Biopolymer–metal complex wool–Pd as a highly active catalyst for Suzuki reaction in water

Abstract

A heterogeneous biopolymer complex wool–Pd catalyst has been applied in water-mediated coupling reactions of aryl iodides and bromides with arylboronic acid. The results showed that the reactions could be conducted in neat water under atmospheric conditions with water-insoluble or even solid aryl halides. More importantly, the catalyst system has the advantages of excellent yields, environmental friendliness, and catalyst recyclability.

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Introduction

The Suzuki–Miyaura cross-coupling reaction is one of the preeminent methods for the carbon(sp2)–carbon(sp2) bond formation.¹ In recent years, this transformation has been extensively used in numerous synthetic ventures,² and also various modifications have been introduced.³

Minimization of chemical waste from the reaction mixture, of which 80% is estimated to be solvents,⁴ is a constant challenge as environmental concerns are increasingly brought into focus. Therefore, the endeavour to replace volatile organic solvents in organometallic catalysis by alternative more practical and environmentally friendly solvents must be a priority.⁵ Although there are some examples reported concerning palladium-catalyzed cross-coupling reactions of aryl halogenates with aryl boronic acids using purely aqueous reaction conditions, most of these reactions were conducted in organic solvents.⁶ Several sulfonated phosphine derivatives and water-soluble ligands have been prepared and used in cross-coupling reactions conducted in water–organic biphasic solvent systems.⁷ These homogeneous catalyst systems suffer from some issues in terms of the separation and recycling of the catalysts. Additionally, they induce contamination of the ligand residue in the products. Therefore, the development of polymer-supported and insoluble transition metal catalysts has attracted a great deal of attention in organic chemistry.⁸ The functionalized Wang resin represents the most commonly used synthetic support, to which phosphines, amines, carbenes etc. were successfully immobilized, and some promising results were achieved.⁹ However, problems still existed. For instance, the requirements of several steps to synthesize supporter itself with appropriate bore diameter, cross-linking degrees, the lower loading of palladium and severe catalyst leaching, and the poor swell property prevents the reactions from taking place in the neat aqueous phase.

In previous research, some natural biopolymers, such as chitosan,¹⁰ cellulose,¹¹ wool,¹²etc. have been used as efficient polymer supports in several important palladium-catalyzed transformations. Among them, we have focused on the natural animal fibers (wool) due to the facts that the animal fibers themselves could be used as a solid phase ligand with no need for further functionalization. Furthermore, the loaded palladium particles could be distributed evenly on the surface of the fibers because of the structurally ordered amino acid chains, so the formation and aggregation of Pd-black could be prevented, which was regarded as the most critical problem to the performance of palladium-catalyzed conversions. In our previous research,¹³ we reported an effective catalyst system composed of wool–palladium (wool–Pd) complex in aqueous media with very small amount of PEG-400 for the Heck reaction using NaAc as a base. The results showed that aryl bromides could carry out the coupling reaction with a variety of alkenes at 80 °C in aqueous media under atmospheric conditions. Encouraged by these results, we have recently applied the catalyst system to the Suzuki–Miyaura cross-coupling reaction.

Herein, we report an effective bio-polymer-supported palladium catalyst in neat water for the Suzuki reaction using K₂CO₃ as a base. The reaction can be conducted under atmospheric conditions without any specific protection, and no phase-transfer agents are necessary. Moreover, the catalyst is easy to separate from the reaction mixture by simple filtration. More importantly, the cheap catalyst was stable, showed negligible metal leaching, and retained good activity for at least three successive runs without any additional activation treatment.

Results and discussion

Synthesis and characterization of the catalyst

The synthesis of the catalyst can be found in our previous research.¹³Fig. 1 shows the powder XRD patterns of the three samples of supported Pd catalysts. The peaks at 2θ values of 40° and 42° in the spectrum of the wool–Pd(0) are related to the (111) and (200) reflection of crystalline Pd(0). This sample was obtained from the treatment of the newly prepared wool–Pd(II) with benzyl alcohol at 70 °C for 6 h. Of note is that in the fresh wool–supported PdCl₂ catalyst, the weak reflections of the crystalline Pd(0) also had peaks at the corresponding 40° and 42°, which was possibly due to the existence of thiol (–SH) groups in the wool fibers. Obviously, after several recycling uses, the representative 2θ values of crystalline Pd(0) also could be observed. The average particle sizes of reused catalyst are almost 60 nm (see ESI).†

Fig. 1 XRD patterns of the three samples of supported Pd catalysts.

