In situ generated and stabilized Pd nanoparticles by N²,N⁴,N⁶-tridodecyl-1,3,5-triazine-2,4,6-triamine (TDTAT) as a reactive and efficient catalyst for the Suzuki–Miyaura reaction in water

Abstract

In situ generated Pd nanoparticles in the presence of N²,N⁴,N⁶-tridodecyl-1,3,5-triazine-2,4,6-triamine (TDTAT) were found to be an efficient catalyst for the Suzuki–Miyaura coupling reaction in water. It seems that TDTAT not only acts as a ligand for stabilization of the produced nanoparticles, but also as a surfactant to facilate the reaction in water, and reduces Pd(II) to Pd(0). The TEM analysis of the reaction mixture showed that Pd nanoparticles with an average size of ∼5 nm are produced, which act as an efficient catalyst in the Suzuki–Miyaura coupling reaction.

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Introduction

Over the past few decades since the discovery of the Suzuki–Miyaura reaction, this reaction has become one of the most important reactions for the synthesis of biaryls.¹ Recent progress in the development of new palladium based catalysts has allowed this reaction to be useful with a broad substrate scope and functional group tolerance.² Palladium catalysts derived from phosphine and N-heterocyclic carbene ligands have been particularly effective in the Suzuki–Miyaura reaction.³ Although these ligands are effective, the reactions were often accomplished in polar organic solvents. Furthermore, most of these reactive ligands are oxygen and moisture sensitive, thus performing the reactions in water-based media is restricted.^3,4 In some cases, to do this coupling reaction in water and under air, some ligands have been used whose synthesis requires several steps.⁵ Recently, much attention has been paid to developing nitrogen-based ligands for application in the Suzuki–Miyaura reaction because their use is not only cost effective but they are also stable in water and in the presence of air.⁶ However, most of the introduced nitrogen based ligands are complicated, and in some cases additives should be used.⁷ Extensive efforts have been made towards the use of less expensive and simple nitrogen ligands in cross-coupling reactions.

On the other hand, much effort has been directed towards using water as a solvent for Suzuki–Miyaura reactions.⁸ One of the methodologies to accomplish a reaction in aqueous media is micellar catalysis.⁹ In order to have a nitrogen ligand with capability of application in aqueous media, very recently, we introduced a series of surfactant-like triazine functionalized ligands for application in Pd-catalyzed Heck and Sonogashira reactions in water.¹⁰ Triazine-based ligands have been considered as efficient ligands in transition-metal catalyzed organic reactions due to their many advantages in comparison with other ligands.¹¹ In the current study we would like to introduce a new application of these ligands in the Suzuki–Miyaura reaction in water. The in situ generated Pd nanoparticles in the presence of these triazine-based ligands in water media catalyze the Suzuki–Miyaura reaction. There are many reports on the use of Pd nanoparticles in cross-coupling reactions such as Suzuki–Miyaura, Heck and Sonogashira reactions.¹² The high surface area and activity of Pd nanoparticles make them effective in different coupling reactions.¹³

Triazine based ligands used in this study can be easily prepared in a single step process using readily available and cheap starting materials (Scheme 1).

Scheme 1 Synthesis of surfactant-like triazine-functionalized ligands.

For the synthesis of the above mentioned ligands, 2,4,6-trichloro-1,3,5-triazine (TCT) was reacted with 3 equivalents of either dodecyl substrates. In the structure of ligands, the triazine ring, which originates from TCT, provides the nitrogen center of ligand and the dodecyl carbon chain participates in the surfactant part.

Results and discussion

The synthesized pseudo surfactant triazine-functionalized ligands (TDTAT, TDOT, TDTT, TDTTS, TTDBS) were used in the Suzuki–Miyaura reaction between 4-bromotoluene and phenylboronic acid in water in order to evaluate their catalytic applicability in this protocol. The reaction conditions including water (as the solvent), PdCl₂ (as the Pd source), and K₂CO₃ (as the base) at 80 °C were applied to check the efficiency of these ligands (Scheme 2). In the absence of a ligand, about 20% of the product was produced, while when using triazine based ligands, the reaction yield was increased remarkably, confirming their important role in the progress of the reaction. Furthermore, the obtained results show that TDTAT and TDTT are more efficient than other ligands and 90% and 85% of the coupled product was obtained in their presence, respectively.

Scheme 2 The Suzuki reaction between 4-bromotoluene and phenylboronic acid in the presence of different triazine based ligands.

It was interesting to know which part of TDTAT, as the most efficient ligand, is effective in the Suzuki–Miyaura reaction under the optimized reaction conditions. In this regard, different fragments of TDTAT, including melamine¹⁴ and dodecyl amine,¹⁵ were used instead of TDTAT in the model reaction. Comparison of the obtained results shows that TDTAT itself is more efficient as a ligand in the Suzuki–Miyaura reaction than its fragments (Scheme 3).

