A mild and recyclable nano-sized nickel catalyst for the Stille reaction in water

Abstract

We report here the first application of nano-sized nickel catalysts (phosphine dendrimer stabilized nickel nano-particles) for the Stille coupling reaction in water. The nickel nanoparticles were found to be highly active and recyclable in the reactions at ambient temperature in water medium, without co-catalyst, affording the biaryls with good to excellent yields. The catalytic process was proven to be heterogeneous, occurring on the surface of nickel nanoparticles based on parallel experiments with molecular complexes and metal leaching tests in recycling experiments as well as TEM morphology of recovered nanocatalysts.

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Introduction

Organic reactions in entirely aqueous environments have been one of the most important choices for organic transformations in green chemistry.¹ Even though reactants and/or catalysts are somehow heterogeneous in water, it was still observed that water dramatically enhanced the reactivity of various organic reactions. This might be attributed to the drive of reactants by water molecules to the organic–aqueous interface, presumably via hydrogen bonding, which reciprocally decreased the entropy of the reaction mixture.^1b Although numerous catalytic reactions have been developed to be carried out in water with significant improvement of their reactivities or selectivities, the employment of metal nanoparticles as catalysts in aqueous media are still limited to certain reactions, such as hydrogenation, oxidation, Heck coupling and Suzuki coupling reactions catalyzed by Pd, Pt, Au nanoparticles etc., while little information related to nickel nanoparticles was documented.²

Nanosized transition metal particles can be used as highly efficient catalysts, which benefit from their large surface-to-volume ratio, leading to their much higher catalytic efficiency in organic transformations compared to that of bulky materials.³ A variety of stabilization methods have been reported, using surfactants,⁴ organic ligands,⁵ polymers,⁶ and dendrimers.⁷ Being well-defined and highly branched three-dimensional macro-molecules, dendrimers have been utilized to encapsulate metal nanoparticles, or serve as wedges, to control the size, shape, solubility, and catalytic activity of the corresponding particles. Research efforts have been devoted to the dendrimer encapsulated nanoparticles (DENs) and dendrimer stabilized nanoparticles (DSNs), containing Cu,⁸ Pd,^3i,9 Au,¹⁰ Rh,^9b,11 Pt,^9a,11a or Ag¹²etc., nevertheless, nickel nanoparticles were rarely applied.¹³ Quite recently, we reported the first synthesis of phosphine dendrimer stabilized nickel nanoparticles and investigated their good catalytic efficiency in Suzuki coupling reactions.¹⁴ It is well known that Fréchet-type dendrons endowed special solubility to the stabilized metal nanoparticles, thus we hypothesized that this hydrophobic environment might act as “nano-reactor or nano-pump” for catalytic substrates in hydrophilic media as described by Crooks and other leading scientists.^8a,15 This property offers a venue to conduct reactions in green media such as water and/or supercritical CO₂, so that the catalytic efficiency might be improved due to the intimacy of substrates and catalytic sites inside this nano-environment. To test our hypothesis, we selected the Stille reaction as the model reaction for the following reasons: (1) the Stille reaction has been one of the most powerful C–C coupling methods to build versatile biaryls, which are necessary moieties for many natural compounds as well as synthetic pharmaceutical compounds.¹⁶ (2) In most cases, the Stille reaction hitherto developed suffers from several restrictions, such as the needs of inevitable copper co-catalysts, regular heating up to 100 °C, and/or low activity for heterocyclic and chlorinated substrates.¹⁷ Consequently, further development of a catalytic system that can overcome these limitations is highly desired. As an extension of our continuing interest in stabilized nano-sized catalysts and applications,^14,18 we herein report the first investigation of dendrimer stabilized nickel nanoparticles in water for the Stille coupling‡ reaction at the ambient temperature without co-catalysts.¹⁹

Results and discussion

The dendrimer stabilized nickel nanoparticles G₃DenP-Ni (1) were prepared and purified as reported,¹⁴ summarized in Scheme 1. With this nano-sized nickel catalyst present, we extensively optimized the catalytic performance for Stille coupling reactions in organic solvents or water. Bromobenzene and trichloro-phenylstannane were selected as the standard substrates, in addition, CsF as the best base in Stille coupling reaction was used directly.^18b,20 As shown in Table 1, in anhydrous solvents, the reaction underwent sluggish conversion even when heated to 60 °C (entries 1–4). Water as an additive into the organic solvents improved the coupling yields up to 77%, suggesting that water can be a good solvent in such a reaction. As expected, the coupling reaction in water was completed within 2 h at 60 °C with a 99% yield, without copper as a co-catalyst (entry 7). Notably, it still obtained an 86% yield at room temperature after increasing the time to 8 h (entry 8). Finally, with 2 mol% nano-sized nickel catalyst, the reaction went through a complete conversion with a 99% yield of biphenyl product within 4 h at r.t. (entry 9). Surprisingly, no conversion was observed for the in situ complex of Ni(G₃DenP)(0)²¹ in water at ambient temperature even after 20 h (entry 10).

