Article pubs.acs.org/OPRD
Catalysts of Suzuki Cross-Coupling Based on Functionalized Hypercross-linked Polystyrene: Influence of Precursor Nature Work from the Organic Reactions Catalysis Society Meeting 2016 Nadezhda A. Nemygina,†,‡ Linda Zh. Nikoshvili,*,† Alexey V. Bykov,† Alexander I. Sidorov,† Vladimir P. Molchanov,† Mikhail G. Sulman,†,§ Irina Yu. Tiamina,† Barry D. Stein,∥ Valentina G. Matveeva,† Esther M. Sulman,† and Lioubov Kiwi-Minsker‡,⊥ Downloaded via IOWA STATE UNIV on January 20, 2019 at 19:23:39 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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Tver Technical University, A.Nikitina Street, 22, 170026 Tver, Russian Federation Tver State University, Zhelyabova Street, 33, 170100 Tver, Russian Federation § A.N. Nesmeyanov Institute of Organoelement Compounds of RAS, Vavilova Street, 28, 119991 Moscow, Russian Federation ∥ Department of Biology, Indiana University, Bloomington, Indiana 47405, United States ⊥ Ecole Polytechnique Fédérale de Lausanne, GGRC-ISIC-EPFL, CH-1015 Lausanne, Switzlerand ‡
S Supporting Information *
ABSTRACT: This paper describes synthesis of Pd-containing catalysts of Suzuki cross-coupling based on amino-functionalized hyper-cross-linked polystyrene at variation of Pd precursor nature (PdCl2, PdCl2(CH3CN)2, or PdCl2(PhCN)2). The investigation of the influence of palladium oxidation state (Pd(II) or Pd(0)) and form (Pdn clusters or Pd nanoparticles) on the rate of Suzuki cross-coupling of 4-bromoanisole and phenylboronic acid is discussed. Developed catalysts are shown to allow achieving conversion of 4-bromoanisole higher than 98% under mild reaction conditions. Independently of the precursor nature, Pd(II) is mainly responsible for observed catalytic activity. However, preliminary reduction of catalysts with H2 results in formation of a large number of Pdn clusters, the contribution of which in the Suzuki reaction becomes predominant.
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INTRODUCTION Palladium-catalyzed Suzuki cross-coupling between aryl halides and arylboronic acids is one of the most widespread and effective methods of synthesis of biaryls, which are in turn important semiproducts in the synthesis of pharmaceuticals, ligands, and polymers.1−3 There are more than three hundred various commercial compounds which can react via Suzuki coupling and a huge diversity of Pd-containing catalysts.4 The latter include the following: (i) homogeneous Pd complexes;5−7 (ii) so-called ligandless catalysts containing Pd2+ or Pd0, which are Pd salts,8,9 Pd deposited on inorganic supports,10−12 Pd deposited on carbon12−15 or nonfunctionalized polymers,16−20 or Pd nanoparticles (NPs) stabilized in magnetically separable nanocomposites;21,22 (iii) Pd immobilized via ionic or covalent interactions on ligand-containing polymers (including functionalized dendrimers,23 carbon nanotubes,24 graphenes,25 resins,26−29 and other polymers30−32) or on magnetic NPs with anchored ligands.31,33−36 Besides, one of the routes to Pd catalyst synthesis, which has recently attracted the attention of the scientific community and is worthy of mention, is the so-called “green” synthesis of Pd NPs using plant extracts,37−40 containing both stabilizing and reducing compounds, or microorganisms.40−42 Catalytic systems obtained via a “green” approach were found to allow high yields of coupling products (more than 95%) at mild © 2016 American Chemical Society
reaction conditions in aqueous medium and with recyclability potential. Among the polymeric materials acting as supports for Pd NPs or complexes, nitrogen-containing polymers are the most prospective ones30,31,43−46 due to the possibility of nitrogen to coordinate metal species and thus to prevent their leaching. However, in order to achieve appropriate catalytic activity, good swelling of the polymeric matrix should be provided.44 Recently, hyper-cross-linked polystyrene (HPS) was shown to be a promising support for Pd NPs for different catalytic applications.47,48 HPS can swell in virtually any solvent,49 revealing high mechanical and thermal stability,50 and allowing controlling metal NPs growth due to the existence of rigid nanocavities. The first application of HPS bearing amino groups (Macronet MN100 type) for the development of the catalysts of Suzuki cross-coupling was reported by Lyubimov et al.51 Recently, we have shown that the catalytic activity of the Pd/ MN100 system can be increased via modification of the procedure of catalyst synthesis, which allowed us to obtain small Pd NPs (about 2−4 nm in diameter) and Pdn clusters revealing high activity in Suzuki cross-coupling.52 In this paper we discuss in more detail the role of Pd precursor nature in provision of the observed activity of the developed Pd/MN100 catalysts in the Suzuki reaction. Three types of precursors and their transformations inside the Received: April 28, 2016 Published: July 20, 2016 1453
DOI: 10.1021/acs.oprd.6b00154 Org. Process Res. Dev. 2016, 20, 1453−1460
Organic Process Research & Development
Article
Figure 1. Scheme of Suzuki cross-coupling of 4-BrAn and PBA.
