Ligand-free Hiyama cross-coupling reaction catalyzed by palladium on carbon

Abstract

A ligand-free Pd/C-catalyzed Hiyama cross-coupling reaction has been developed. A variety of aryl bromides were efficiently cross-coupled with aryltriethoxysilanes with only 0.5 mol% of 5% Pd/C. The protocol would be practical for use as an economical synthetic method for the construction of biphenyl derivatives.

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Introduction

The Hiyama cross-coupling reaction,¹ a palladium-catalyzed carbon–carbon bond formation between organosilanes and organohalides or their equivalents, is drawing increasing attention as one of the useful synthetic methods for the construction of asymmetrical biphenyls or multi-substituted alkenes that are structural components of various functional materials, such as pharmaceuticals, agrochemicals, and natural products. The stability and low toxicity of organosilanes^2–4 and the facile conversion of silicon waste generated with reaction which progress to harmless SiO₂ by incineration⁵ have made the Hiyama coupling attractive from environmental and user-friendly points of view.

Hiyama coupling has generally been achieved by the combined use of a homogeneous palladium catalyst and a phosphine ligand.^6,7 Recently, the phosphine ligand-free cross-coupling reactions have enthusiastically been investigated because of their toxicity and cost burden.⁸ Only a few ligand-free Hiyama coupling reactions using a homogeneous palladium catalyst, such as palladium(II) chloride (PdCl₂),^9apalladium(II) acetate [Pd(OAc)₂],^9b and palladacycles,¹⁰ or heterogeneous palladium nanoparticles which are not commercially available, have been reported in the literature.¹¹ Heterogeneous catalysts have recently been recognized as convenient alternatives in organic synthesis due to their stability and ease of handling.¹²Palladium on carbon (Pd/C), a widely used heterogeneous catalyst for hydrogenation,¹³ has, in particular, been employed for various kinds of reactions including carbon–carbon, carbon–nitrogen, and carbon–oxygen bond formations¹⁴ due to its easy access and low cost. Recently, Novák and co-workers¹⁵ and our group¹⁶ independently reported the effective use of Pd/C and phosphine ligands for the Hiyama coupling. This paper describes the first ligand-freePd/C-catalyzed Hiyama cross-coupling reaction between a variety of aryl halides and aryltriethoxysilanes.

Results and discussion

During the course of our previous study on the Pd/C-catalyzed Hiyama cross-coupling using a phosphine ligand [(4-FC₆H₄)₃P],¹⁶ it was surprisingly found that the 10% Pd/C-catalyzed^17,18 coupling reaction between 4-bromonitrobenzene and phenyltriethoxysilane in toluene proceeded without ligands to give the corresponding 4-nitrobiphenyl in 65% yield (Table 1, Entry 1). The reaction progress under ligand-free conditions was greatly affected by the solvent. The reaction hardly took place in DMF or MeCN (Entries 2 and 3), and the desired 4-nitrobiphenyl was obtained in moderate to good yields in EtOH, THF, 2-butanone, o-xylene, and toluene (Entries 1 and 4–7). TBAF·3H₂O was found to be specifically effective as an activator of the organosilane probably due to the high solubility in toluene (Entry 1), while the reaction never proceeded without a fluoride source or with the use of metal fluorides, such as LiF, KF, and CsF (Entries 8–11).

Table 1 Screening of solvents and fluoride sources for the Pd/C-catalyzed Hiyama coupling reaction^a

Entry	Solvent	F source	Yield (%)^b
a Unless otherwise noted, reactions were carried out using 10% Pd/C (5.0 mol%, 25 μmol), 4-nitrobromobenzene (0.50 mmol), phenyltriethoxysilane (0.75 mmol), the fluoride source (1.0 mmol), and solvent (1.0 mL) at reflux for 24 h. b Determined by ^1H NMR spectroscopy using 1,4-dioxane as an internal standard. c The reaction was carried out at 120 °C.
1	toluene	TBAF·3H2O	65
2^c	DMF	TBAF·3H2O	5
3	MeCN	TBAF·3H2O	14
4	EtOH	TBAF·3H2O	44
5	THF	TBAF·3H2O	52
6	2-butanone	TBAF·3H2O	55
7^c	o-xylene	TBAF·3H2O	58
8	toluene	none	0
9	toluene	LiF	0
10	toluene	KF	0
11	toluene	CsF	0