Scheme 1 The preparation of G₃DenP-Ni by two-phase reduction.

Table 1 Stille coupling of bromobenzene and organotin reagent catalyzed by G₃DenP-Ni^a

Entry	Solvent/base	t (h)	T/°C	Yield (%)^b
a 1 mmol bromobenzene, 1.1 mmol trichlorophenylstanne, 2.2 mmol base, 1 mol% G3DenP-Ni, 5 mL solvent, N2. b Isolated yield based on aryl halide. c 2 mol% G3DenP-Ni was used. d 2 mol% Ni(G3DenP)(0) complexes were used instead of G3DenP-Ni.
1	anhydr. Dioxane/CsF	20	25	No reaction
2	anhydr. THF/CsF	20	25	No reaction
3	anhydr. Dioxane/CsF	6	60	35
4	anhydr. DMF/CsF	6	60	48
5	DMF-H2O (10 : 1, v/v)/CsF	6	60	77
6	THF-H2O (10 : 1, v/v)/CsF	6	60	65
7	H2O/CsF	2	60	99
8	H2O/CsF	8	25	86
9 ^c	H2O/CsF	4	25	99
10^d	H2O/CsF	20	25	No reaction

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Table 2 The Coupling Reactions of Aromatic and Heterocyclic Halides with Various Organotins Catalyzed by G₃DenP-Ni^a

Entry	R	X/Z	Ar	t (h)	Yield (%)^b
a 1 mmol aryl halide, 1.1 mmol organotin, 2.2 mmol CsF, 2 mol% G3DenP-Ni, 5 mL deionised water, N2. b Isolated yield based on aryl halide.
1	p-COOMe	Br/C	Ph	3	99
2	p-CHO	Br/C	Ph	3	99
3	p-N(CH3)2	Br/C	Ph	4	98
4	p-NO2	Br/C	Ph	3	99
5	o-CH3	Br/C	Ph	4	97
6	o,o′-dimethyl	Br/C	Ph	5	95
7	o-CH3O	Br/C	Ph	4	95
8	p-COOMe	Cl/C	Ph	5	98
9	p-CH3O	Cl/C	Ph	6	93
10	p-CF3	Cl/C	Ph	5	97
11	o,o′-dimethyl	Cl/C	Ph	8	91
12	H	2-Br/N	Ph	6	89
13	H	2-Cl/N	Ph	8	86
14	H	3-Cl/N	Ph	8	87
15	6-CH3	3-Cl/N	Ph	10	85
16	H	2-bromo furan	Ph	6	80
17	H	2-bromo thiophene	Ph	7	91
18	H	2-chloro thiophene	Ph	10	83
19	p-COOMe	Cl/C	p-MeC6H4	3	98
20	p-COOMe	Cl/C	p-CF3C6H4	5	95
21	o-CH3	Cl/C	Thiophene	6	90
22	p-CH3O	Cl/C	Thiophene	6	85
23	o-CH3	Cl/C	Vinyl	6	92
24	H	2-Cl/N	Thiophene	8	63

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To further verify the generality of the nano-sized nickel catalyst in the Stille coupling reaction, we explored the substrate scopes from aromatic to heterocyclic halides under the optimized conditions. Treatment of trichlorophenylstannane with a series of brominated or chlorinated aromatics furnished the corresponding coupling products in good to excellent yields (Table 2, entries 1–11). The catalytic efficiency was not significantly affected by the substituents on the aromatic ring of the halides. Both electron-rich and electron-deficient substituents were highly effective, albeit at slightly lower yields compared to when di-ortho substitutes or chlorinated aromatic substrates were employed (entries 6, 8–11). Further extension of scopes to heteroatom-containing substrates were also investigated. At the expense of time up to 10 h, brominated and chlorinated heterocyclic derivatives afforded good yields when treated with trichlorophenylstannane under the optimized conditions (entries 12–18). The coupling reactions of chlorinated aromatics with various organotin reagents, such as trichloro(p-methyl) phenylstannane, trichloro(p-trifluoromethyl)phenylstannane, 2-(trichlorostannyl)thiophene and vinyltrichlorotin, gave decent yields from 85 to 98% (entries 19–23). To be noted, the vinyltrichlorotin was proven to be a reliable substrate which used to be sluggish in our previous nano-sized palladium (G₃DenP-Pd) catalyzed Stille coupling reactions (entry 23).^18b Furthermore, a moderate yield obtained from coupling of 2-chloropyridine with 2-(trichlorostannyl)thiophene also guaranteed the versatility of our nanosized nickel catalyst (entry 24).