Besides, for all the synthesized catalysts, preliminarily reduction in hydrogen flow at 275 °C for 2 h was carried out (the catalysts were designated as Pd/MN100-1-R, Pd/MN100-2-R, and Pd/MN100-3-R, respectively). Catalytic Testing. Testing of HPS-based catalysts was carried out in a 60 mL isothermal glass batch reactor under vigorous stirring at the temperature 60 °C. The total volume of the liquid phase was 30 mL. EtOH/water mixture at the volumetric ratio of 5:1 was used as a solvent. NaOH was used as a base at the quantity of 1.5 mmol. In each experiment the quantity of 4-BrAn was equal to 1 mmol, and 1.5 molar excess of PBA was used. Catalyst loading in each experiment was 50 mg. Before the catalyst addition in the reactor, in each experiment the blank test (duration of 60 min) was carried out in order to ensure that the reaction did not proceed in the absence of catalyst. Samples of reaction mixture were periodically taken and analyzed via GC-MS (Shimadzu GCMS-QP2010S) equipped with a capillary column HP-1MS (30 m × 0.25 mm i.d., 0.25 μm film thickness). Helium was used as a carrier gas at the pressure 74.8 kPa and linear velocity 36.3 cm/s. The oven temperature was programmed: 120 °C (0 min) → 10 °C/min (160 °C) → 25 °C/min (300 °C) → 300 °C (2.4 min). The temperature of the injector, interface, and ion source was 260 °C, ranging from 10 up to 500 m/z. The concentrations of the reaction mixture components were calculated using the internal standard calibration method (diphenylamine was used as an internal standard). Catalytic activity was defined as R = N4‑BrAn × NPd−1 × τ−1 × X, where N4‑BrAn and NPd are number of moles of 4-BrAn and Pd, respectively; X is conversion of 4-BrAn, and τ is the reaction time for achieving conversion X. Catalyst Characterization. Pd/HPS catalysts were characterized by liquid nitrogen physisorption, X-ray fluorescence analysis (XFA), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Liquid nitrogen physisorption was carried out using Beckman Coulter SA 3100 (Coulter Corporation, USA). Prior to the analysis, samples were degassed in a Becman Coulter SA-PREP at 120 °C in vacuum for 1 h. The weight of each sample was above 0.1 g. The following models were used for calculation of specific surface area (SSA) and pore size distribution: Langmur, Brunauer−Emmett−Teller (BET), t-Plot, and Barrett−Joyner− Halenda (BJH). Pore size distribution was measured in the range 3−200 nm. Microporosity was estimated using the t-plot model. X-ray fluorescence analysis (XFA) was carried out to determine the Pd content. It was performed with a Zeiss Jena VRA-30 spectrometer (Mo anode, LiF crystal analyzer, and SZ detector). Analyses were based on the Co Kα line and a series of standards prepared by mixing 1 g of polystyrene with 10−20 mg of standard compounds. The time of data acquisition was constant at 10 s.
polymeric matrix of MN100 during the catalyst synthesis and testing were investigated: PdCl2, PdCl2(CH3CN)2, and PdCl2(PhCN)2. The catalytic properties of synthesized Pd/ MN100 samples were evaluated in Suzuki cross-coupling of 4bromoanisole (4-BrAn) and phenylboronic acid (PBA) to yield 4-methoxybiphenyl (4-MBP) (biphenyl (BP), formed as a result of PBA homocoupling, was the side product (see Figure 1)).
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EXPERIMENTAL SECTION Materials. HPS Macronet MN100 (Purolite Int., UK), bearing amino groups, was washed with distilled water and acetone and dried under vacuum as described elsewhere.53 4Bromoanisole (4-BrAn, ≥98%) was purchased from Merck KGaA. 4-Methoxybiphenyl (4-MBP, >99%) was purchased from Tokyo Chemical Industry Co. Ltd. Phenylboronic acid (PBA, 95%), diphenylamine (99%), biphenyl (99.5%), palladium(II)chloride (PdCl2, 99%), bis(acetonitrile)palladium(II)chloride (PdCl2(CH3CN)2, >99%), bis(benzonitrile)palladium(II)chloride (PdCl2(PhCN)2, 95%), tetrahydrofuran (THF, ≥99.9%), ethanol (EtOH, ≥99.8%), methanol (MeOH, 99.8%), sodium chloride (NaCl, ≥99%), sodium carbonate (Na2CO3, ≥99.5%), and sodium hydroxide (NaOH, ≥98%) were obtained from Sigma-Aldrich. All chemicals were used as received. Distilled water was purified with an Elsi-Aqua water purification system. Catalyst Synthesis. A series of Pd-containing HPS-based catalysts was synthesized via a wet-impregnation method according to the procedure described elsewhere.53 In a typical experiment, 1 g of pretreated, dried, and crushed (
