

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ACS Sustainable Chemistry & Engineering

Polymer-supported bis-1,2,4-triazolium ionic tag framework for an efficient Pd(0) catalytic system in biomass derived -valerolactone. Federica Valentini,a‡ Hamed Mahmoudi,b‡ Lucia Anna Bivona,c Oriana Piermatti,a Mojtaba Bagherzadeh,b Luca Fusaro,c Carmela Aprile,c Assunta Marrocchia and Luigi Vaccaroa* aLaboratory

of Green S.O.C. – Dipartimento di Chimica, biologia e Biotecnologie, Università

degli Studi di Perugia, Via Elce di Sotto 8, 06123 – Perugia – I; E-mail: [email protected]; Web: www.dcbb.unipg.it/greensoc. bChemistry

Department, Sharif University of Technology, Tehran, P.O. Box 11155-3615, Iran

cLaboratory

of Applied Material Chemistry (CMA), University of Namur, 61 rue de Bruxelles,

5000 Namur, Belgium

KEYWORDS : heterogeneous catalysis• cross-coupling • polymeric supports• green solvents

ABSTRACT A resin-bound 1,2,4-triazolium ionic tag has been used as support for the preparation of solid palladium nanoparticles (Pd(0)-POLI-TAG-Pd). Owing to the pincer-type architecture of the triazolium ligand, the stabilization of a high amount of palladium nanoparticles (16 wt%) has been possible. The catalytic system has been fully characterized and used in low amounts (i.e. 0.1 mol% of palladium loading) in representative Heck-Mizoroki cross-coupling processes. A

1 ACS Paragon Plus Environment

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negligible release of the metal was demonstrated, and a high activity was obtained over more runs. Besides, the protocol has been optimized for the use of safe biomass-derived γ-valerolactone reaction medium.

Introduction One of the most effective strategies for reducing environmental pollution associated with chemical production is the use of efficient catalytic reactions.1,2 Transition metal-catalyzed carboncarbon bond formation reactions have gained an ever-essential role in organic chemistry.2-12 Particularly, the cross-coupling reactions promoted by palladium catalysts have become fundamental to access selectively a wide range of complex molecules including pharmaceuticals,2 agrochemicals,2 and organic semiconductors for optoelectronic devices applications.3 However, despite the extensive application of transition metals for catalysis at the development stages, their commercial application may be hampered, owing to the difficulties in their separation. For instance, strict regulatory guidelines exist for the amount of toxic palladium residues in pharmaceutical ingredients, which must be typically kept lower than 5 ppm.13,14 Moreover, metal contamination in organic semiconductors may prevent gauging the full potential of this latter, negatively influencing the device performance.15-18 The product contamination by metal also limits the catalysts lifetime, which represents an environmental and economic damage. In 2013, the European Union identified twenty metals, that include palladium, with the imminent threat of failing to meet demand and, in most cases, they have a high cost.19-22


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Therefore, it is important to improve metal removal and recycle processes to avoid product contamination as well as the loss of precious exhaustive resource into the product or waste stream. In this context, many efforts have been directed towards the immobilization of palladium on various supports, e.g. silica, alumina, organic polymers, zeolites, carbon-based materials (e.g. multi-walled nanotubes), and magnetic particles,23-30 or the use of metal-organic framework (MOF) structures,31 on the premise that heterogeneous catalysts can be easily separated from the reaction mixture and reused.32-35 Besides, it is well-established that imidazolium salts are able to stabilize transition metal nanoparticles in respect of aggregation/agglomeration by exploiting the coordinative capacity of the organic ionic tags.36-38 Anchoring ionic liquid (IL)-like structures either onto inorganic or organic solid materials represent therefore a good strategy to obtain innovative metal catalyst supports to access systems which combine the benefits of heterogeneous catalysis – i.e. easy separation and recycling – with low levels of palladium leaching into the products.39,40 Thus, in continuation of our endeavors in developing efficient, sustainable protocols for crosscoupling reactions,41-49 here we introduce the first example of a novel Pd(0) catalytic system based on the use of our new POLystyrene-Ionic TAGs (POLI-TAGs) resins. These polymers features a cross-linked structure obtained by using a large 1,4-bis(4-vinylphenoxy)benzene cross-linker which replace divinylbenzene used in classic Merrifield’s resins (see Scheme 1). In the broad context of heterogeneous catalysis, organic polymer-anchored systems are considered among the most versatile ones.50 Particularly, a key attraction is the availability of efficient, group- tolerant methods, enabling the controlled synthesis of diverse ranges of polymer