Scheme 3 Effect of TDTAT and its fragments in the Suzuki–Miyaura reaction in water.

Since the carbon chain connected to the triazine ring can have a significant effect on the efficiency of the TDTAT ligand, its length was changed by using different amines and three new ligands were synthesized (Scheme 4). The application of these ligands in the Suzuki–Miyaura reaction showed the important role of the carbon chain as the surfactant part of the ligand. It was observed that by decreasing the carbon chain length in the triazine-based ligands, the efficiency of the ligand in the Suzuki–Miyaura reaction was decreased. As a result, TDTAT was recognized as the most efficient ligand in the Suzuki–Miyaura reaction among the studied ligands (Scheme 4).

Scheme 4 Effect of carbon chain in the ligands on Suzuki–Miyaura reaction of 4-bromotoluene in water.

After selection of TDTAT as the most appreciate ligand for this transformation, other reaction parameters were studied in order to obtain the optimum conditions. Different bases, and also the required amount of ligand and Pd, were studied and the obtained results are shown in Table 1.

Table 1 Optimization of Suzuki–Miyaura reaction using TDTAT ligand^a

Entry	TDTAT (w%)	Base (equiv.)	Time (h)	Yield^b (%)
a Reaction conditions: 4-bromotoluene (1 mmol), phenylboronic acid (1.2 mmol), H2O (3 mL), PdCl2 (1.5 mol%).b Isolated yield.c Reaction was performed at room temperature.d Reaction was accomplished under reflux conditions.e 1.2 mol% Pd was used.f 1.0 mol% Pd was used.g 0.5 mol% Pd was used.
1	3	K2CO3 (2)	12	90
2	3	NaOH (2)	12	85
3	3	NaOH (3)	12	88
4	3	K3PO4 (2)	12	81
5	3	Cs2CO3 (2)	12	83
6	3	Et3N (2)	12	80
7	3	K2CO3 (2.5)	12	91
8	3	K2CO3 (2)	24	10^c
9	3	K2CO3 (2)	6	92^d
10	2	K2CO3 (2)	12	78^e
11	3	K2CO3 (2)	16	83^f
12	3	K2CO3 (2)	24	75^g
13	4	K2CO3 (2)	12	89

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Some bases, including K₂CO₃, Et₃N, NaOH and Cs₂CO₃, were tested in the model reaction and K₂CO₃ was selected as the best one (Table 1, entries 1–6). Furthermore, 2.0 mmol of base was sufficient for this reaction (Table 1, entry 7). Different temperatures were also checked and 80 °C was selected as the most suitable one (Table 1, entries 8 and 9). Then, the amount of ligand and Pd were optimized and 1.2 mol% of Pd and 3.0 w% of ligand were sufficient to perform the reaction (Table 1, entries 8–14). The optimized reaction conditions for the Suzuki–Miyaura reaction between 4-bromotoluene and phenylboronic are shown in Scheme 5.

Scheme 5 Optimized conditions for the Suzuki–Miyaura reaction of 4-bromotoluene and phenylboronic acid in the presence of TDTAT ligand.

The optimized conditions were then applied to other aryl electrophiles. It was observed that this methodology is efficient for aryl halides (X = Cl, Br, I) and also for other aryl C–O electrophiles, such as tosylates and triflates (Table 2). The amount of homocoupling product was also investigated. The obtained results showed that a negligible amount of biphenyl was produced during the reaction, representing the good applicability of this catalyst system for the synthesis of biaryls.

Table 2 Using different aryl electrophiles in the Suzuki–Miyaura reaction^a

Entry	X	Time (h)	Yield 3c^b (%)	Yield 3a^b (%)
a Reaction conditions: 4-bromotoluene (1 mmol), phenylboronic acid (1.2 mmol).b GC yield.
1	Cl	24	60	4.5
2	Br	12	90	2.3
3	I	6	92	1.8
4	OTs	24	65	3.1
5	OTf	12	88	2.4

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The scope of this methodology was evaluated in the Suzuki–Miyaura reaction of other substrates and the results are shown in Scheme 6. As shown, this methodology is applicable for different aryl halides with both electron-donating (3b–e) and electron-withdrawing groups (3i,j). (4-Bromophenyl)methanol was used as a coupling partner and biaryl 3f was synthesized in good yields (hydroxy group remained unchanged). Bihalide substrates were also used and selectivity was observed between reactive halide (compounds 3g,h). 5-Bromopyrimidine was also tested under the optimized conditions and compound 3k was produced in 85% yield, demonstrating the applicability of heterocyclic substrates in this methodology. (4-Fluorophenyl)boronic acid also resulted in the corresponding products in good yields (3l–o). Terphenyl (3p)^8f was synthesized in 90% isolated yield using a double coupling reaction between 1,4-diiodobenzene and phenylboronic acid. The reaction between 1-(benzyloxy)-4-bromobenzene and phenylboronic acid resulted in the production of compound 3r in 82% yield. The structural diversity of this reaction was further increased using an aryl halide containing a xanthene moiety, leading to the formation of 3s in 80% yield. Overall, this methodology is suitable for the synthesis of diverse biaryls through the coupling reaction of different aryl electrophiles and arylboronic acids.

