- Email: [email protected]
Palladium nanoparticles stabilized by aqueous vesicles self-assembled from a PEGylated surfactant ionic liquid for the chemoselective reduction of nitroarenes
Catalysis Communications 99 (2017) 57–60
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Palladium nanoparticles stabilized by aqueous vesicles self-assembled from a PEGylated surfactant ionic liquid for the chemoselective reduction of nitroarenes
MARK
Zhu-bing Xu, Guo-ping Lu⁎, Chun Cai School of Chemical Engineering, Nanjing University of Science and Technology, Xiaolingwei 200, Nanjing 210094, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Aqueous vesicles PEGylated surfactant ionic liquid Palladium nanoparticles Hydrogen transfer Aromatic amine
Vesicles self-assembled from an aqueous PEGylated surfactant ionic liquid solution can be applied for stabilizing palladium nanoparticles, which prove to be an efficient catalytic system for chemoselective hydrogen transfer of nitroarenes using hydrazine hydrate as a hydrogen source. The particle sizes of vesicles are decreased with the increase of ionic liquid's concentrations and relatively small particle sizes are beneficial to the reduction. Moreover, the aqueous catalytic system still stays in reactor by simple extraction, and is reused without further treatment.
1. Introduction Metal nanoparticles stabilized in ionic liquids (MNPs@ILs) are promising catalysts for transformations [1–2]. Although MNPs@ILs can be recycled by simple extraction, ILs are relatively expensive solvents than other solvents that are widely applied in industry. Moreover, the environmental fate of ionic liquids is a complex situation, such as their modes of toxicity, biodegradation pathways and behavior concerning biosorption [3]. Thus, the use of catalytic amount of ILs as stabilizers for the formation of MNPs in a cheap, green solvent is a greener alternative to applying ILs as both stabilizers and solvents. ILs can be supported on solid materials via covalent bonding for immobilization of MNPs [4–5]. In these approaches, a small amount of ILs is required to stabilize the MNPs, and the properties of ILs enhance their catalytic performance. Like other supported MNPs [6], these MNPs can be easily reused by simple filtration but washing steps and extra solutions (organic solvents or acidic/basic aqueous solutions) are always the norm. Additionally, the reaction systems (mediums and additives) are often discarded in these strategies. Ideally, the catalyst solution remains in the reactor and is reused with a fresh batch of reactants without further treatment [7]. The use of water as a solvent opens the way to biphasic extraction of products and recycling of the catalytic systems [8]. Lipshutz's group has reported that recyclable aqueous TPGS-750-M micelles can stabilize the MNPs (Pd, Ni and Fe) for catalyzing organic synthesis [9,10]. Meanwhile, there has been a tremendous increase of interest in the behavior of surfactant ionic liquids in water. Although there are several reports on aqueous
⁎
Corresponding author. E-mail address: [email protected] (G.-p. Lu).
http://dx.doi.org/10.1016/j.catcom.2017.04.051 Received 8 December 2016; Received in revised form 9 March 2017; Accepted 15 April 2017 Available online 26 May 2017 1566-7367/ © 2017 Elsevier B.V. All rights reserved.
micelles formed by ILs and micellar catalysis in aqueous-ionic liquid systems [11–13], only a few examples of vesicles have been observed in aqueous-ionic liquid systems [14,15] and no report has applied them as reaction medium. Actually, vesicles may have better stabilization and catalytic activity than micelles in some transformations [16–18]. Therefore, we reason that the metal NPs can be stabilized by aqueous vesicles generated by surfactant ILs with a combination of steric and electronic stabilization, which may be an efficient catalyst for organic reactions. Functionalized arylamines that play an important role in agrochemicals, pharmaceuticals, fine chemicals, dyes, pigments, material science and biotechnology [19], are mainly produced by chemoselective hydrogenation of the corresponding nitroarenes [20–22]. The use of active metals or sulfides results in serious pollution [23,24], and hydrogen reduction methods suffer from the requirement of specialized equipment [25,26]. The reductions by hydrazine monohydrate are one of valid solutions to these issues, but high temperature is always required [27–29]. Based on these results, we disclosed a recyclable aqueous catalytic system for the chemoselective reduction of nitroarenes using hydrazine monohydrate as the reducer under relatively mild conditions, in which the Pd NPs are stabilized in vesicles fabricated by self-assembly of a PEGylated surfactant IL. To the best of our knowledge, this is the first example that aqueous vesicles self-assembled from a PEGylated surfactant ionic liquid are employed for stabilizing Pd NPs and as the reaction medium. Although the recyclability of the catalyst is less satisfactory than supported MNPs, but the catalyst still has some advantages compared with supported MNPs, including easy to
Catalysis Communications 99 (2017) 57–60
Z.-b. Xu et al.
preparation and simple recycling process.