The binding energies of wool, PdCl₂ and wool–Pd complex were obtained by XPS analysis (Table 1). The binding energy of the Pd_3d 3/2 and Pd_3d 5/2 in the wool–Pd complex increase 0.75 eV and 0.87 eV, respectively; the change of Pd_3d binding energy means the decrease of its electron density. Little change of the binding energy of the Cl_2p was observed, this means there are unreactive Cl though the chemical bond formed in this process. There are two kinds of nitrogen-containing group, –NH–CO– and –NH₂ in wool, and their N_1s binding energies are different. Such data in wool–Pd are also differed from those in wool. The difference of the N_1s binding energies between –NH–CO– in wool and –NH–CO– in wool–Pd is 0.32 eV, and that between –NH₂ in wool and –NH₂ in wool–Pd is 0.55 eV. In the same way, there are three kinds of S-containing group, –SO₃H, –SH and –S–S– in wool, and their S_2p binding energies are different. The difference of S_2p binding energy between –SO₃H in wool and –SO₃H in wool–Pd is only 0.31 eV, that between –SH in wool and –SH in wool–Pd is 0.8 eV and that between –S–S– in wool and –S–S– in wool–Pd is 0.7 eV. The difference of O_1s binding energy between wool and wool–Pd also could not be detected. These results show that coordination or ionic bonds are formed by the connection of N atoms (in –NH₂) and S atoms (in –SH and –S–S–) with Pd atoms in the wool–Pd complex. (Scheme 1)

Table 1 XPS data for the wool, wool–Pd complex and salt PdCl₂

XPS Peaks		Binding energy (eV)			ΔEb (eV)
		PdCl2	Wool	Wool–Pd complex
Pd3d	Pd3d 3/2	342.80		343.55	+0.75
	Pd3d 5/2	337.43		338.30	+0.87
Cl2p		198.55		198.80	+0.25
N1s	NHCO		400.05	399.80	−0.32
	NH2		400.37	399.25	−0.55
S2p	SO3H		168.18	167.87	−0.31
	S–S		165.05	165.75	+0.70
	SH		163.80	163.00	−0.80
O1s			531.93	532.12	+0.19

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Scheme 1 The possible structure of wool–Pd.

Suzuki reaction in water

To explore the optimal reaction conditions, the coupling of bromobenzene (1a) with phenylboronic acid (1b) was determined as the model reaction (Table 2). The preliminary results indicated that temperature and base had a significant effect on the coupling reaction. The lower temperature disfavored the transformation, (Table 2, entries 1–2). Higher yields of cross-coupling product could be achieved at elevated temperature (75 °C), however, over-heated reaction mixture led to the self-coupling of phenylboronic acid to form biphenyl.¹⁴ (Table 2, entry 4). Under the optimized reaction temperature, several bases were examined, such as NaAc, NaOH, Na₂CO₃, K₂CO₃ and pyridine. Notably, the most commonly used base K₂CO₃ was effective in providing the corresponding cross-coupling product with better yield 99% (Table 2, entry 8). We also found that an appropriate amount of catalyst was necessary for the transformation, with 0.55% catalyst, 1c was obtained in 99% isolated yield. Minimization of catalyst amount to 0.45%, 1c was achieved with 85% yield and 188 TON value. Notably in the aqueous phase, the gradually formed 1c precipitated during stirring. After consumption of 1a, 1c was colleted by simple filtration, obviating the need for additional chromatograph separation steps. Finally, the general reaction condition is optimized as bromobenzene (0.2 mmol), phenylboronic acid (0.22 mmol), K₂CO₃ (0.3 mmol), 1 mg of wool–Pd complex catalyst (0.55 mol%), stirred in 5 mL water at 75 °C under atmosphereic conditions.

Table 2 Optimization of cross-coupling reaction of bromobenzene and phenylboronic acid^a

Entry	Base	T/°C	Cat. (mol%)	Yield^b (%)	TON (mol mol^−1)
a Reaction conditions: Bromobenzene (0.2 mmol), phenylboronic acid (0.22 mmol), K2CO3 (0.3 mmol), water (5 mL), under atmospheric conditions, time (5 h). b Yield according to GC.
1	NaAc	35	0.55	8	15
2	NaAc	55	0.55	26	47
3	NaAc	75	0.55	53	96
4	NaAc	85	0.55	43	78
5	Pyridine	75	0.55	21	38
6	NaOH	75	0.55	78	141
7	Na2CO3	75	0.55	81	147
8	K2CO3	75	0.55	99	180
9	K2CO3	75	0.45	85	188
10	K2CO3	55	0.11	77	140
11	K2CO3	25	0.11	18	33