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structures as well as the easy incorporation of the catalytic centers. Another important aspect is the tunability of the morphological properties of the organic polymer structure. We focused on the preparation of gel-type polymers – i.e. with no permanent porosity – as the most adequate support for immobilizing a pincer-type ionic tag ligand.51-53 In our catalyst design rationale, 1,2,4-triazolium motif was selected for the ionic tag component as it is still poorly explored in the field.54 Next, the pincer architecture may have the advantage to help achieving a high Pd loading.55-57 This enable the use of a smaller mass of solid support compared to the substrates for running the reactions, which is essential in view of large-scale applications. In this regard, it is important to note that, with a few exceptions, the ILs-based materials enable the stabilization of poor amounts (0.1-6 wt%) of catalytically active metal. One of the main drawbacks of classic protocols for cross-coupling reaction is the use of dipolar aprotic solvents (e.g. NMP, DMA, DMF), which rank high in the list of harmful chemicals.41-49, 5860

Besides, these solvents are able to coordinate the catalyst metal, thereby leading to its dissolution, which means that a careful choice of the reaction medium may enhance the possibility for catalyst recyclability while reducing the contamination of the product or waste stream.15 Bearing all this in mind, we focused on the use of a safer reaction medium, i.e. biomass-derived γ-valerolactone (GVL).61,62 This solvent may be produced via catalytic hydrogenation of levulinic acid, deriving from lignocellulosic biomass degradation.63-69 GVL represents a major derivative of levulinic acid and it is among the top-ten platform chemicals produced from biomasses. In addition, considering that solvents are generally used in large volumes by chemical industry, the


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use novel biomass-derived solvents such as GVL, represents an important research endeavor able to further promote the market of biomass derived platform fine-chemicals. Indeed, it has been recently reported that GVL is an effective replacement for common dipolar aprotic solvents in the presence of both homogeneous70,71 and heterogeneous catalysts.45,46 It has also been proven that, in some cases, the use of GVL in combination with heterogeneous catalysts, enables a far better control of palladium catalyst leaching into the final products compared to more conventional polar solvents.45,46 Our findings have demonstrated that the newly designed catalytic material is an excellent system for a clean Heck-Mizoroki cross-coupling reaction, which enables to prepare pure p-arylene vinylenes in high yields. It will be shown that GVL enables a better control of metal leached into product compared to conventional dipolar aprotic. Experimental Section All chemicals were purchased and used without any further purification unless otherwise noted. GVL was kept over molecular sieves 4 Ǻ; its water content was 0.29 ± 0.04 wt %, determined by METROHM 684 KF Coulometer. 4-Vinylbenzylchloride (VBC) and styrene were extracted three times with a 5% w/w NaOH solution to remove the polymerization inhibitor (tert-butyl catechol). Dibenzoylperoxide was re-crystallized from methanol. The 1H- and 13C NMR spectra in solution were recorded on a Bruker 400 MHz spectrometer (1H at 400 MHz and 13C at 100.6 MHz) at 298 K, and the data are reported in the SI, Characterization Data section. Solid-state

13C

CP-MAS

NMR spectra was recorded on a Bruker 500 MHz spectrometer with samples packed in zirconia rotors spinning at different kHz. Carbon and nitrogen contents were determined by combustion analysis in a Fisons EA 1108 elemental analyzer. TEM images were obtained using a FEI Tecnaї



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10 system with an operating voltage of 80 kV. Samples were dispersed in ethanol and deposited on a carbon coated copper grid. The X-ray photoelectron spectroscopy (XPS) analyses were performed on a Thermo Scientific K-Alpha instrument. A flood gun (very low energy) was used to avoid possible charging effects instrument. This spectrometer uses a monochromatic Al Kα Xray source (1486.6 eV) and a hemispherical deflector analyser (SDA) working at constant pass energy (CAE). Loading and leaching of palladium were measured by MP-AES 4210. GLC analyses were performed by using Hewlett-Packard HP 5890 SERIES II equipped with a capillary column DB-5MS (30 m, 0.32 mm), a FID detector and helium as gas carrier. GC-EIMS analyses were carried out by using a Hewlett-Packard HP 6890N Network GC system/5975 Mass Selective Detector equipped with an electron impact ionizer at 70 eV. Melting points were measured on a Büchi 510 apparatus.