Table 1 Optimization of reaction conditionsa.
2. Experimental section 2.1. The reduction of nitroarenes catalyzed by Pd NPs To an oven-dried reaction flask with Teflon coated stir bar purged with argon, Pd(OAc)2 0.01 mmol, IL4 0.08 mmol, KOH (or K2CO3) 0.30 mmol, nitroarene 1 1.0 mmol and degassed HPLC grade water 1.5 mL were added, and the mixture was stirred for 5 min under argon at room temperature. Then, 0.5 mL of hydrazine hydrate aqueous solution (~ 5.0 mmol) was syringed into the flask at the same temperature. After stirring for additional 5 min, the reaction was heat to 50 °C, and stirring for 8 h under argon. After completion, the reaction mixture was extracted by methyl tertiary butyl ether (MTBE) (3 × 2 mL), and the organic layer was collected and filtered through a bed of silica gel layered over Celite. The volatiles were removed in vacuo to afford the product 2. In some cases, further column chromatography on silica gel was required to afford the pure desired products. 2.2. Recycling aqueous catalytic system After reaction completion, the mixture was then extracted with MTBE (3 × 1 mL). The organic layer was collected by syringes and filtered through a bed of silica gel layered over Celite. The volatiles were removed in vacuo to afford the product 2b. To the IL4-palladium mixture (aqueous phase), 1b 1.0 mmol was added, followed by hydrazine hydrate (3.0 mmol, 80 wt% aqueous solution) at room temperature and the reaction stirred under argon for 8 h at 50 °C. The extraction cycle was then repeated for the separation of 2b. 3. Results and discussion
Entry 1c 2c 3c 4c 5 6 7
IL IL1 IL2 IL3 IL4 IL4 IL4 IL4
Base (x) / / / / / / K2CO3 (0.30)
Yield (%)b 0c 0c 11c 26c 54 88 99
8 9 10 11 12 13 14 15 16 17 18 19 20 21
IL4 IL4 IL4 IL4 IL4 IL4 IL4 IL4 / IL4 IL4 IL4 IL4 IL4
/ K2CO3 (0.25) LiOH (0.25) NaOH (0.25) KOH (0.25) KOH (0.30) KOH (0.30) KOH (0.30) KOH (0.30) KOH (0.40) t-BuONa (0.25) NEt3 (0.25) KHMDS (0.25) KOH (0.30)
29 48 trace 75 92 96, 78d, 45e 90f 68c 28 98 53 39 77 71g
a Reaction conditions: 1a or 1b 1.0 mmol, ionic liquid 0.08 mmol, base × mmol, N2H4·H2O y mmol, H2O 2 mL, Ar, 8 h. b Yields were determined by GC with 1,3-dimethoxybenzene (100 μL) as the internal standard, which was added to the mixture after the reaction was complete. c At room temperature. d 3 equiv of N2H4·H2O is used. e 1 equiv of N2H4·H2O is used. f At 40 °C. g At 80 °C.
Initially, four PEGylated imidazole-based ILs were synthesized by a three-step process (See SI). The contrast experiments were performed by selecting the reduction of 4-nitrotoluene in the presence of Pd(OAc)2 and hydrazine hydrate in IL1-4 aqueous solutions (Table 1, entries 1–4), and IL4 provided the best result. A good yield could be obtained by increasing the amount of hydrazine hydrate and heating to 50 °C (entry 6). However, only a poor yield of p-methoxyaniline 2b was gained under identical conditions (entry 8). Thus, various bases were screened to further improve the unsatisfactory results (entries 7, 9–12, 18–20), because base can enhance the solubility of Pd(OAc)2 in water and may capture the C-2 hydrogen of imidazole ring in IL4 to form Nheterocyclic carbene palladium complex [30,31]. KOH proved to be the best choice and 0.30 equiv. was the optimized amount of usage (entry 13). The reaction was inhibited in the absence of IL4 since the stability of Pd NPs may be weakened (entries 16). The temperature (entries 13–15, 21) and the amount of hydrazine hydrate (entry 13) were also optimized, and 50 °C and 5 equiv. of hydrazine hydrate were the best option in the reduction of 1b. To investigate the relationship between the aqueous PEGylated imidazole-based ILs' solutions and the reaction yields, the average particle sizes of aggregates formed by ILs in water were detected by dynamic light scattering (DLS). The particle sizes are inversely proportional to the alkyl chain's length of ILs [16–18,32] (Fig. S1 in SI) and the concentrations of aqueous IL4 solutions (Fig. 1). The aggregates may be vesicles owing to the relatively large range of their particle sizes [14,15]. Meanwhile, the reaction yields were enhanced with the decreasing of the particle sizes (Fig. 1), presumably due to the larger specific surface area and the better stabilization for Pd NPs [33]. Transmission electron microscopy (TEM) allowed us to further confirm the structure of aggregates formed by IL4 (40 mM) (Fig. 2a) and the combination mode between Pd nanoparticles and vesicles (Fig. 2b). The TEM specimens were prepared by dipping a polymercoated copper grid into aqueous solutions of IL4 (40 mM). The TEM
Fig. 1. The average particle sizes of vesicles formed by different concentrations of aqueous IL4 solutions (10–70 mM) determined by DLS. (b) The yields (GC yields) of the reduction of 1b in different concentrations of aqueous IL4 solutions.