Scheme 6 Suzuki reaction of different aryl halides, tosylates and triflates with arylboronic acids under the optimized conditions. Reactions conditions: aryl electrophile (1.0 mmol), arylboronic acid (1.2 mmol), K₂CO₃ (2.0 mmol), H₂O (3.0 mL). Yields are isolated.

Formation of the emulsion droplets in the reaction mixture was established by optical microscopy (Fig. 1). The optical image of dissolved TDTAT in water (Fig. 1a) shows the formation of the material in separate form and different shapes. It seems that the particles of TDTAT are orientated in colloidal forms in water. After completion of the reaction, the reaction mixture was analyzed by optical microscopy (Fig. 1b). The emulsion droplets containing black species were observed in this image. The emulsion droplets for TDTAT ligand in the reaction mixture are in the micrometer range (1–5 μm). The black species are Pd particles, which are dispersed in good manner. It seems that these emulsion droplets containing Pd(0) nanoparticles can act as the reaction reactor in water.

Fig. 1 Optical micrograph of (a) TDTAT ligand in water and (b) reaction mixture of Suzuki–Miyaura reaction in the presence of TDTAT ligand.

Fig. 2 UV-visible spectra of Pd(II) and produced Pd(0) in the presence of TDTAT ligand.

The UV spectrum of a mixture of PdCl₂/TDTAT/H₂O after heating at 80 °C shows the disappearance of the band around 304 and 420 nm, indicating the conversion of Pd(II) to Pd(0) under these conditions (Fig. 2).

In addition, the reaction mixture was analyzed by TEM image and the obtained results show the formation of Pd nanoparticles in the average range of 5 nm (Fig. 3). The TDTAT ligand can coordinate with Pd nanoparticles through nitrogen atoms to stabilize them electrostatically and prevents the aggregation of the produced Pd(0) nanoparticles sterically via the dodecyl chain.

Fig. 3 TEM image of the reaction mixture after completion of the reaction from two different positions and magnifications.

It seems that the TDTAT ligand in the presence of water can provide good interactions between its non-polar parts and polar parts with water to produce droplets that can possibly act as reaction reactors. In Fig. 4, we proposed a plausible scheme for the interactions between TDTAT molecules in water.

Fig. 4 Proposed interaction between TDTAT molecules in water creating the reaction reactors and providing suitable conditions for stabilization of produced Pd(0) nanoparticles.

The reusability of this catalyst system in the model reaction was also checked and it was reusable for at least 5 more times (Fig. 5).

Fig. 5 Recycling of catalyst system for reaction between 4-bromotoluene and phenylboronic acid under optimized conditions.

This issue also revealed that the generated Pd nanoparticles are efficiently stabilized by TDTAT so that the catalytic activity remained approximately unchanged after 5 uses.

Conclusions

In conclusion, we have developed an efficient methodology for the Suzuki–Miyaura reaction with water as the solvent using a surfactant-like triazine functionalized ligand. In the presence of N²,N⁴,N⁶-tridodecyl-1,3,5-triazine-2,4,6-triamine (TDTAT) as the ligand, Pd(0) nanoparticles in the range of 5 nm were produced to act as the active catalyst in the Suzuki–Miyaura coupling reaction of aryl halides as well as tosylates and triflates. The TDTAT ligand not only provides efficient conditions (production of emulsion droplets in the micrometer range) to accomplish the reaction in water but also facilitates the stabilization of in situ generated palladium nanoparticles during the reaction progress. The TDTAT ligand prevents aggregation of Pd nanoparticles electrostatically and sterically through both the triazine ring and its dodecyl chain. The Suzuki–Miyaura reactions of different substrates were checked under the optimized conditions and a range of biaryl compounds were obtained in good to excellent yields.

Experimental section

Chemicals were purchased from Merck and Aldrich chemical companies. All the chemicals and solvents were used as received without purification. For recording ¹H NMR and ¹³C NMR spectra we used a Bruker Avance DRX (250 MHz), and samples were dissolved in pure deuterated DMSO-d₆ or CDCl₃ with tetramethylsilane (TMS) as the internal standard. FT-IR spectroscopy (Shimadzu FT-IR 8300 spectrophotometer) was employed for characterization of the compounds. Transmission electron microscopy (TEM) was carried out using a TEM apparatus (CM-10-Philips, 100 kV). Melting points were determined in open capillary tubes in a Barnstead Electrothermal 9100 BZ circulating oil melting point apparatus. The reaction monitoring was accomplished by TLC on silica gel PolyGram SILG/UV254 plates.