Preparation of SP-Cl support 1 The resin was prepared following a previously reported procedure.53 Styrene: 2.98 g (28.6 mmol); 4-Vinylbenzylchloride: 5.36 g (35.1 mmol); 1,4-bis(4vinylphenoxy)benzene: 0.409 g (1.3 mmol); dibenzoylperoxide: 178 mg (0.72 mmol, 2% wt/wt). Yield: 80% (6.78 g, 3.41 mmol Cl per g). Preparation of SP-3. In a 25 mL-round bottom flask equipped with a magnetic stirrer, 3,3bis(1H-1,2,4-triazol-1-yl)propan-1-ol (2) (0.243 g, 1.25 mmol) was dissolved in 5 mL of dry DMF under N2 atmosphere. Sodium hydride (0.150 g, 3.75 mmol, 60% in mineral oil) was subsequently added at 0 °C. The mixture was kept under stirring for 30 min, then the reaction temperature was raised up to 25 °C and SP-Cl 1 was added (0.500 g, 1.25 mmol). The reaction mixture was then warmed at 60 °C for 24 h, and subsequently filtered. The solid fraction was washed with water and


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acetone, then dried under vacuum, to afford 0.686 g of the target compound SP-3. The loading was determined by elemental analysis C: 72.49; H: 6.99; N: 14.45 (1.72 mmol/g).

Quaternization of SP-3. In a stainless-steel vessel equipped with a magnetic stirrer, 5 mL of iodomethane were added to SP-3 (0.650 g), and the reaction mixture was stirred at 90 °C for 20 h. Subsequently, the mixture was filtered, washed with methanol and acetone, and finally dried under vacuum to afford SP-4 as a yellow solid. The loading was determined by elemental analysis C: 47.74; H: 4.92; N: 9.16 (1.09 mmol/g). Synthesis of catalyst SP-Pd-4. A 50 mL-round-bottom flask equipped with a magnetic stirrer was charged with PdCl2 (0.198 g, 1.12 mmol) and NaCl (1.439 g, 24.64 mmol), in 15 mL of water. The mixture was then heated at 80 C until PdCl2 dissolution. Subsequently, the solution was cooled at RT and added dropwise to a suspension of SP-4 (0.600 g) in water (10 mL). The resulting mixture was kept under stirring at RT for 2 h, then filtered under reduced pressure, washed with water and methanol and dried in air. The resulting material (0.400 g) was suspended in anhydrous ethanol (10 mL) at 0 °C and added dropwise with a solution of NaBH4 (0.120 g, 3.16 mmol) in 5mL of anhydrous ethanol. The reaction mixture was kept under stirring for 2 h, then filtered under reduced pressure, washed with methanol and ethanol, and dried under vacuum. The catalyst SP-Pd-4 was obtained as a black solid. The palladium loading was determined by MP-AES (16 wt%).

Typical procedure for the Heck-Mizoroki reaction. A 8 mL screw capped vial equipped with a magnetic stirrer was charged with the aryl iodide (2 mmol), the terminal alkene (1.2 eq, 2.4 mmol), PS-TEA (1.5 eq, 3 mmol, 0.937 g), catalyst SP-Pd-4 (0.1 mol%), and 2.5 mL GVL (0.8 M). The



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reaction mixture was then stirred at 130 °C for 3 h or 4 h (see Results and discussion). The conversion of aryl iodide into product(s) was determined by GLC analyses.

