Regioselective Heck reaction catalyzed by Pd nanoparticles immobilized on DNA-modified MWCNTs

Abstract

This is the first report of regioselective Heck reaction of aryl iodides with 2,3-dihydrofuran using heterogonous nanocatalyst. Herein, palladium nanoparticles is immobilized on DNA-modified multi walled carbon nanotubes and characterized by FT-IR spectroscopy, UV-vis spectroscopy, field emission scanning electron microscopy, X-ray diffraction, transmission electron microscopy, inductively coupled plasma and elemental analysis. DNA as a well-defined structure and biodegradable natural polymer generate the palladium catalyst which showed high activity in common and regioselective Heck reaction in excellent yields and good selectivity under ligand-free and mild reaction conditions. Moreover, the catalyst could be recovered and reused at least nine times without any considerable loss of its catalytic activity.

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Introduction

The Heck reaction is one of the most important carbon–carbon bond forming reactions in organic synthesis and in the pharmaceutical, agrochemical, and fine chemical industries.^1–3 The palladium-catalyzed bond forming reaction between aryl halides and olefins bearing electron-withdrawing substituents (EWG, e.g. –COOR, –CN) is well studied.^4–8 In contrast, the Heck coupling involving an olefin bearing a hydrogen atom in the allylic position has proved to be a more challenging substrate. This reaction can be accompanied by migration of the carbon–carbon double bond leading to products mixture.^9–18 Reactions of this type have been intensively investigated in some variants with respect to the kind of used substrate such as arylation of dihydrofuran (DHF). Most conventional methods of palladium-catalyzed Heck reaction were limited to using phosphine ligands and homogeneous route. These approaches employing aryl triflate substrate and numerous phosphine backbone ligands such as amide-based phosphite-oxazolines,¹¹ atropisomeric P,O-ligands,¹⁶ benzylically substituted P,N-ligands,¹⁷ and diphosphine-oxazoline ferrocenyl¹⁸ in presenting of palladium salts.

In the typical procedure of Heck arylation of DHF, aryl triflate was used. However, the main drawback of this substrate is the easy hydrolysis of triflate, limiting its application in the presence of water. Thus, aryl halides are cheaper and more easily available substrates. There are a few reports about application of aryl halides in the Heck coupling. Zhou and co-workers reported the arylation of DHF with common aryl bromides and chlorides catalyzed by spiro bisphosphine oxides as a ligand of Pd catalysts. This method gave a high conversion and good olefinic ratio but suffer from some limitations such as harsh reaction conditions, long reaction times and using expensive ligands.¹⁴ Lauer and co-workers recently reported conditions for achieving excellent vinyl selectivity in the Heck coupling of cyclic olefins with aryl bromides using neopentyl phosphine ligands.¹⁵

Despite the wide application of these homogeneous palladium catalysts, their separation is very difficult and their recycling is almost complicated. Moreover, in concern to toxicity of palladium residuals, acceptable limits of palladium traces in pharmaceuticals were set usually as ppm level. Application of heterogonous catalysts can solve this problem. Furthermore, due to economical and environmental factors, catalytic systems without expensive and toxic phosphine ligands are favourable. Numerous P-free conditions for Heck reaction of DHF has been reported to obtain selective products through Jeffry method which used ionic media in the reaction.^19–24 This P-free catalytic processes in spite of various advantages such as better commercial and ecological properties in compared to phosphine/palladium catalysts, exhibited lower selectivity. Therefore, more convenient selective approaches using a palladium catalyst are required.

Development of effective and practical catalytic methods, under more economic and greener conditions, has been a topic of great interest during recent years.^25–30 Learning from the nature, employing biomolecules is highly favourable in chemical transformations. In the past ten years, DNA has also emerged as a powerful tool for the synthesis of various biomaterials and biocatalysts.^31–38 This molecule has several properties that make it a very promising scaffold for designing a hybrid catalyst, which is chemically stable and commercially available natural DNA is inexpensive. Furthermore its solubility and biodegradability make it an attractive material in catalyst field, giving it green credentials. Transition metal cations, such as Ag,³⁹ Au,⁴⁰ Co,^41,42 Cu,⁴³ Pt (ref. 44) and Pd (ref. 45 and 46) can be chelated via dative bonding in such defined sites along the DNA lattice and these metal nanoparticles can be stabilized in the catalytic system.