grid was dried for 0.5 h at room temperature, and then subjected to TEM observation. A lot of water is in the inner core of aggregates (dark areas in Fig. 2a) and a hydrophobic region around the core is also observed (gray areas in Fig. 2a), so the aggregates formed by IL4 were vesicles [16–18]. The Pd NPs may be in vesicles or enwrapped by IL4 58
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Fig. 2. (a) TEM micrograph of vesicles formed by IL4 in water (40 mM).a (b) TEM micrograph of vesicles formed by IL4 with Pd NPs in water (40 mM).a (c) TEM micrograph of Pd NPs in aqueous vesicles of IL4 (40 mM) using Pd(OAc)2 as the precursor and hydrazine hydrate as the reducing agent. a On a polymer-coated copper grid, stained with an aqueous solution of phosphotungstic acid (2 wt%).
group (2g, 2h) as starting materials because aminophenols have better solubility in water than other anilines. Nitroarenes substituted with easily reducible functional groups such ester, cyano and vinyl groups (2k, 2l, 2n) were also selectively reduced to the corresponding anilines with excellent yields. Different substituted heterocyclic nitroarenes were reduced to the corresponding anilines in good to excellent yields without affecting the heterocyclic ring (2p–2r). Studies were also conducted to assess the potential for recycling of the reaction medium and transfer hydrogenation of 4-methoxyl nitrobenzene 1b was selected as the model reaction. After completion of reaction, the product undergoes in-flask extraction with minimum amounts of an organic solvent (MTBE). Remaining in the water are the PEGylated surfactant ionic liquid, KOH, N2H4·H2O and the palladium catalyst. Addition of fresh N2H4·H2O (3.0 equiv, 80 wt% aqueous solution) leads to an active catalyst ready for re-introduction of the starting material. The process could be repeated 4 times without an obvious change in yields, but fresh Pd(OAc)2 (0.5 mol%) and KOH (0.1 equiv) was added to retain the catalytic activity of the system since the third recycle (Fig. 3).