Results and Discussion In the following sections, we first report the synthesis, structural and physical properties of the designed catalyst as defined by elemental analysis,

13C

CP-MAS NMR spectra, X-ray

photoelectron spectroscopy, and transmission electron microscopy (TEM). The catalytic activity for the proposed system is first reported and discussed in terms of reaction medium and palladium catalyst leaching features. Next, on the identification of the optimal conditions, the applicability of the catalyst to a broad scope of substrates is reported. Catalyst recover/recycle characteristics are discussed.

Catalyst synthesis and characterization. The synthesis of the catalytic material is depicted in Scheme 1. In the first step, the nucleophilic substitution reaction between chloromethyl-modified polystyrene-type support SP-Cl 1 and 3,3di(1H-1,2,4-triazol-1-yl)propan-1-ol (2) in the presence of NaH in dimethylformamide (DMF) gave the material SP-3. The loading of bis-triazole units corresponds to 1.72 mmol/g, as determined by elemental analysis (see Experimental). It is worth mentioning here that chloromethylated resin SP-Cl 1 was prepared in high yield (80%) by suspension copolymerization of styrene and 4-vinyl-benzylchloride in the presence of 1,4bis(4-vinylphenoxy)benzene cross-linker.22 Next, the 3,3-di(1H-1,2,4-triazol-1-yl)propan-1-ol (2) was prepared from 1,2,4-triazole by reaction with methyl propiolate under solvent-free conditions


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to afford methyl 3,3-di(1H-1,2,4-triazol-1-yl)propanoate intermediate, which was subjected to reduction with LiAlH4, to provide the target compound (see Supporting Information). With SP-3 material in hand, the resin-bound pincer-type triazolium salt SP-4 was prepared through quaternization reaction by treatment with iodomethane at 90 °C for 20 h (Scheme 1). The elemental analysis revealed a loading of bis-triazolium salt of 1.09 mmol/g (see Experimental). A structural analysis of both SP-3 and the corresponding quaternization product SP-4 was assessed by 13C cross polarization magic angle spinning (CP-MAS) NMR investigation. Since the position of the isotropic chemical shift is not influenced by variation of the spinning frequencies, CP-MAS spectra at different spinning speeds (8 KHz and 10 KHz) were recorded to unambiguously identify the signals related to the SP-3 and SP-4 solids. In any case, a different pulse sequence known as “total suppression of spinning side bands” (TOSS) was employed as well. Figure 1(a) shows the CP-TOSS spectra of both samples. As can be clearly observed in the figure, the quaternarization induces, in the aromatic region, a shift at low frequencies of the most deshielded signal. This modification is accompanied by the strong decrease of the peak assigned to the tertiary carbon close to the two heterocyclic rings (at around 66 ppm) together with a modification of the signal at around 40 ppm. Non-quaternary and non-methyl suppression (NQS) NMR experiments performed on both SP-3 and SP-4 (Figure 1b) allowed confirming the presence of methyl groups (36 ppm) in the triazolium based solid.



Figure 1. 13C CP NMR spectra of SP-3 (black line) and SP-4 (red line) obtained using a TOSS (a) and a NQS (b) sequence.

SP

N N N

HO

+ Cl

2

SP-Cl - 1

CH3I 90°C

N N N

O

NaH, DMF 60°C 0°C

SP

SP

N N

2.NaBH4, EtOH N I + N N

N+

I-

Pd(0)

SP

N N

N+

I-

O Ph

Ph

x y z

N

SP-Pd-4

O

SP

N N

O

SP-4

Cl

N

SP-3

N+ N I N 1. [PdCl4]2-

O

N N

x

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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y Cl

Cl

z

SP-Cl - 1

Scheme 1. Synthesis of the catalytic material SP-Pd-4.