In the majority of the DNA-based catalytic transformations investigated to date, it have been used effectively in some transformations such as the Diels–Alder, (oxa)-Michael addition and Friedel–Crafts alkylation reactions.⁴⁷ A notable exception, however, is the DNA-based catalytic Suzuki reaction using graphene/Pd hybrid catalyst without using ligand in water and under aerobic conditions which reported by Qu and co-workers.⁴⁸ They indicated that a highly charged polyelectrolyte and the negatively charged phosphate groups on the DNA backbone can provide DNA-modified materials with high dispersibility in aqueous solutions. Thus, these properties of natural DNA make it an ideal mediator to build carbon-based palladium catalysts. Meanwhile, application of nano-supports in this case because of high activity and environmental accept-ability, is preferred.

In recent years, carbon nanotubes (CNTs) have aroused significant interests of scientists all over the world since; it exhibits unique features such as large surface, intrinsic low mass and easy surface modifications which might be promising candidates as catalysts or supports.^49–52

Modification of its activity, including immobilization of DNA and palladium nanoparticles, make a unique and recoverable catalyst. Based on the fascinating structure of CNTs material, as well as the large capability of loading organic molecules, we reason that the CNTs can be used as an efficient support for DNA allowing for desirable catalytic properties. Single-stranded salmon testes of DNA (ss-DNA) can be loaded on the MWCNTs surface via non-covalent and simple route by mixing them in an aqueous solution, giving the excellent DNA/MWCNTs catalyst for organic synthesis. In this hybrid, the aromatic nucleobases in DNA can interact through π–π stacking with carbon basal surface^53,54 and stabilize palladium nanoparticles. Synthesis and study on the structure of DNA-modified materials such as MWCNTs, SWCNTs and graphene have been reported in the literature, and their applications in drug delivery, biosensors and high-performance modern materials have also been investigated.⁵⁵ However, it's catalytic applications are rare and to the best of our knowledge, olefin arylation employing immobilized palladium on DNA-modified MWCNTs, have not been reported previously.

In continuation of our recent investigations on the application of heterogeneous catalytic systems in cross-coupling reactions;^56–60 In this report, palladium is supported on the ss-DNA-functionalized MWCNTs materials to give a new and highly stable palladium catalyst (Pd/DNA@MWCNTs).

Results and discussion

The catalyst preparation procedure was shown in Scheme 1. This process were monitored by various analysis.

Scheme 1 Synthesis of the catalyst Pd/DNA@MWCNTs.

As shown in FT-IR spectrum depicted in Fig. S1 (ESI†). The characteristic peaks of C–N stretching (1264 cm⁻¹), C–H stretching (3014, 2985, 2900 cm⁻¹) N–H stretching (3100–3500 cm⁻¹), C=C (1507 cm⁻¹) and C=N (1620 cm⁻¹) to aromatic nucleobases which assignable to DNA. The peak at 1643 cm⁻¹ is observed due to carbonyl group stretching vibration.

A sharp band appeared at 2221 cm⁻¹ which can be due to phosphate group of DNA. Very similar FT-IR spectra to previously reports have been observed.⁶¹

The structural properties of the synthesized catalyst was analysed by XRD and compared with the pure MWCNTs. The XRD pattern in Fig. S2† shows characteristic peaks of amorphous MWCNTs and the appearance of new peaks in pattern of catalyst attributed to metal Pd species. The nitrogen content of Pd/DNA@MWCNTs, determined by elemental analysis, was found to be 0.99 mmol g⁻¹ of catalyst. The reduction of oxidized-MWCNTs and the formation Pd/DNA@MWCNTs hybrids can be monitored by UV-vis spectroscopy (Fig. S3†). The UV-vis spectrum of oxidized-MWCNTs contains a strong absorption band at 202 nm, which is corresponded to C–C π → π* and C–O n → π* transitions. After reaction, the absorption peaks of Pd/DNA@MWCNTs composites red shift from 202 to 213 nm, suggesting that the electronic conjugation within carbon nanotube is restored after the reaction. The disappearance of the absorption of H₂PdCl₄ at 378 nm indicated Pd²⁺ has been reduced to Pd(0) nanoparticles. The morphology of the surfaces of Pd/DNA@MWCNTs were studied by field emission scanning electron microscopy (FE-SEM) and compared it to pure MWCNTs images (Fig. 1).