because the dark areas that may be water or Pd NPs are only found in vesicles and the irregular aggregates of IL4 (gray areas in Fig. 2b). Therefore, there may be three stabilizing effects of IL4 on Pd NPs, including (i) electrostatic stabilization (cations and anions of ILs) [34]; (ii) steric protection (PEG chain) [35]; (iii) the formation of N-heterocyclic carbene palladium complex [30,31]. The TEM images (Fig.2c) showed the Pd NPs were generated and stabled by the aqueous vesicles of IL4. The TEM images (Fig. S2f in SI) showed Pd NPs in vesicles are aggregated obviously after reusing four times. Obvious aggregation of Pd NPs is also found in aqueous vesicles of IL4 (10 mM) (Fig. S2 g in SI), so the larger particle sizes of vesicles may enhance the aggregation of Pd NPs, which goes against the reactivity of Pd NPs (more details on TEM images see Fig. S2 in SI). SEM was also used to further confirm the vesicles. Some spherical particles can be observed, which may be vesicles (more details on SEM images see Fig. S3 in SI). With the optimized conditions in hands, various nitroarenes were chosen to establish the scope and generality of this protocol (Scheme 1). The steric hindrance effects of nitrobenzenes had few influences on the reaction. Meanwhile, nitrobenzenes containing both electron-withdraw and electron-donating groups could provide satisfactory yields. No dehalogenation occurred in the cases of 2d, 2e and 2o which could be attributed to the mild conditions and weak leaving abilities of F and Cl atoms, but a 18% of dehalogenation product was detected in the case of 2f. Slight lower isolated yields were gained using nitroarenes with OH
4. Conclusions In summary, we disclose the first example that Pd NPs are stabilized in aqueous vesicles derived by a new PEGylated surfactant ionic liquid for the chemoselective transfer hydrogenation of nitroarenes using hydrazine hydrate as a hydrogen donor. The particle sizes of vesicles are inversely proportional to the concentrations of IL4 in water and smaller particle sizes are beneficial to the reaction. The newly
Fig. 3. Recycle studies. a Conditions: 1b 1.0 mmol, IL4 0.08 mmol, KOH 0.30 mmol, N2H4·H2O 5.0 mmol, H2O 2.0 mL, Ar, 8 h. IL4 was recycled in all runs, and fresh N2H4·H2O (3.0 equiv) was added in each recycle. b Isolated yields. c The yields (blue) were obtained without employing additional Pd(OAc)2. The yields (red) were obtained by adding fresh Pd(OAc)2 (0.5 mol%) and KOH (0.1 equiv). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Scheme 1. The reduction of nitroarenes in aqueous vesicles of IL4.a,b Conditions: 1 1.0 mmol, IL4 0.08 mmol, KOH 0.30 mmol, N2H4·H2O 5.0 mmol, H2O 2 mL, Ar, 50 °C, 8 h. b Isolated yields. c The use of K2CO3 (0.30 mmol) instead of KOH.
59
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[9] E.D. Slack, C.M. Gabriel, B.H. Lipshutz, Angew. Chem. Int. Ed. 53 (2014) 14051–14054. [10] S. Handa, Y. Wang, F. Gallou, B.H. Lipshutz, Science 349 (2015) 1087–1091. [11] M. Blesic, M.H. Marques, N.V. Plechkova, K.R. Seddon, L.P.N. Rebelo, A. Lopes, Green Chem. 9 (2007) 481–490. [12] G. Singh, T.S. Kang, J. Phys, Chem. B 119 (2015) 10573–10585. [13] Y. Liu, L. Qiao, Y. Xiang, R. Guo, Langmuir 32 (2016) 2582–2590. [14] F. Gayet, J.-D. Marty, A. Brûlet, N.L.-d. Viguerie, Langmuir 27 (2011) 9706–9710. [15] G. Li, S. Zhang, N. Wu, Y. Cheng, J. You, Adv. Funct. Mater. 24 (2014) 6204–6209. [16] J.H. Fendler, Acc. Chem. Res. 13 (1980) 7–13. [17] G. Hamasaka, T. Muto, Y. Uozumi, Angew. Chem. Int. Ed. 50 (2011) 4876–4878. [18] E.N. Towns, A.N. Parikh, D.P. Land, J. Phys, Chem. C 119 (2015) 2412–2418. [19] A.M. Tafesh, J.A. Weiguny, Chem. Rev. 96 (1996) 2035–2052. [20] F.A. Westerhaus, R.V. Jagadeesh, G. Wienhöfer, M.M. Pohl, J. Radnik, A.-E. Surkus, J. Rabeah, K. Junge, H. Junge, M. Nielsen, Nat. Chem. 5 (2013) 537–543. [21] R.V. Jagadeesh, A.-E. Surkus, H. Junge, M.-M. Pohl, J. Radnik, J. Rabeah, H. Huan, V. Schünemann, A. Brückner, M. Beller, Science 342 (2013) 1073–1076. [22] J.-w. Zhang, G.-p. Lu, C. Cai, Catal. Commun. 84 (2016) 25–29. [23] J. Kroemer, C. Kirkpatrick, B. Maricle, R. Gawrych, M.D. Mosher, Tetrahedron Lett. 47 (2006) 6339–6341. [24] M.-f. Lv, G.-p. Lu, C. Cai, Asian J. Org. Chem. 4 (2015) 141–144. [25] Z. Wei, J. Wang, S. Mao, D. Su, H. Jin, Y. Wang, F. Xu, H. Li, Y. Wang, ACS Catal. 5 (2015) 4783–4789. [26] E. Boymans, S. Boland, P.T. Witte, C. Müller, D. Vogt, ChemCatChem 5 (2013) 431–434. [27] M. Shokouhimehr, J.E. Lee, S.I. Han, T. Hyeon, Chem. Commun. 49 (2013) 4779–4781. [28] Z. Zhao, H. Yang, Y. Li, X. Guo, Green Chem. 16 (2014) 1274–1281. [29] C.-G. Chao, D.E. Bergbreiter, Catal. Commun. 77 (2016) 89–93. [30] T. Yasukawa, H. Miyamura, S. Kobayashi, Chem. Sci. 6 (2015) 6224–6229. [31] R. Visbal, M.C. Gimeno, Chem. Soc. Rev. 43 (2014) 3551–3574. [32] J.N. Israelachvili, D.J. Mitchell, B.W. Ninham, Generally, increasing the alkyl chain length can reduce the aggregation number, resulting in the decreasing the particle size of vesicles, J. Chem. Soc. 72 (1976) 1525–1568. [33] M.K. Kawamuro, H. Chaimovich, E.B. Abuin, E.A. Lissi, I.M. Cuccovia, J. Phys. Chem. 95 (1991) 1458–1463. [34] M. Planellas, R. Pleixats, A. Shafir, The cations and anions of ILs can stabilize the metal NPs by electrostatic protection Adv, Synth. Catal. 354 (2012) 651–662. [35] M.J. MacLeod, J.A. Johnson, The PEG chain can stabilize the metal NPs by steric protection, J. Am. Chem. Soc. 137 (2015) 7974–7977.