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To get the supported palladium nanoparticles, SP-4 was stirred with an aqueous solution of PdCl42- for 2 h. After filtration and washings with methanol, the supported PdCl42- intermediate species was reduced with with NaBH4 in dry ethanol. Following this procedure, a highly loaded palladium catalyst SP-Pd-4 featuring a metal loading of 16 wt% was afforded, as determined by Microwave Plasma Atomic Emission Spectroscopy (MP-AES). Information about the surface chemical composition and the oxidation state of the metal were obtained by X-ray photoelectron spectroscopy (figure 2) setting the reference signal C 1s at 284.6 eV. The Pd3d region is characterized by well separated spin-orbit components, on the other hand the peaks have asymmetric peak shape. The XPS data analysis was performed considering the asymmetry of the signals. The reduction step was very efficient and gave an high amount of Pd(0) (close to 100%). This is a crucial aspect for the good activity of the catalyst. Pd(II) Pd(0)

Pd 3d5/2 1,0

Pd 3d3/2

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,5

0,0 346

344

342

340

338

336

334

332

330

Binding energy (eV)

Figure 2. XPS spectrum of catalyst SP-Pd-4 (blue line) in comparison with Pd(II) precursor (red line). Transmission electron microscopy (TEM) allows us to determine the particle size distribution of the palladium nanoparticles on the polymeric support. The mean particles size of Pd(0) nanoparticles, estimated on more than 100 measurements, was about 7.5 nm. However, due to poor stability of the polymeric support under the electron beam, TEM investigation at high



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magnification was not always possible. For this reason, the presence of smaller particles cannot be completely excluded. Palladium nanoparticles are well stabilized from triazolium ligand and homogeneously distributed on the surface (Figure 3 and Figure S3, SI).

Figure 3. TEM images of palladium nanoparticles stabilized on the polymer surface.

Heck-Mizoroki coupling reaction. To verify the activity of SP-Pd-4 as a catalyst for Heck-Mizoroki coupling, we investigated first the model reaction between iodobenzene 5a and methyl acrylate 6a (Table 1). Furthermore, we decided to carry out this reaction by employing a supported base, namely, diethylaminomethyl−polystyrene (PS-TEA). Indeed, a heterogeneous base can trap the HI byproduct and can be easily removed by filtration, which enables the product isolation with no lengthy purification protocol.47


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We found that using a low amount of SP-Pd-4 (0.1 mol%), only a slight excess of PS-TEA (1.5 equiv), and high reactant concentration (0.8M) at 130 °C, an excellent substrates reactivity was achieved in GVL after short time (3 h) and a very low 1.2 ppm of Pd-leaching (Table 1, entry 2). Not unexpectedly, by employing homogeneous triethylamine base we found a much higher product 7a contamination (Table 1, entry 1). Indeed, polystyrene-bonded amino groups may act as capping agents for the Pd nanoparticles, thereby leading to a reduced Pd leaching. 47 Next, we investigated the issue of the palladium leaching for SP-Pd-4 both when using GVL as the reaction medium, and in the presence of traditional dipolar aprotic media. Thus, 5a and 6a showed high reactivity in representative DMA, DMF and NMP, as well as in the acetonitrile azeotrope (Table 1, entries 3-7). On the other hand, with GVL the lowest leaching of palladium was recorded (Table 1).

Table 1. Screening of solvents and data for palladium leaching for Heck-Mizoroki coupling between 5a and 6a.a

entry

medium

C c (%)

Pd (ppm)e

1b

GVL

>99

17.0

2

GVL

>99

1.2d

3

DMA

>99

2.0

4

DMF

>99

1.3

5

NMP

97

1.4

6

CH3CN/H2O az.

>99

2.6



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aReaction

conditions: iodobenzene 5a (2.0 mmol), methyl acrylate 6a (2.4 mmol), PS-TEA (1.5 equiv), catalyst SP-Pd-4 (0.1 mol%), medium (2.5 mL), 130 °C, 3 h. bHomogeneous triethylamine base was used. cConversion (C) was measured by GLC analyses. dIsolated yield: 90%. eDetermined by MP-AES; average values over three measurements. It is worth-noting that in the same process and conditions, when employing palladium nanoparticles supported on inorganic zirconium phosphates and phosphonates (i.e. Pd/α-ZrPK and Pd@ZPGly-15)72 or on imidazolium salts supported on silica gel (SILLPs)47 a higher metal contamination is achieved (6 ppm, 2 ppm and 4.5 ppm vs 1.23 ppm, respectively), pointing out the chemical stability of the newly designed palladium source immobilized on an organic solid material. We also focused our attention on the work-up procedure optimization. The SP-Pd-4 catalyst and PS-TEA were removed by a simple filtration of the reaction mixture. The pure product could be isolated by extraction with n-heptane, without the need for column chromatography. Next, we explored the catalyst scope, and the reaction between several (hetero)aryl-iodides 5ae and representative acrylates (6a,b) or styrenes (8a,b) was performed, using the conditions optimized for the Heck-Mizoroki coupling between 5a and 6a. It should be mention here that 4chlorostyrene 8b was chosen bearing in mind that the tolerability of the coupling procedure towards halo- groups is synthetically useful since the products could be easily further modified. In all cases nearly quantitative conversions and generally good-to-excellent isolated yields were enabled (Table 2 and Table 3). When 2-iodothiophene (5e) reacted with 6a (Table 2, entry 8), a small percentage (4%) of homocoupling product was also obtained; in this case, products 7i was purified by column chromatography.