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During the reaction indicated in Table 1, Entry 1, the generation of a trace amount of 4-ethoxynitrobenzene by the cross-coupling of the ethoxide anion generated from phenyltriethoxysilane with 4-bromonitrobenzene^19,20 was confirmed by ¹H NMR spectroscopy and GC-MS, although such an undesirable reaction was effectively suppressed by the addition of acetic acid (1.5 equiv) as a proton source, and the desired biphenyl was obtained in 77% yield (Table 2, Entry 2). On the other hand, the addition of H₂O (Entry 3) or benzoic acid (Entry 4) was not effective for the reaction progress. Furthermore, the use of 5% Pd/C together with 1.5 equiv of acetic acid instead of 10% Pd/C significantly enhanced the reaction, and the desired 4-nitrobiphenyl was obtained in 81% yield (Entry 6), but the reaction efficiency was reduced by the decrease (Entry 7) or increase (Entry 8) in the amount of acetic acid.

Table 2 Effect of proton sources on the Pd/C-catalyzed Hiyama coupling reaction

Entry	Pd/C^a	additive	additive quantity	Yield (%)^b
a 5.0 mol% of Pd/C was used. b Determined by ^1H NMR spectroscopy using 1,4-dioxane as an internal standard.
1	10%	—	—	65
2	10%	acetic acid	1.5 equiv	77
3	10%	H2O	5.6 equiv (50 μL)	60
4	10%	benzoic acid	1.5 equiv	62
5	5%	—	—	68
6	5%	acetic acid	1.5 equiv	81
7	5%	acetic acid	1.0 equiv	75
8	5%	acetic acid	2.0 equiv	70

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5% Pd/C was indispensable for the reaction progress (Table 3, Entry 1). The usage of 5% Pd/C could markedly be reduced from 5 mol% to only 0.5 mol% with a slight increase in the yield of the desired cross-coupling product presumably due to the suppression of the homo-coupling of the aryl halide (Entries 2–5). Further reduction to 0.1 mol% led to a slight decrease in the reaction efficiency (Entry 6). When the reaction temperature was lowered to 80 °C from reflux conditions (120 °C), the cross-coupling reaction was significantly suppressed and 40% of the starting 4-bromonitrobenzene was recovered (Entry 7).

Table 3 Effect of the dose of 5% Pd/C on the Hiyama cross-coupling reaction

Entry	5% Pd/C (mol%)	Yield (%)^a
a Determined by ^1H NMR spectroscopy using 1,4-dioxane as an internal standard. b The reaction was carried out at 80 °C. Unreacted 4-bromonitrobenzene was recovered in 40% yield.
1	0	0
2	5	81
3	2	80
4	1	88
5	0.5	88
6	0.1	82
7^b	0.5	58

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The cross-coupling reaction between a wide range of aryl halide and aryltrialkoxysilane derivatives was next investigated under the optimized reaction conditions (Table 3, Entry 5). 4-Bromoacetophenone was effectively cross-coupled with phenyltrimethoxysilane (Table 4, Entry 1) and phenyl, p-tolyl-, p-chloro, and p-methoxyphenyltriethoxysilanes to give the corresponding biphenyl derivatives in good to excellent yields (Entries 2–5). Aryl bromides possessing either an electron-withdrawing (nitro, acetyl, cyano, formyl,²¹carboxylic acid, fluoro group) or an electron-donating functionality (methyl or methoxy group), were found to be good substrates for the cross-coupling (Entries 6–14) with rare examples such as 2-bromoanisole (Entry 15). On the other hand, aryl chlorides were poorly reacting substrates under the present reaction conditions (Entries 16 and 17).