developed procedures are environmentally friendly in that only limited amounts of water are invested, and workup entails only an in-flask extraction with a minimal amount of a single, recoverable organic solvent. After extraction, the aqueous vesicles with Pd NPs remains in the reactor and is reused with a fresh batch of reactants without further treatment. Furthermore, this chemistry provides a new aqueous system for stabilizing MNPs, in which ionic liquids can be further modified to enhance the recyclability of the catalyst. The synthesis of other ionic liquids for stabilizing MNPs is ongoing in our group. Acknowledgments We gratefully acknowledge the Natural Science Foundation of China (21402093) and Jiangsu (BK20140776), Chinese Postdoctoral Science Foundation (2016T90465, 2015M571761) for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2017.04.051. References [1] K.L. Luska, P. Migowski, W. Leitner, Green Chem. 17 (2015) 3195–3206. [2] J. Dupont, J.D. Scholten, Chem. Soc. Rev. 39 (2010) 1780–1804. [3] M. Petkovic, K.R. Seddon, L.P.N. Rebelo, C. Silva Pereira, Chem. Soc. Rev. 40 (2011) 1383–1403. [4] N.M. Patil, T. Sasaki, B.M. Bhanage, ACS Sustain. Chem. Eng. 4 (2016) 429–436. [5] K.L. Luska, P. Migowski, S. El Sayed, W. Leitner, Angew. Chem. Int. Ed. 54 (2015) 15750–15755. [6] B. Paul, D.D. Purkayastha, S.S. Dhar, S. Das, S. Haldar, J. Alloy, Composites 681 (2016) 316–323. [7] R.A. Sheldon, Green Chem. 9 (2007) 1273–1283. [8] G. La Sorella, G. Strukul, A. Scarso, Green Chem. 17 (2015) 644–683.
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Palladium nanoparticles stabilized by aqueous vesicles self-assembled from a PEGylated surfactant ionic liquid for the chemoselective reduction of nitroarenes
MARK
Zhu-bing Xu, Guo-ping Lu⁎, Chun Cai School of Chemical Engineering, Nanjing University of Science and Technology, Xiaolingwei 200, Nanjing 210094, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Aqueous vesicles PEGylated surfactant ionic liquid Palladium nanoparticles Hydrogen transfer Aromatic amine
Vesicles self-assembled from an aqueous PEGylated surfactant ionic liquid solution can be applied for stabilizing palladium nanoparticles, which prove to be an efficient catalytic system for chemoselective hydrogen transfer of nitroarenes using hydrazine hydrate as a hydrogen source. The particle sizes of vesicles are decreased with the increase of ionic liquid's concentrations and relatively small particle sizes are beneficial to the reduction. Moreover, the aqueous catalytic system still stays in reactor by simple extraction, and is reused without further treatment.