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To demonstrate the scope of the catalyst system, different aryl halides were cross-coupled with phenylboronic acids (Scheme 2). The results showed that the catalytic system was applicable to various aryl iodides and bromides and tolerant to a broad range of functional groups, including of H, NO₂, NH₂, OR, OH, COMe and CHO (Scheme 2, 1c–11c). Using 0.055 mol% catalyst, the coupling of electron-donating aryl iodides and phenylboronic acids provided the corresponding biaryl products in >91% yields. A temperature of 75 °C was essential for the successful coupling reaction of electron-withdrawing group (–NO₂) substituted substrates; the excellent yield of 96% (4c) and 98% (8c) were obtained within 5 h. When different temperatures were screened, 2c and 4c could be produced at even lower temperatures of 55, 45 and 25 °C, however, longer reaction time was required. At 25 °C, 2c and 4c were achieved in 10% and 20% isolated yields respectively within 5 h. Of particular importance was that the coupling of substrates containing hydrophilic functional groups, which are insoluble in organic solvents and are present in many pharmaceutically interesting compounds, were particularly efficient partners, for instance, 2c, 5c, 6c and 9c resulted in 94, 92, 94 and 91% yields, respectively. Of note, the insoluble biaryl products precipitated almost quantitatively in the aqueous phase, obviating the need for additional separation and protection/deprotection steps. Aryl bromides with diverse electron-donating and electron-withdrawing substituents delivered the cross-coupled products in high yields using 0.55 mol% catalyst at 75 °C for longer reaction time. Relatively, electron deficient substrates performed better than those with electron-rich functional groups. Typically, 3c and 11c were isolated in 90% yields, 5c, 9c, 10c and 16c were given with 76, 79, 69 and 78% yields, respectively. Successful application to hindered substrate combinations, however, has been scarcely reported. To ascertain whether our palladium catalyst system could address this limitation, in a similar fashion, the hindered substrate of aryl bromides fragments were evaluated against different arylboronic acids. 2,4-Dimethoxy aryl bromide was successfully coupled to form an ortho-substituted biaryl in an excellent yield with only 0.55 mol% Pd (Scheme 2, 13c and 19c). Obviously, the electron-rich arylboronic acids performed well under the optimized reaction conditions, giving the corresponding products with excellent yields. For the more electron-deficient substrates, 4-Cl arylboronic acid predominantly coupled to CHO, OH, Me, OEt, NO₂, COMe substituted aryl bromides with good to excellent yields. Due to the ubiquity of heterocycles in biologically active molecules we focused our attention on using heteroaryl halides as substrates. Typically, high catalyst loading is required to overcome catalyst deactivation through heteroatom coordination. With 1 mol% of wool–Pd, pyridine, thiofuran substrates all afforded their cross-coupled products in moderate yields (14c/65%, 15c/42%, 20c/72%, 21c/50%).

Scheme 2 Scope of the Suzuki reactions with wool–Pd complex catalyst.^a

It has been known that hydrophobic interactions can have a significant influence on organic reactions.¹⁵ It was discovered in several organic reactions in water that such reactions often proceed with much higher rates than in organic solvents. The accelerating effect of water has been ascribed to a number of factors, including the hydrophobic effect as well as hydrogen bonding between water molecules and reactants. In our reaction protocols, the difference between the non-hydrophobic and hydrophobic conditions was remarkable (Table 3). The isolated yields of 11c were significantly lower for reaction in THF or THF–water than in the aqueous phase, despite the aqueous reactions being heterogeneous. We reasoned that in water hydrophobic surfaces associate strongly as a result of the tendency of water to exclude nonpolar species and thus minimize the Gibbs energy of solvation.¹⁵ In other words, reactions in water may be predisposed to favor transition states that optimize hydrophobic interactions. The great efficiency of the water-modulated reaction attributed to the hydrophobic effect because of the biologically lipophilic fibers-wool.

Table 3 Hydrophobic effect in Suzuki reaction

THF : H2O (v : v)	H2O	1 : 1	2 : 1	3 : 1	5 : 1	10 : 1	THF
Yield (%)	90	89	64	38	34	3	n.r.

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THF, DMSO and toluene are the most commonly used reaction media in Suzuki coupling reactions.¹⁶ However, in our disclosed catalyst system such non-protic solvents with better solubility, higher boiling point and stronger polarity disfavor the transformation (Fig. 2). A further solvent screening experiment revealed that except for water, protic solvents, such as MeOH and EtOH are very effective in promoting the C–C bond formation. The observations raise an interesting question. Could the cheap and environmentally friendly alcohols be used as solvent? It is well known that alcohols are used to act as receptors for halide anions, and in particular MeOH, EtOH and H₂O are good hydrogen bond donors (HBD) as judged by their high E^N_T values.¹⁷ This belief was strengthened by a study of the coupling reaction in more than 10 solvents, which demonstrated that hydrogen bond donating, protic solvents indeed accelerate C–C bond formation. As shown in Fig. 2, the higher the E^N_T values, the faster this reaction becomes. Hydrogen bond donors always play an essential role in the arylation of aromatic halides, which probably promotes the dissociation of the intermediate I, resulting from the oxidative addition of halide with palladium species and thus formation of intermediate II or III. After liberation from I, X⁻ serves as an electron-pair donor (EPD) and is stabilized by HBDs, such as H₂O, MeOH and EtOH (Fig. 3).