Fig. 1 FE-SEM images of (a) MWCNTs and (b) Pd/DNA@MWCNTs.

As can be seen the nanotubes are aggregated and has retained their nanotube nature upon functionalization with DNA and palladium. Further characterization of Pd/DNA@MWCNTs was performed by transmission electron microscopy (TEM). The TEM images of Pd/DNA@MWCNTs showed well-defined spherical Pd particles dispersed as small dots on immobilization of DNA which surround around MWCNTs (Fig. 2). The size distribution of Pd nanoparticles was about 5–8 nm, indicating that palladium nanoparticles did not aggregate upon immobilization on MWCNTs. All these observations display that the natural DNA polymer is a good host and ligand for palladium nanoparticles. The palladium content of the catalyst, measured by ICP, showed a value of 1.27% (0.12 mmol g⁻¹ of MWCNTs).

Fig. 2 TEM images of Pd/DNA@MWCNTs.

After structure characterization of the catalyst, its catalytic activity was investigated in Mizoroki–Heck reaction (Table 1). In order to obtain the optimum experimental conditions, the reaction of 4-bromoacetophenone with styrene was considered as a model reaction.

Table 1 Optimization of the Heck reaction of 4-bromoacetophenone with styrene^a

Entry	Solvent	Base (equiv.)	T (°C)	Yield^b (%)
a Reaction conditions: 1 mmol 4-bromoacetophenon, 1.5 mmol styrene, 2 mL of solvent, for 3 h.b GC and isolated yield.c 5 h.d 24 h.
1	DMSO	KOH (3)	80	75
2	NMP	KOH (3)	80	87
3	DMF	KOH (3)	80	63
4	MeCN	KOH (3)	80	48
5	H2O : EtOH	KOH (3)	80	50
6	DMF/H2O (1 : 2)	KOH (3)	80	92
7	DMF/H2O (1 : 2)	K2CO3 (3)	80	84
8	DMF/H2O (1 : 2)	NaOH (3)	80	79
9	DMF/H2O (1 : 2)	Li2CO3 (3)	80	72
10	DMF/H2O (1 : 2)	KOH (1.5)	80	69
11	DMF/H2O (1 : 2)	KOH (3)	50	91^c
12	DMF/H2O (1 : 2)	KOH (3)	r.t.	40^d

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The effects of the reaction conditions such as the type of base, solvent and temperatures were tested. According to the experimental results and the essential goal of green chemistry to reduce consumption of energy, the best conditions were chosen using 4-bromoacetophenone (1 mmol), styrene (1.5 mmol), KOH (3 mmol) and 5 mg of catalyst in DMF/H₂O (1 : 2) at 50 °C (Table 1, entry 11). Using the optimized reaction conditions, a variety of structurally divergent aryl iodides and bromides was coupled with olefins to generate the desired coupling products in 83–96% yields (Table 2). The results showed that aryl halides with either electron withdrawing or electron-donating substituents reacted with olefins rapidly and generated the coupled products in excellent yields at mild reaction conditions.