1. Introduction Metal nanoparticles stabilized in ionic liquids (MNPs@ILs) are promising catalysts for transformations [1–2]. Although MNPs@ILs can be recycled by simple extraction, ILs are relatively expensive solvents than other solvents that are widely applied in industry. Moreover, the environmental fate of ionic liquids is a complex situation, such as their modes of toxicity, biodegradation pathways and behavior concerning biosorption [3]. Thus, the use of catalytic amount of ILs as stabilizers for the formation of MNPs in a cheap, green solvent is a greener alternative to applying ILs as both stabilizers and solvents. ILs can be supported on solid materials via covalent bonding for immobilization of MNPs [4–5]. In these approaches, a small amount of ILs is required to stabilize the MNPs, and the properties of ILs enhance their catalytic performance. Like other supported MNPs [6], these MNPs can be easily reused by simple filtration but washing steps and extra solutions (organic solvents or acidic/basic aqueous solutions) are always the norm. Additionally, the reaction systems (mediums and additives) are often discarded in these strategies. Ideally, the catalyst solution remains in the reactor and is reused with a fresh batch of reactants without further treatment [7]. The use of water as a solvent opens the way to biphasic extraction of products and recycling of the catalytic systems [8]. Lipshutz's group has reported that recyclable aqueous TPGS-750-M micelles can stabilize the MNPs (Pd, Ni and Fe) for catalyzing organic synthesis [9,10]. Meanwhile, there has been a tremendous increase of interest in the behavior of surfactant ionic liquids in water. Although there are several reports on aqueous
⁎
Corresponding author. E-mail address: [email protected] (G.-p. Lu).
http://dx.doi.org/10.1016/j.catcom.2017.04.051 Received 8 December 2016; Received in revised form 9 March 2017; Accepted 15 April 2017 Available online 26 May 2017 1566-7367/ © 2017 Elsevier B.V. All rights reserved.
micelles formed by ILs and micellar catalysis in aqueous-ionic liquid systems [11–13], only a few examples of vesicles have been observed in aqueous-ionic liquid systems [14,15] and no report has applied them as reaction medium. Actually, vesicles may have better stabilization and catalytic activity than micelles in some transformations [16–18]. Therefore, we reason that the metal NPs can be stabilized by aqueous vesicles generated by surfactant ILs with a combination of steric and electronic stabilization, which may be an efficient catalyst for organic reactions. Functionalized arylamines that play an important role in agrochemicals, pharmaceuticals, fine chemicals, dyes, pigments, material science and biotechnology [19], are mainly produced by chemoselective hydrogenation of the corresponding nitroarenes [20–22]. The use of active metals or sulfides results in serious pollution [23,24], and hydrogen reduction methods suffer from the requirement of specialized equipment [25,26]. The reductions by hydrazine monohydrate are one of valid solutions to these issues, but high temperature is always required [27–29]. Based on these results, we disclosed a recyclable aqueous catalytic system for the chemoselective reduction of nitroarenes using hydrazine monohydrate as the reducer under relatively mild conditions, in which the Pd NPs are stabilized in vesicles fabricated by self-assembly of a PEGylated surfactant IL. To the best of our knowledge, this is the first example that aqueous vesicles self-assembled from a PEGylated surfactant ionic liquid are employed for stabilizing Pd NPs and as the reaction medium. Although the recyclability of the catalyst is less satisfactory than supported MNPs, but the catalyst still has some advantages compared with supported MNPs, including easy to
Catalysis Communications 99 (2017) 57–60
Z.-b. Xu et al.
preparation and simple recycling process.
Table 1 Optimization of reaction conditionsa.