Fig. 2 The effect of solvent on Suzuki reaction.

Fig. 3 An equilibrium of Pd(Ar)(X) in Suzuki reaction.

We also found that the decrease in the arylation rate with increasing bromide concentration, which suggested that the Br⁻ generated from each arylation cycle must be effectively scavenged. The trapping of the bromide anions most likely arises from possible hydrogen bonding between ROH and Br⁻, rendering the equilibrium in favor of the Pd(II) cations (Fig. 3, II or III). Evidence in support of this equilibrium comes from reactions in the presence of added KBr. As shown in Fig. 4, the arylation of phenylboronic acid with 4-bromobenzaldehyde was notably slower when 1 equiv of Br⁻ was introduced into the reaction system. With the increase of volume ratio of THF, the yields of 11c were reduced gradually. This is consistent with there being an equilibrium, which is shifted towards the left on addition of the halide ions (Fig. 3). Similar observations have been made in arylation reactions such as the Heck reaction. Thus, we reasoned that introduction of a potential hydrogen bond donor could enhance the reaction rates. Especially, with the property of hydrophobic effect and as also a hydrogen bond donor, water is screened as the most effective reaction media for the Suzuki coupling reaction.

Fig. 4 The effect of KBr on Suzuki reaction.

Recycling of the catalyst

An important point concerning the use of heterogeneous catalysts is its lifetime, particularly for industrial and pharmaceutical applications of the Suzuki reaction. For the recycling study, the reaction was performed with 1a and 1b, maintaining the same reaction conditions as described above. The successive operations were carried out as follows: after the end of the reaction, the mixture was cooled down to room temperature and filtrated. Then the filtrated aqueous phase and the used wool–Pd were added to the next run. TLC monitored the reaction progress, and the conversion and product selectivity were determined using GC analysis. This indicated that the catalyst can be recycled more than 3 times, and the total TON value is 680 (Fig. 5).

Fig. 5 Recycling test of wool–Pd complex catalyst.

Experimental section

Apparatus

All NMR spectra are recorded on MERCURY (400 MHz for ¹H NMR, 100 MHz for ¹³C NMR) spectrometers; chemical shifts are expressed in ppm (δ units) relative to TMS signal as internal reference in CDCl₃. Gas chromatography (GC) analysis was performed on a Shimadzu GC-2010 equipped with a 15 m × 0.53 mm × 1.5 μm RTX-1 capillary column and an oxyhydrogen flame detector. X-ray photoelectron spectroscopy analysis (XPS) measurements were recorded on a PHI5702 photoelectron spectrometer. Scanning electron microscopy (SEM) was performed with field-emission scanning electron microscopy (FESEM, JSM-6701F, Japan) at an accelerating voltage of 5.0 kV. X-ray diffraction (XRD) of samples was performed on a diffractometer (D/Max-2400, Rigaku) advance instrument using Cu-Kα radiation (k = 1.5418 Å) at 40 kV, 100 mA.

General experimental procedure for the Suzuki reaction

In air, aryl halide (0.2 mmol), arylboronic acid (0.22 mmol), K₂CO₃ (0.3 mmol), 5 mL of distilled water, and 2 mg of wool–Pd complex catalyst (Pd 0.0022 mmol) were combined in a 10 mL round-bottomed flask. The reaction mixture was magnetically stirred and the temperature was maintained at 75 °C with an oil bath. Reaction progress was monitored by TLC. After reaction was completed, the reaction mixture was cooled to room temperature and filtrated. The solid was washed with water (3 × 5 mL), then with EtAc (3 × 5 mL), and the organic phase was combined. The catalyst was separated by filtration, washed with water, and dried in vacuum. The combined organic layer was dried with anhydrous MgSO₄, and the solvent was removed under reduced pressure to give the product.

Conclusion

In summary, we have developed an efficient catalyst system for Suzuki–Miyaura cross-coupling reactions. Our catalyst system had the following advantages: (1) using a natural biopolymer as a solid ligand and catalyst support, and the catalyst was synthesized via a facile method; (2) the reaction conditions were mild and neat water was used as the solvent; (3) the catalyst was reusable and easy to separate; (4) the product could be obtained by simple filtration. Further efforts to study the detailed mechanism and extend the application of the system to other coupling transformations are under way in our laboratory.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy20126g

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