Table 2 Heck reaction of aryl halides with olefins^a

Entry	X	RC6H4X	Olefin	Time (h)	Yield^b (%)
a The reaction was carried out with aryhalide (1.0 mmol), olefin (1.5 mmol) and KOH (3 mmol) in DMF/H2O (1 : 2) in the presence of 5 mg of catalyst at 50 °C.b Isolated yield.
1	I	H	Methyl acrylate	1	95
2	I	4-NO2	Methyl acrylate	1	93
3	I	4-OMe	Methyl acrylate	1	96
4	Br	H	Methyl acrylate	2	95
5	Br	4-NO2	Methyl acrylate	2	89
6	Br	4-Cl	Methyl acrylate	2	92
7	Br	2-Cl	Methyl acrylate	2	90
8	Br	3-Cl	Methyl acrylate	2	93
9	Br	4-OMe	Methyl acrylate	2	94
10	Br	3-Me	Methyl acrylate	2	91
11	Br	4-CN	Methyl acrylate	2	96
12	Br	4-Br	Methyl acrylate	2	92
13	Br	2-Me	Methyl acrylate	2	84
14	I	1-Naphthalene	Methyl acrylate	2	87
15	Br	4-COH	Methyl acrylate	2	90
16	Br	H	Styrene	2	96
17	Br	H	Styrene	5	87
18	Br	4-COMe	Styrene	5	91
19	Br	4-COMe	4-Me-styrene	5	74

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Encouraged by the obtained results in the Mizoroki–Heck cross-coupling, the efficiency of the catalyst (Pd/DNA@MWCNTs) was investigated in the Heck reaction of an electron-reach olefin (DHF) with aryl iodides. Initially, the Heck cross-coupling of 4-methoxyiodobenzene with 2,3-dihydrofuran was chosen as a model. The results are summarized in Table 3.

Table 3 Optimization conditions of the Heck reaction of 4-methoxyiodobenzene with 2,3-dihydrofuran^a

Entry	Solvent	Tem. (°C)	Cat. (mg)	Conv.% (2 + 3)^b	Ratio 3/4^c
a Unless otherwise noted, 4-methoxyiodobenzene (0.5 mmol), 2,3-dihydrofuran (1 mmol), solvent (3 mL) and Li2CO3 (2 mmol), 24 h.b Conversion percentages was determined by GC.c The selectivity was measured by GC.^18d 48 h.
1	MeCN	80	10	86	47 : 53
2	DMF	80	10	93	68 : 32
3	DMSO	80	10	87	49 : 51
4	MeCN	50	10	78^d	58 : 42
5	MeCN	110	10	97	55 : 45
6	THF	50	10	92^d	87 : 13
7	THF	50	5	54^d	62 : 38
8	THF	50	20	84^d	56 : 44
9	THF	50	10 (Pd/MWCNTs)	14	—

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The model reaction was first performed in the presence of different solvents such as acetonitrile (MeCN), dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF), among them THF was found to be the most effective solvent in conversion and selectivity. The obvious improvement in the conversion (81%) and selectivity was achieved for the reaction at 50 °C, (Table 3, entry 6). Higher reaction temperature (80 and 110 °C) even though gave better yield but resulted lower regioselectivity. Therefore, all reactions were carried out at 50 °C. The effects of the amount of catalyst in the model reaction were also explored; the best result was obtained using 10 mg of catalyst.

The Pd/DNA@MWCNTs hybrid materials could catalyze arylation reaction of 2,3-dihydrofuran with yields more than 67%, while the palladium supported on MWCNTs (Pd/MWCNTs) displayed a yield of just 14% (Table 3, entry 9).

The high activity of our catalyst can be ascribed to the good synergistic effects of palladium nanoparticles with DNA which grafted on MWCNTs. With the optimum trade-off between activities and selectivity, the conditions of using THF as solvent and 10 mg of catalyst at 50 °C was selected as the best conditions. We then examined the generality and versatility of this supported palladium-catalyzed, regioselectivity arylation reaction of 2,3-dihydrofuran cross-coupling of aryl iodides. The results are illustrated in Table 4. Some varieties of aryl iodides bearing either electron-rich or electron-deficient substitutes reacted efficiently with 2,3-dihydrofuran and gave the corresponding products in moderate to good yields and selectivity. In comparison to the known activity of palladium-base catalyst designed to obtain selective products (all of them are homogenous), this catalyst has presented comparable activity and selectivity. The Heck reactions of 2,3-dihydrofuran with a number of para-substituted phenyl bromide including 4-Br, 4-Cl, 4-CN, and 4-COMe were also investigated. Unfortunately, all of these reactions failed to form the Heck products.