2. Experimental section 2.1. The reduction of nitroarenes catalyzed by Pd NPs To an oven-dried reaction flask with Teflon coated stir bar purged with argon, Pd(OAc)2 0.01 mmol, IL4 0.08 mmol, KOH (or K2CO3) 0.30 mmol, nitroarene 1 1.0 mmol and degassed HPLC grade water 1.5 mL were added, and the mixture was stirred for 5 min under argon at room temperature. Then, 0.5 mL of hydrazine hydrate aqueous solution (~ 5.0 mmol) was syringed into the flask at the same temperature. After stirring for additional 5 min, the reaction was heat to 50 °C, and stirring for 8 h under argon. After completion, the reaction mixture was extracted by methyl tertiary butyl ether (MTBE) (3 × 2 mL), and the organic layer was collected and filtered through a bed of silica gel layered over Celite. The volatiles were removed in vacuo to afford the product 2. In some cases, further column chromatography on silica gel was required to afford the pure desired products. 2.2. Recycling aqueous catalytic system After reaction completion, the mixture was then extracted with MTBE (3 × 1 mL). The organic layer was collected by syringes and filtered through a bed of silica gel layered over Celite. The volatiles were removed in vacuo to afford the product 2b. To the IL4-palladium mixture (aqueous phase), 1b 1.0 mmol was added, followed by hydrazine hydrate (3.0 mmol, 80 wt% aqueous solution) at room temperature and the reaction stirred under argon for 8 h at 50 °C. The extraction cycle was then repeated for the separation of 2b. 3. Results and discussion
Entry 1c 2c 3c 4c 5 6 7
IL IL1 IL2 IL3 IL4 IL4 IL4 IL4
Base (x) / / / / / / K2CO3 (0.30)
Yield (%)b 0c 0c 11c 26c 54 88 99
8 9 10 11 12 13 14 15 16 17 18 19 20 21
IL4 IL4 IL4 IL4 IL4 IL4 IL4 IL4 / IL4 IL4 IL4 IL4 IL4
/ K2CO3 (0.25) LiOH (0.25) NaOH (0.25) KOH (0.25) KOH (0.30) KOH (0.30) KOH (0.30) KOH (0.30) KOH (0.40) t-BuONa (0.25) NEt3 (0.25) KHMDS (0.25) KOH (0.30)
29 48 trace 75 92 96, 78d, 45e 90f 68c 28 98 53 39 77 71g
a Reaction conditions: 1a or 1b 1.0 mmol, ionic liquid 0.08 mmol, base × mmol, N2H4·H2O y mmol, H2O 2 mL, Ar, 8 h. b Yields were determined by GC with 1,3-dimethoxybenzene (100 μL) as the internal standard, which was added to the mixture after the reaction was complete. c At room temperature. d 3 equiv of N2H4·H2O is used. e 1 equiv of N2H4·H2O is used. f At 40 °C. g At 80 °C.
Initially, four PEGylated imidazole-based ILs were synthesized by a three-step process (See SI). The contrast experiments were performed by selecting the reduction of 4-nitrotoluene in the presence of Pd(OAc)2 and hydrazine hydrate in IL1-4 aqueous solutions (Table 1, entries 1–4), and IL4 provided the best result. A good yield could be obtained by increasing the amount of hydrazine hydrate and heating to 50 °C (entry 6). However, only a poor yield of p-methoxyaniline 2b was gained under identical conditions (entry 8). Thus, various bases were screened to further improve the unsatisfactory results (entries 7, 9–12, 18–20), because base can enhance the solubility of Pd(OAc)2 in water and may capture the C-2 hydrogen of imidazole ring in IL4 to form Nheterocyclic carbene palladium complex [30,31]. KOH proved to be the best choice and 0.30 equiv. was the optimized amount of usage (entry 13). The reaction was inhibited in the absence of IL4 since the stability of Pd NPs may be weakened (entries 16). The temperature (entries 13–15, 21) and the amount of hydrazine hydrate (entry 13) were also optimized, and 50 °C and 5 equiv. of hydrazine hydrate were the best option in the reduction of 1b. To investigate the relationship between the aqueous PEGylated imidazole-based ILs' solutions and the reaction yields, the average particle sizes of aggregates formed by ILs in water were detected by dynamic light scattering (DLS). The particle sizes are inversely proportional to the alkyl chain's length of ILs [16–18,32] (Fig. S1 in SI) and the concentrations of aqueous IL4 solutions (Fig. 1). The aggregates may be vesicles owing to the relatively large range of their particle sizes [14,15]. Meanwhile, the reaction yields were enhanced with the decreasing of the particle sizes (Fig. 1), presumably due to the larger specific surface area and the better stabilization for Pd NPs [33]. Transmission electron microscopy (TEM) allowed us to further confirm the structure of aggregates formed by IL4 (40 mM) (Fig. 2a) and the combination mode between Pd nanoparticles and vesicles (Fig. 2b). The TEM specimens were prepared by dipping a polymercoated copper grid into aqueous solutions of IL4 (40 mM). The TEM
Fig. 1. The average particle sizes of vesicles formed by different concentrations of aqueous IL4 solutions (10–70 mM) determined by DLS. (b) The yields (GC yields) of the reduction of 1b in different concentrations of aqueous IL4 solutions.
grid was dried for 0.5 h at room temperature, and then subjected to TEM observation. A lot of water is in the inner core of aggregates (dark areas in Fig. 2a) and a hydrophobic region around the core is also observed (gray areas in Fig. 2a), so the aggregates formed by IL4 were vesicles [16–18]. The Pd NPs may be in vesicles or enwrapped by IL4 58
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Fig. 2. (a) TEM micrograph of vesicles formed by IL4 in water (40 mM).a (b) TEM micrograph of vesicles formed by IL4 with Pd NPs in water (40 mM).a (c) TEM micrograph of Pd NPs in aqueous vesicles of IL4 (40 mM) using Pd(OAc)2 as the precursor and hydrazine hydrate as the reducing agent. a On a polymer-coated copper grid, stained with an aqueous solution of phosphotungstic acid (2 wt%).