Given the high efficacy of these nano-sized catalysts in the Stille coupling reaction, their recyclability was then investigated as the separation and recycling of catalysts are still the key steps in heterogenous catalyses to ensure reusability and avoid undesired contaminations in the final products. As previously described, Fréchet-type dendrimers endowed special solubility to the nanosized nickel catalysts, which are not soluble in hexane, ethyl ether and alcohols. Hence, hexane was used to extract coupling products and precipitate nickel catalysts. For recycling experiments, the coupling of p-nitrobromobenzene with trichlorophenylstannane in the presence of 2 mol% G₃DenP-Ni was chosen as a model reaction. Upon the completion of catalytic process, a large amount of hexane was added into the above mixtures, the catalysts were quantitatively precipitated and then collected by centrifugation. The left organic phase (hexane) was then collected to determine the yield. The recovered catalysts were dried in vacuo and used directly in the next run. Following this standard procedure, the nanosized nickel catalysts could be reused for seven runs with the similar activities, as the yields of the reactions were 99%, 99%, 98%, 99%, 97%, 99%, and 98%, respectively (Fig. 1). Delightfully, the leaching of nickel in aqueous solution for each run was below the detection threshold of ICP analysis. TEM images of recovered nano-sized nickel catalysts became consistent with the results of ICP analysis, as no obvious “Ostwald ripening” was observed even after six runs (please see ESI† for the morphology of nanoparticles after recycling). These results testified our hypothesis, indicating that the catalytic cycles took place heterogeneously on the surface of nickel nanoparticles surrounded by the nano-environment of dendrimers. This helps elucidate in a hot debate regarding the catalytic performance of metal nanoparticles as “heterogeneous or homogeneous”.²²

Fig. 1 The recovery and recycling of G₃DenP-Ni catalyst (2 mol%) in the coupling of p-nitrobromobenzene with trichlorophenylstannane.

Conclusions

In conclusion, we developed the first application of nano-sized nickel catalysts for the Stille coupling reaction in water. The catalytic efficacy was found to be greatly improved in this green media without co-catalysts, affording biaryls with good to excellent yields. It is worth mentioning that in this system, the catalytic cycle occurred heterogeneously on the surface of nickel nanoparticles and inside the nano-environment of dendrimers, which were evidenced by parallel experiments with molecular complexes and metal leaching tests in recycling experiments as well as TEM images of recovered nanoparticles. This may open up new possibilities to develop novel asymmetric catalysis of dendrimer stabilized metal nanoparticles.

Acknowledgements

This project is supported by the National Scientific Research Innovation Foundation (No.HIT.NSRIF 98332112) in the Harbin Institute of Technology, and partially from the National Natural Science Foundation of China (No. 21002019). L. W. also thanks the Harbin Institute of Technology for startup financial support.

Notes and references

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21. Ni(G₃DenP)(0) was prepared from Ni(COD)₂ with G₃DenP ligand (1 : 1) in THF, and used in situ.
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Footnotes

† Electronic supplementary information (ESI) available: Experimental procedures and product characterizations. See DOI: 10.1039/c2cy00466f

‡ General procedures for G₃DenP-Ni catalyzed Stille coupling reaction and catalyst recycling: To a 10 mL vial under argon were added aryl halides (1 mmol), organotin (1.1 mmol), CsF (2.2 mmol), 2 mol% G₃DenP-Ni and 5 mL degassed deionised water. The reaction mixture was then stirred for the time listed in Table 2 at r.t. After the reaction was complete and monitored by TLC, maximum recovery was obtained by adding 50 mL hexane to extract the product and precipitate the catalysts. The nano-sized catalyst was recovered by centrifugation, followed by 5 mL hexane washing three times, and drying over vacuum, which can be used directly in the next run. The remaining organic phase containing products was separated from the aqueous solution, and proceeded to determine the coupling yield after silica chromatography.

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