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Longer reaction times were generally needed to afford complete substrate conversion when employing styrenes 8a-b. In all cases, a small percentage of gem-byproduct 10 a-l was also observed. Bearing in mind our interest in minimizing the waste associated to a chemical process, we also optimized the work-up protocol. Importantly, after filtering off the SP-Pd-4/PS-TEA solid mixture, the product was easily isolated by (1) precipitation from the reaction mixture induced by the addition of a minimal amount of water, or (2) extraction with n-heptane, depending on the formation of solid or oily products, respectively. We also studied the recovery and reuse of the catalyst for the representative reaction of 5a 6a. Interestingly, the mixture of catalyst SP-Pd-4 and PS-TEA could be quantitatively recovered and reused representatively for five consecutive runs, with the latter undergoing regeneration in between runs to remove the trapped hydriodic acid by-product.

Table 2. Heck-Mizoroki reactions between aryl iodides 5a-e and acrylates 6a-b.a

entry

Ar

R

Product

Isolated Yield (%)b

1

Ph

n-butyl

7b

80

2

4-CH3-C6H4

CH3

7c

95

3

4-CH3-C6H4

n-butyl

7d

75

4

4-OCH3-C6H4

CH3

7e

73

5

4-OCH3-C6H4

n-butyl

7f

82

6

4-COCH3-C6H4

CH3

7g

80



Page 16 of 32

7

4-COCH3-C6H4

n-butyl

7h

67

8

2-iodothiophene

CH3

7i

63c,d

9

2-iodothiophene

n-butyl

7l

72d

aReaction

conditions: aryl iodide 5a-e (2.0 mmol), acrylates 6a-b (2.4 mmol), PS-TEA (1.5 equiv), catalyst SP-Pd-4 (0.1 mol%), GVL (2.5 mL), 130 °C, 3 h. bIsolated yields of the pure product after extraction with heptane (oils) or filtration (solids), and without any further purification step. cAfter column chromatography. dTraces of 2,2’-bithiophene byproduct (< 4%) were revealed by GLC analyses. Two different protocols were investigated to regenerate PS-TEA, with the aim of identifying the one not affecting catalyst chemical integrity and efficiency. Thus, by treatment of the solid mixture of catalyst SP-Pd-4 and PS-TEA with triethylamine in GVL at room temperature for 30 min, and subsequent filtration/drying processes, the base was successfully regenerated. Besides, the palladium leaching was quantified by MP-AES analysis after each reaction run, to evaluate the lifetime of the catalyst. We constantly observed a negligible leaching level of palladium in solution with an average value of ~1 ppm (Figure 4, green line). The good reusability of the catalytic system was also confirmed by the good reaction yields obtained in five consecutive runs (Figure 4, green bars). A turnover number (TON) of 4870 and a turnover frequency (TOF) of 325 h-1 were achieved, which are expected to increase for further recycling.