Synthesis and characterization of ligands

The procedure for synthesis of other ligands and their spectral data are reported in our previous work.¹⁰

Synthesis of N²,N⁴,N⁶-tributyl-1,3,5-triazine-2,4,6-triamine (TBTAT). Into a canonical flask (50 mL), a mixture of butan-1-amine (3.1 mmol), TCT (1.0 mmol), and KOH (3.0 mmol) was stirred in THF (10 mL) at room temperature overnight. After completion of the reaction, as indicated by TLC, the mixture was filtered and washed exhaustively with CH₂Cl₂. The solvent was removed under a vacuum. Recrystallization from ethanol gave the product as a white solid. ¹H NMR (250 MHz, CDCl₃): δ (ppm) = 0.97 (t, J = 5 Hz, 9H), 1.27–1.40 (m, 6H), 1.53–1.63 (m, 6H), 3.31–3.45 (m, 6H), 5.42 (t, J = 50 Hz, 3H). ¹³C NMR (62.5 MHz, CDCl₃): 14.1, 22.2, 29.7, 41.4, 163.5. Anal. calcd for C₁₅H₃₀N₆: C, 61.19; H, 10.27; N, 28.54. Found: C, 61.10; H, 10.22; N, 28.46.

Synthesis of N²,N⁴,N⁶-trioctyl-1,3,5-triazine-2,4,6-triamine (TOTAT). The procedure is similar to that used for TBTAT. ¹H NMR (250 MHz, CDCl₃): δ (ppm) = 0.97 (t, J = 5 Hz, 9H), 1.33 (brs, 30H), 1.61–1.72 (m, 6H), 3.32–3.46 (m, 6H), 5.42 (t, J = 50 Hz, 3H). ¹³C NMR (62.5 MHz, CDCl₃): 14.1, 22.6, 26.9, 28.9, 29.3, 29.6, 31.8, 41.4, 163.4. Anal. calcd for C₂₇H₅₄N₆: C, 70.08; H, 11.76; N, 18.16. Found: C, 70.01; H, 11.65; N, 18.07.

General procedure for the Suzuki–Miyaura reaction using the TDTAT ligand. A mixture of aryl halide/tosylate/triflate (1 mmol), arylboronic acid (1.2 mmol), K₂CO₃ (2.0 mmol), PdCl₂ (1.2 mol%, 0.021 g) and TDTAT/water (3.0 mL, 3 w% TDTAT) was added to a conical flask (10 mL) and stirred at 80 °C. The reaction was monitored by TLC analysis. Stirring was continued until the consumption of the starting materials based on the reaction time (Scheme 6). After completion of the reaction, the mixture was cooled down to room temperature. The organic compound was extracted with ethyl acetate (3 × 5 mL) from the aqueous layer and dried over anhydrous Na₂SO₄, filtered, and concentrated under vacuum. The organic mixture was then purified by silica gel column chromatography employing n-hexane/ethyl acetate as the eluent, affording the pure product.

Typical procedure for the large-scale Suzuki–Miyaura reaction using TDTAT ligand. A flask equipped with a magnetic stirring bar was charged with 4-bromotoluene (10.0 mmol, 1.71 g), phenylboronic acid (12.0 mmol), K₂CO₃ (10.0 mmol), PdCl₂ (1.2 mol%, 0.21 g) and TDTAT/water (30 mL, 30 w% TDTAT). The mixture was heated in an oil bath at 80 °C for 15 h. After completion of the reaction monitored by GC or TLC analysis, the reaction mixture was cooled to room temperature. The organic compound was extracted with ethyl acetate from the aqueous layer and dried over anhydrous Na₂SO₄, filtered, and concentrated under vacuum. The organic mixture was then purified by silica gel column chromatography using n-hexane/ethyl acetate as the eluent to obtain the corresponding pure coupled product. Yield: 84% (1.41 g). ¹H NMR (250 MHz, CDCl₃): δ (ppm) = 2.39 (s, 3H), 7.25 (d, J = 5 Hz, 2H), 7.32–7.62 (m, 7H). ¹³C NMR (62.5 MHz, CDCl₃): 21.1, 127.0, 127.1, 128.7, 129.5, 137.1, 138.3, 141.1.

Acknowledgements

The authors would like to acknowledge the support of this work by Shiraz University research council and the grant from the national elite foundation of Iran.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Spectral data and copies of ¹H, ¹³C NMR for synthesized compounds. See DOI: 10.1039/c5ra24120k

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