Table 4 The Hiyama cross-coupling of various aryl halides and aryltriethoxysilanes

Entry	X	R^1	R^2	Yield (%)^a
a Isolated yield. b Phenyltrimethoxysilane was employed. c 1 M solution of 3-bromoanisole in toluene was used. d 48 h.
1	Br	4–Ac	H^b	87
2	Br	4–Ac	H	81
3	Br	4–Ac	Cl	85
4	Br	4–Ac	Me	84
5	Br	4–Ac	OMe	90
6	Br	4–NO2	H	85
7	Br	4–CN	H	65
8	Br	4–CHO	H	80
9	Br	2–CHO	H	80
10	Br	4–COOH	H	54
11	Br	4–F	H	77
12	Br	4–Me	H	65
13^c	Br	3–OMe	H	66
14^d	Br	4–OMe	H	77
15	Br	2–OMe	H	12
16	Cl	4–NO2	H	7
17	Cl	3–OMe	H	0

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The reuse test of 5% Pd/C using 4-bromonitrobenzene and phenyltriethoxysilane as substrates was examined. 5% Pd/C could be reused until the second run without significant loss of catalyst activity, while the reaction efficiency was drastically reduced in the third run (Table 5).²² The results suggested that the palladium species could be leaching out from the carbon support. The palladium leaching in the reaction media was then measured by an inductively coupled plasma-atomic emission spectrometric analysis (iCAP 6500 Duo, Thermo Scientific). While a very low level of Pd-leaching (0.97% of total Pd amount of the 5% Pd/C) was detected in the filtered reaction media of the 5% Pd/C (5 mol%)-catalyzed cross-coupling reaction between 4-bromonitrobenzene and phenyltriethoxysilane without AcOH (Table 2, Entry 5),²³ 60 ppm of Pd-leaching was observed in the presence of AcOH (Table 3, Entry 5), suggesting that Pd-leaching was facilitated by AcOH.²⁴ To confirm the catalyst activity of the leached palladium species, we prepared two types [(1) and (2)] of filtrates without substrates (4-bromonitrobenzene and phenyltriethoxysilane).²⁵ (1) 5% Pd/C (0.5 mol%) was stirred with AcOH (1.5 equiv) in refluxing toluene under argon for 24 h and the mixture was filtered with a 0.45 μm membrane filter after cooling to afford Filtrate 1. (2) Filtrate 2 was prepared by the pretreatment of 5% Pd/C with AcOH and TBAF·3H₂O (2.0 equiv) in refluxing toluene in a manner similar to Filtrate 1. Although a mixture of 4-bromonitrobenzene and phenyltriethoxysilane in the Filtrates 1 and 2 was heated at reflux for 24 h in the absence of 5% Pd/C, no reaction was confirmed by ¹H NMR spectroscopic analysis (Table 6, Entries 1–3).

Table 5 Reuse test of 5% Pd/C

Entry	Run	Yield (%)^a
a Determined by ^1H NMR spectroscopy using 1,4-dioxane as an internal standard.
1	1	80
2	2	88
3	3	25

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Table 6 Effect of leached palladium species on the present Hiyama cross-coupling reaction

Entry	Filtrate X	Reagents	Yield (%)^a
a Determined by ^1H NMR spectroscopy. b 2 equiv of TBAF·3H2O were further added to the cross-coupling reaction mixture in Filtrate 2.
1	Filtrate 1	AcOH (1.5 equiv) toluene (1.0 mL)	0
2	Filtrate 2	AcOH (1.5 equiv) TBAF·3H2O (2.0 equiv) toluene (1.0 mL)	0
3^b	Filtrate 2	—	0

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Next, we confirmed the catalyst activity of the filtrate with substrates. Since Djakovitch et al. have recently reported a so-called hot-filtration method to confirm the catalyst activity of the leached palladium species,²⁶ the catalyst activities of two types of filtrates prepared by the hot-filtration and filtration after cooling were compared. The 5% Pd/C-catalyzed cross-coupling reaction between 4-bromonitrobenzene and phenyltriethoxysilane in the presence of TBAF·3H₂O (2 equiv) and AcOH (1.5 equiv) was carried out in boiling toluene. After 30 min, the mixture was filtered without cooling with a glass fiber filter (<1.0 μm). The filtrate containing 14% of the desired cross-coupling product (4-nitrobiphenyl) was further heated at reflux for an additional 5.5 h to increase the formation of 4-nitrobiphenyl to 48% yield (Fig. 1, ■, Hot-filtration), although the reaction efficiency was slightly decreased in comparison to the reaction without filtration (♦, Standard run). Even though the use of the filtrate prepared by the filtration after cooling to rt, the reaction efficiency achieved a level nearly equal (52% yield) to the result of the hot-filtration by refluxing for an additional 5.5 h (▲, Filtration after cooling). These results suggest that (1) the reduced catalyst activity of the reuse test (Table 5) should be attributed to the palladium leaching into the reaction media; (2) coexistence of the substrates, 4-bromonitrobenzene and phenyltriethoxysilane, and acetic acid would enhance the leaching of the palladium species from 5% Pd/C; (3) leached Pd possesses catalyst activity toward the Hiyama cross-coupling reaction; (4) fresh 5% Pd/C should assume a key role as a palladium donor in the present ligand-free Hiyama cross-coupling reaction.