Table 4 Heck reactions of aryl halides with 2,3-dihydrofuran

Entry	Substrate	Yield^a (%)	Ratio 3/4^b
a Isolated yield of major isomer.b Determined by GC.^18
1	2a	94	77 : 23
2	2b	92	87 : 13
3	2c	97	97 : 3
4	2d	89	79 : 21

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It was originally proposed that the mechanism of the selective Heck reaction proceed via a kinetic resolution. As shown in Scheme 2, it seems that the negatively charged phosphate groups on the DNA backbone as a ligand bind to the olefin product tightly.

Scheme 2 Possible mechanism for arylation of 2,3-dihydrofuran in the presence of palladium catalyst.

The strong bonding permits for the hydridopalladium species to undergo reversible insertion/elimination to give the desired products; while, the previous reports have been emphasized that phosphine ligands with steric hindrance are essential in the performance of this regioselective intermolecular Heck reaction.

It is noteworthy that the application heterogonous palladium catalyst in such reactions has not been previously reported. Furthermore, this method offers several advantages such as high product yields, mild reaction conditions, excellent regioselectivity, clean reaction profiles and operational simplicity. Therefore, this synthetic methodology can be considered as a useful practical achievement in the preparation of these important heterocyclic compounds.

The separation and reusability of noble metal catalysts are the trends of the catalysis industry and green chemistry, not only for lowering costs, but also for avoiding pollution. To gain insight into this issue, the catalyst reusability experiments in the Mizoroki–Heck reaction of 4-bromoacetophenone with styrene were carried out. At the end of each reaction, the catalyst was separated by centrifugation (in 8000 rpm for 6 min), washed with acetone and water for several times and dried in vacuum overnight at 60 °C. After that, the catalyst was reused in model reaction. As reported in Table 5, the catalyst was reused several times (nine consecutive runs were checked) without significant loss of activity. Wonderful recyclability of palladium heterogonous catalyst based on MWCNTs were reported before.⁶²

Table 5 Reusability of the catalyst

Run	Yield^a	Run	Yield^a
a Isolated yield.
1	91	6	89
2	90	7	90
3	90	8	89
4	90	9	84
5	90	—	—

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The structure of the catalyst was demonstrated after the last recovery using ICP and CHN analyses (N = 8.51%) and the results showed that only a very small amount of palladium metal was removed from the catalyst (ICP = 0.11 mmol g⁻¹); the morphologies of the recovered catalysts were also examined by TEM (Fig. S4†), the typical images indicate that Pd nanoparticles are well dispersed on MWCNTs surfaces after nine cycles and high conversion is kept at about 84%. Additionally, to explore the leaching of these catalysts, two similar control experiments were performed in each case. To this end, the reaction between 4-bromoacetophenone and styrene was carried out simultaneously in two different vessels under the exactly the same conditions. In each case, the reaction progress was monitored using gas chromatography. After 2.5 hours, the catalyst was separated from one of these vessels using centrifuge (in 8000 rpm for 6 min), while the other remained unchanged. Thereafter, both reactions continued under the same conditions. After another 2.5 hours, both reactions was stopped and their completion was again examined using GC. The results showed that the reaction in the absence of the catalyst did not proceeded any more, while completion of the reaction in the other vessel was improved. This observation confirmed that no leaching occurred in the reaction medium during the process. In addition, the leaching of palladium from the Pd/DNA@MWCNTs catalyst was measured by ICP-OES, and it was found that no leaching of palladium occurred during the reaction. This clearly revealed that the catalyst is stable under the reaction conditions and can be recovered and reused. All these results demonstrate that the Pd/DNA@MWCNTs catalyst exhibits significantly higher catalytic activity and selectivity in regioselective intermolecular Heck reaction, showing more environmental and economic advantages over the wide range of homogenously pervious reported catalysts.

Conclusions

In summary, this work provides a novel environmental friendly and economical route for dramatically efficient and regioselective arylation of dihydrofuran using a DNA-modified MWCNTs-based Pd hybrid catalyst. In addition, our catalytic systems show high activity for Heck reaction under mild and aerobic conditions. To the best of our knowledge, this is the first report on using heterogonous catalyst for regioselective Heck reaction of aryl iodides and dihydrofuran.

