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Synlett 2007(20): 3209-3213  
DOI: 10.1055/s-2007-1000816
LETTER
© Georg Thieme Verlag Stuttgart · New York

Polymer-Incarcerated Palladium with Active Phosphine as Recoverable and Reusable Pd Catalyst for the Amination of Aryl Chlorides

Takeshi Inasaki, Masaharu Ueno, Shinpei Miyamoto, Shu Kobayashi*
Department of Chemistry, School of Science and Graduate School of Pharmaceutical Sciences, The University of Tokyo, The HFRE Division, ERATO, Japan and Science Technology Agency (JST), Hongo, Bunkyo-ku, Tokyo 1130033, Japan
Fax: +81(3)56840634; e-Mail: shu_kobayashi@chem.s.u-tokyo.ac.jp;
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Publication History

Received 4 October 2007
Publication Date:
21 November 2007 (online)

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Abstract

A new type of polymer-incarcerated (PI) Pd catalyst bearing an active phosphine ligand has been developed. This catalyst was prepared easily according to a conventional PI method (microencapsulation and cross-linking of polymer chains), and TEM images of PI Pd showed the formation of small Pd clusters dispersed on the polymer support without formation of Pd aggregates. The catalyst showed high activity in amination reactions of various aryl chlorides with amines. The reactions proceeded without addition of any external phosphine ligands, and the catalyst was recovered quantitatively by simple filtration and reused at least three times without loss of activity.

Key words

heterogeneous catalysis - polymer - aminations - palladium - aryl chlorides

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Preparation of Copolymer 1: Styrene (2.24 g, 21.6 mmol), 2-[(2-phenylallyloxy)methyl]oxirane (630 mg, 3.32 mmol), tetraethyleneglycol mono-2-phenyl-2-propenyl ether (1.03 g, 3.32 mmol), dicyclohexyl-2-[4′-(4-styrylmeth­-oxy)phenyl]phenylphosphine (2.40 g, 4.97 mmol) and 2,2′-azobis(isobutyronitrile) (54.4 mg, 0.332 mmol) were mixed in CHCl3 (4.2 mL). The mixture was stirred at reflux for 27 h under argon and was then cooled to r.t. The resulting polymer solution was poured slowly into hot deoxygenated hexane and the precipitated polymer was collected by filtration and washed with hexane. The polymer so obtained was dissolved in CHCl3, and the resulting solution was poured slowly into MeCN. The precipitated polymer was filtered and washed with MeCN several times and dried in vacuo to afford the desired copolymer 1 (3.67 g, 58%). The molar ratio of the monomer was determined by 1H NMR analysis {styrene-2-[(2-phenylallyloxy)methyl]oxirane-tetraethyleneglycol mono-2-phenyl-2-propenyl ether-dicyclohexyl-2-[4′-(4-styrylmethoxy)phenyl]phen­-ylphosphine = 65:7:6:22}. 31P NMR (162 MHz, CDCl3): δ = -12.9, 48.5. Mw: 11520, Mn: 8470, Mw/Mn = 1.36 (GPC).
Preparation of PI Pd 2: Copolymer 1 (3.66 g) was dissolved in THF (20 mL) and the solution was refluxed under argon. A solution of palladium acetate (177 mg, 0.79 mmol) in THF (20 mL) was added dropwise to the polymer solution, and the reaction mixture was refluxed under argon for 3 h, and then cooled to r.t. Hexane (80 mL) was then slowly added to the mixture. The mixture was left to stand at r.t. for 1 h, and the precipitated catalyst capsules were washed with hexane several times and dried at r.t. in vacuo. Next, the catalyst capsules were stirred at 120 °C for 4 h under reduced pressure to cross-link the microencapsulated palladium. The cross-linked solid was then washed with THF, toluene and hexane, and then dried in vacuo to give the polymer incarcerated palladium (PI Pd 2, 3.54 g); loading value of palladium metal: 0.19 mmol/g; the ratio of phosphorus atoms in the polymer to palladium atoms (P/Pd): 5.71.
Typical Experimental Procedure for Phosphinated PI Pd-Catalyzed Amination (Table 2, entry 2): 4-Chloro­-toluene (76.0 mg, 0.6 mmol), aniline (111.8 mg, 1.2 mmol), PI Pd 2 (63.5 mg, 0.012 mmol, 2 mol%), and t-BuONa (132.6 mg, 1.38 mmol) were combined in toluene (2 mL) under argon. The mixture was stirred for 8 h at reflux. After cooling to r.t., the mixture was diluted with hexane (6 mL), filtered and rinsed with hexane and toluene. The combined organic layers were concentrated in vacuo, and the volume of the residue was adjusted to 50 mL using hexane. The solution was then divided into two halves.
One portion was placed in a 50-mL test tube and the solvent was removed by evaporation. To this mixture was added sulfuric acid (1.0 mL), and the mixture was heated at 180 °C. Then nitric acid (0.5 mL) was added dropwise to decompose the organic residue. After cooling to r.t., the solution was adjusted to 25 mL by H2O and then the amount of palladium metal was measured by ICP analysis to determine the leaching of palladium.
The other portion of the reaction mixture was taken, and the solvents were removed by evaporation. The residual crude product was purified by preparative TLC on silica gel (hexane-EtOAc, 10:1) to afford N-(4-methylphenyl)aniline (53.3 mg, 97%). 1H NMR (400 MHz, CDCl3): δ = 2.29 (s, 3 H), 5.58 (s, 1 H), 6.87 (t, J = 7.6 Hz, 1 H), 6.99-7.02 (m, 4 H), 7.08 (d, J = 8.0 Hz, 1 H), 7.24 (dd, J = 7.6, 8.0 Hz, 2 H). 13C NMR (100 MHz, CDCl3): δ = 20.7, 116.9, 118.9, 120.3, 129.3, 129.9, 130.9, 140.3, 143.9.
Typical Recycling Procedures: 4-Chlorotoluene (144.4 mg, 1.14 mmol), morpholine (197.6 mg, 2.27 mmol), PI Pd 2 (120.4 mg, 0.023 mmol, 2 mol%), and t-BuONa (250.6 mg, 2.61 mmol) were combined in toluene (3.8 mL) under argon. The mixture was stirred for 4 h at 100 °C. After cooling to r.t., the mixture was diluted with hexane (10 mL) and filtered. The collected catalyst was further washed with hexane and toluene, and then filtered. The combined organic washing solutions were subjected to the procedures described above for product isolation and detection of palladium leaching. The catalyst was rinsed with H2O and MeOH and was then dried in vacuo. The catalyst was reused without any other purification step.