Fig. 1 Time-course study on filtrates after the removal of the Pd/C. Standard run (♦): The reaction of 4-bromonitrobenzene and phenyltriethoxysilane was carried out in refluxing toluene for 6 h without filtration. Hot-filtration (■): 5% Pd/C was removed by filtration without cooling after 0.5 h of the reaction at reflux and the resulting filtrate was refluxed again. Filtration after cooling (▲): 5% Pd/C was removed by filtration after cooling at rt after 0.5 h of the reaction at reflux, and the resulting filtrate was refluxed again up to 6 h.

Conclusions

In summary, we have developed a ligand-free Pd/C-catalyzed Hiyama cross-coupling reaction for an effective preparation method of a variety of biphenyl derivatives. To the best of our knowledge, this is the first application of Pd/C as a catalyst in the absence of ligands for the Hiyama cross-coupling reaction. Because only a small amount of 5% Pd/C (0.5 mol%) is required for the efficient reaction progress, an industrial application of the protocol is expected.

Experimental section

General method

Dry-type 10% Pd/C (K-type) or dry-type 5% Pd/C (K-type) was supplied by the N.E. Chemicat Corporation (Tokyo, Japan). All reagents and solvents were purchased from commercial sources and used without further purification. Flash column chromatography was performed using Silica Gel 60 N (Kanto Chemical Co., Inc., 63–210 μm spherical, neutral). ¹H NMR and ¹³C NMR spectra were recorded on a JEOL AL 400 (400 MHz for ¹H NMR spectroscopy and 100 MHz for ¹³C NMR spectroscopy) or ECA 500 spectrometer (500 MHz for ¹H NMR spectroscopy and 125 MHz for ¹³C NMR spectroscopy). Chemical shifts (δ) are expressed in ppm and are internally referenced (0.00 ppm for TMS for CDCl₃ for ¹H NMR spectroscopy and 77.0 ppm for CDCl₃ for ¹³C NMR spectroscopy). Mass spectra were obtained on a JEOL JMS-SX102A or a JEOL T100TD instrument. GC-MS analysis of the side reaction was conducted by a JEOL Jms Q1000GC Mk2.

General procedure for the ligand-free hiyama cross-coupling reaction with heterogeneous Pd/C as the catalyst

A mixture of the aryl halide (500 μmol), the aryltriethoxysilane (750 μmol), TBAF·3H₂O (316 mg, 1.0 mmol), 5% Pd/C (5.4 mg, 2.5 μmol) and acetic acid (43.0 μL, 750 μmol) in toluene (1.00 mL) was stirred at 120 °C under Ar. After the consumption of the aryl bromide was observed by TLC analyses, the reaction mixture was diluted with EtOAc (10 mL) and water (10 mL) and passed through a Celite pad (3.0 cm) to remove the catalyst. EtOAc (50 mL) and water (50 mL) were added to the filtrate, and the layers were separated. The aqueous layer was extracted with EtOAc (10 mL), and the combined organic layers were washed with brine (20 mL), dried over MgSO₄, filtered, and concentrated in vacuo. The residue was purified by flash column chromatography on silica gel (hexane/EtOAc) to give the corresponding biphenyl (Table 4).

Acknowledgements

We thank the N.E. Chemicat Corporation for the gift of the Pd/Cs.

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c1ra00776a/

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