Table 3. Heck-Mizoroki reactions between aryl iodides 5a-e and styrenes 8a-b.a


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entry

Ar

R1

product

Isolated

9/10/11c

Yield (%)b 1d

Ph

H

9a

76

91/9/0

2

Ph

Cl

9b

91

95/5/0

3

4-CH3-C6H4

H

9c

84

90/10/0

4

4-CH3-C6H4

Cl

9d

73

94/6/0

5

4-OCH3-C6H4

H

9e

83

86/14/0

6

4-OCH3-C6H4

Cl

9f

92

91/9/0

7d

4-COCH3-C6H4

H

9g

91

93/7/0

8d

4-COCH3-C6H4

Cl

9h

88

95/5/0

9

2-iodothiophene

H

9i

73

88/6/6

10

2-iodothiophene

Cl

9l

78

90/4/6

aReaction

conditions: aryl iodide 5a-l (2.0 mmol), styrenes 8a-b (2.4 mmol), PS-TEA (1.5 equiv), catalyst SP-Pd-4 (0.1 mol%), GVL (2.5 mL), 130 °C, 4 h. bIsolated yields of the clean mixture of products 9/10/11 after filtration, and without any further purification step. cDetermined by GC or 1H-NMR analyses. d Reaction time: 3 h On the other hand, when the regeneration was carried out by means of an inorganic base (i.e. K2CO3) in GVL/H2O (3:1 v/v), a somewhat higher level of palladium leaching in between the subsequent runs was observed (Figure 4, blue line). Not unexpectedly, this led to a faster decrease in the conversion values (Figure 4, blue bars) over the time, and to a lower TON and TOF values (4520 and 301 h-1 , respectively). On these bases, we opted for the triethylamine/GVL system to regenerate the PS-TEA. It is worth-mentioning here that GVL used for PS-TEA regeneration was distilled off and recovered with a purity comparable to the fresh solvent (NMR analysis), thus being reusable.


100

5

80

4

60

3

40

2

20

1

0

Page 18 of 32

Pd leaching (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Conversion (%)


0 1

2

3

4

5

run

Figure 4. Comparison between conversion and palladium catalyst leaching for five consecutive reaction cycles using triethylamine (green bars/green line, respectively) and potassium carbonate (blue bars/blue line, respectively) for the PS-TEA regeneration step. To monitor the amount of waste produced by the process we calculated the E-factor73 which is defined as the kg of waste produced per kg of the desired product. Under this condition E-factor values are in the range of 19-66.

Conclusions We reported the synthesis and characterization of the first example of a novel class of highloaded heterogeneous palladium catalysts, namely POLI-TAGs-Pd. The catalytic system displayed a high activity, enabling the synthesis of a large set of p-arylene vinylenes in good-to-excellent isolated yields working with only 0.1 mol% of Pd loading, and in the presence of GVL as safe reaction medium. The high Pd content (16 wt%) allowed us to use a small mass of solid support compared to the substrates for running the reactions, which is a key feature in view of large-scale applications. In addition, the catalyst demonstrated a high activity and stability over several runs,


Page 19 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with negligible metal leaching. The results presented here are highly encouraging towards the development of new polymeric ionic tags-supported palladium catalysts. ASSOCIATED CONTENT Supporting Information. Typical procedures, E-factor calculations, synthetic scheme for the preparation of 2, 13C solid-state NMR, full characterization for all the compounds and copies of the 1H and

13C

NMR spectra. This material is available free of charge via the Internet at

http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Luigi Vaccaro - Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia Via Elce di Sotto, 8; Perugia, Italia. Fax: +39 075 5855560; Tel: +39 075 5855541; E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Funding Sources AMIS, through the program “Dipartimenti di Eccellenza - 2018-2022” ACKNOWLEDGMENT F.V., O.P., A.M., L.V. gratefully acknowledge the Università degli Studi di Perugia and MIUR for financial support to the project AMIS, through the program “Dipartimenti di Eccellenza - 2018-



Page 20 of 32

2022”. The authors acknowledge Dr. Pierre Louette for his support in the XPS analysis. This research used resources of the “Plateforme Technologique Physico-Chemical Characterization” – PC2 located at the University of Namur.

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BRIEFS A novel tailor-made heterogeneous catalytic system based on polystyrene-ionic tags and Pd(0) (Pd(0)-POLITAG) for the effective Heck cross-coupling with minimal metal leaching.


Polymer-supported bis-1,2,4-triazolium ionic tag framework for an

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