Experimental section

General

All the reagents and solvents were purchased from Merck chemical company. Salmon testes DNA was purchased from Sigma-Aldrich Chem Co. (USA). The state of palladium was determined by X-ray diffractometer (XRD, Xpert MPD), X-ray diffractometer using CuKα radiation. The size and morphology of catalyst were observed using transmission electron microscopy (TEM) EM208S micro-scope with an accelerating voltage of 100 kV. The palladium quantity on the carriers was measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Varian vistampx). Reaction yields were analysed by gas chromatography (GC, Agilent Technologies). The selectivity was also investigated using GC. The FT-IR spectra were recorded on a Jasco-680 (Japan) spectrophotometer in KBr pellets and reported in cm⁻¹. UV-vis spectroscopy was carried out with a JASCO V-550 UV/vis spectrometer. Scanning electron micrographs of the support and catalyst were taken on a SEM EM 3200 instrument. The NMR of the products was recorded on a 500 MHz Bruker-advance in CDCl₃.

General procedure for the preparation of Pd on DNA-functionalized MWCNTs (Pd/DNA@MWCNTs)

Multi walled carbon nanotubes oxide was synthesized using solution of H₂SO₄ and H₂O₂ (0.3 g of the as-received MWCNTs was dispersed in 25 mL of the piranha (mixture of sulphuric acid 96 wt% and hydrogen peroxide 30 wt% in ratio 70 : 30) in a 100 mL round bottom flask equipped with a condenser and dispersion was kept for 5 h. After that, the resulting dispersion was diluted in water and filtered. Then the resulting solid was washed up to neutral pH and the sample was dried in vacuum at 40 °C overnight). The DNA-functionalized MWCNTs have been prepared using Qu's strategy follows the steps described in Scheme 1.⁴⁸ The DNA was heated at 95 °C for 1 h to obtain ss-DNA. The CO₂H-MWCNTs dispersion was mixed with single-stranded DNA (10 mL, 2 mg mL⁻¹), and NaBH₄ was added (8 μL, 75 wt%; NaBH₄/MWCNTs-O); the mixture refluxed at 100 °C for 1 h. Then, the solution was centrifuged in 8000 rpm for 6 min and washed several times with double-distilled water and dried in vacuum overnight at 60 °C to afford the desired catalyst.

General procedure for the Heck reaction of aryl halides with olefins

Aryl halide (1 mmol) was added to a stirred mixture of KOH (3 mmol), olefin (1.5 mmol) and DMF : H₂O (1 : 2, 3 mL), followed by adding 5 mg of catalyst. The mixture was then stirred at 50 °C in an oil bath, and the extent of the reaction was monitored by TLC (thin-layer chromatography, n-hexane/ethyl acetate, 5 : 1) and gas chromatography (GC). After completion of the reaction, the mixture was diluted with dichloromethane and water. The organic layer was washed with brine, dried over anhydrous MgSO₄, and concentrated under reduced pressure. Finally, the product was isolated by chromatography on a column of silica gel (n-hexane/ethyl acetate, 5 : 1) to obtain the corresponding products in 74–96% yields. The products were characterized by comparing their physical properties m.p., IR, ¹H, ¹³C NMR spectra with those found in the literature (ESI data†).

General procedure for the Heck reaction of aryl iodide with 2,3-dihydrofuran

The Pd/DNA@MWCNTs (10 mg) was dispersed in THF (2 mL) by sonication and then aryl iodide (1 mmol), 2,3-dihydrofuran (1.5 mmol) and Li₂CO₃ (1.5 mmol) was added and stirred at 50 °C for 48 h. After completion, the reaction mixture was diluted with hexane (5 mL) and the catalyst was separated by centrifuging (8000 rpm for 6 min). The isomeric ratio of the product was determined by GC before purification. In final, the residue was purified by column chromatography (n-hexane/ethyl acetate, 6 : 1) to afford the products. All the products are known and their structures were secured on the basis of their analytical and/or spectral data, compared with literature data (ESI data†).

Acknowledgements

Financial support from the Isfahan University of Technology (IUT), Iran is appreciated.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11737f

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