group (2g, 2h) as starting materials because aminophenols have better solubility in water than other anilines. Nitroarenes substituted with easily reducible functional groups such ester, cyano and vinyl groups (2k, 2l, 2n) were also selectively reduced to the corresponding anilines with excellent yields. Different substituted heterocyclic nitroarenes were reduced to the corresponding anilines in good to excellent yields without affecting the heterocyclic ring (2p–2r). Studies were also conducted to assess the potential for recycling of the reaction medium and transfer hydrogenation of 4-methoxyl nitrobenzene 1b was selected as the model reaction. After completion of reaction, the product undergoes in-flask extraction with minimum amounts of an organic solvent (MTBE). Remaining in the water are the PEGylated surfactant ionic liquid, KOH, N2H4·H2O and the palladium catalyst. Addition of fresh N2H4·H2O (3.0 equiv, 80 wt% aqueous solution) leads to an active catalyst ready for re-introduction of the starting material. The process could be repeated 4 times without an obvious change in yields, but fresh Pd(OAc)2 (0.5 mol%) and KOH (0.1 equiv) was added to retain the catalytic activity of the system since the third recycle (Fig. 3).
because the dark areas that may be water or Pd NPs are only found in vesicles and the irregular aggregates of IL4 (gray areas in Fig. 2b). Therefore, there may be three stabilizing effects of IL4 on Pd NPs, including (i) electrostatic stabilization (cations and anions of ILs) [34]; (ii) steric protection (PEG chain) [35]; (iii) the formation of N-heterocyclic carbene palladium complex [30,31]. The TEM images (Fig.2c) showed the Pd NPs were generated and stabled by the aqueous vesicles of IL4. The TEM images (Fig. S2f in SI) showed Pd NPs in vesicles are aggregated obviously after reusing four times. Obvious aggregation of Pd NPs is also found in aqueous vesicles of IL4 (10 mM) (Fig. S2 g in SI), so the larger particle sizes of vesicles may enhance the aggregation of Pd NPs, which goes against the reactivity of Pd NPs (more details on TEM images see Fig. S2 in SI). SEM was also used to further confirm the vesicles. Some spherical particles can be observed, which may be vesicles (more details on SEM images see Fig. S3 in SI). With the optimized conditions in hands, various nitroarenes were chosen to establish the scope and generality of this protocol (Scheme 1). The steric hindrance effects of nitrobenzenes had few influences on the reaction. Meanwhile, nitrobenzenes containing both electron-withdraw and electron-donating groups could provide satisfactory yields. No dehalogenation occurred in the cases of 2d, 2e and 2o which could be attributed to the mild conditions and weak leaving abilities of F and Cl atoms, but a 18% of dehalogenation product was detected in the case of 2f. Slight lower isolated yields were gained using nitroarenes with OH
4. Conclusions In summary, we disclose the first example that Pd NPs are stabilized in aqueous vesicles derived by a new PEGylated surfactant ionic liquid for the chemoselective transfer hydrogenation of nitroarenes using hydrazine hydrate as a hydrogen donor. The particle sizes of vesicles are inversely proportional to the concentrations of IL4 in water and smaller particle sizes are beneficial to the reaction. The newly
Fig. 3. Recycle studies. a Conditions: 1b 1.0 mmol, IL4 0.08 mmol, KOH 0.30 mmol, N2H4·H2O 5.0 mmol, H2O 2.0 mL, Ar, 8 h. IL4 was recycled in all runs, and fresh N2H4·H2O (3.0 equiv) was added in each recycle. b Isolated yields. c The yields (blue) were obtained without employing additional Pd(OAc)2. The yields (red) were obtained by adding fresh Pd(OAc)2 (0.5 mol%) and KOH (0.1 equiv). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Scheme 1. The reduction of nitroarenes in aqueous vesicles of IL4.a,b Conditions: 1 1.0 mmol, IL4 0.08 mmol, KOH 0.30 mmol, N2H4·H2O 5.0 mmol, H2O 2 mL, Ar, 50 °C, 8 h. b Isolated yields. c The use of K2CO3 (0.30 mmol) instead of KOH.
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