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Synlett 2007(16): 2579-2583  
DOI: 10.1055/s-2007-986639
LETTER
© Georg Thieme Verlag Stuttgart · New York

Palladium-Doped Mixed Oxides as ‘Slow-Release’ Catalysts for Suzuki Couplings

Uli Kazmaier*a, Saskia Hähna, Torsten D. Weissb, Ralf Kautenburgerc, Wilhelm F. Maierb
a Institut für Organische Chemie, Universität des Saarlandes, 66123 Saarbrücken, Germany
b Lehrstuhl für Technische Chemie, Universität des Saarlandes, 66123 Saarbrücken, Germany
c Institut für Anorganische und Analytische Chemie und Radiochemie, Universität des Saarlandes, 66123 Saarbrücken, Germany
Fax: +49(681)3022409; e-Mail: u.kazmaier@mx.uni-saarland.de;
Further Information

Publication History

Received 28 June 2007
Publication Date:
28 August 2007 (online)

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Abstract

Palladium-containing sol-gel materials are efficient ­catalysts for Suzuki couplings. Especially good results are obtained under microwave irradiation in water. At least in this solvent, ­homogeneous palladium species are responsible for the catalytic ­activity.

Key words

cross-couplings - boronic acids - microwave irradiation - sol-gel catalysts - Suzuki couplings

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Preparation of Catalysts: The catalysts were prepared by a modified sol-gel method. The following precursors, Cu(NO3)2·3H2O, Fe(NO3)3·9H2O, La(NO3)3·6H2O, Ni(NO3)2·6H2O, and Pd(NO3)2·H2O, were dissolved in distilled H2O and mixed with ethylene glycol and nitric acid. The molar ratio was M/H2O/ethylene glycol/HNO3 = 1:40:20:4 (M as the sum of metal ions). The solutions were dried and calcined in air for 12 h at 80 °C, for 60 h at 105 °C and for 5 h at 400 °C with a heating rate of 6 °C/ h each. After calcination the powders were ground and sieved.

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General Procedure for Suzuki Couplings in EtOH-Toluene: To a two-phase system of toluene (4 mL), EtOH (1.3 mL) and 2 M K2CO3 (2 mL), aryl halide (1.0 mmol), boronic acid (1.1 mmol) and catalyst (for mol% of Pd see Table [1] ) were added and the mixture was heated to reflux for the specified time. After cooling to r.t., H2O was added and the aqueous layer was extracted with Et2O (3 ×). After drying (Na2SO4) and evaporation of the solvent, the crude product was purified by column chromatography (silica gel, hexanes-EtOAc).
4-Benzyloxy-4′-methoxybiphenyl (3): According to this general procedure 3 was obtained from 4-bromoanisole (187 mg) and 4-benzyloxyphenylboronic acid (251 mg) as a white solid; mp 170 °C. 1H NMR (500 Hz, CDCl3): δ = 7.45 (m, 6 H), 7.40 (dd, J = 7.3, 7.7 Hz, 2 H), 7.34 (t, J = 7.3 Hz, 1 H), 7.04 (d, J = 8.7 Hz, 2 H), 6.96 (d, J = 8.7 Hz, 2 H), 5.11 (s, 2 H), 3.85 (s, 3 H). 13C NMR (500 Hz, CDCl3): δ = 158.7, 157.9, 137.0, 133.7, 133.4, 128.6, 128.0, 127.7, 127.5, 115.1, 114.2, 70.1, 55.3. Anal. Calcd for C20H18O2 (290.36): C, 82.73; H, 6.25. Found: C, 82.99; H, 6.45. HRMS: m/z [M+] calcd for C20H18O2: 290.1307; found: 290.1340.

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General Procedure for Suzuki-Couplings in Water under Microwave Irradiation: In a glass tube were placed aryl halide (1 mmol), phenylboronic acid (183 mg, 1.5 mmol), catalyst (1.2 mol%), 2 M K2CO3 (2 mL) and H2O (4 mL). The sealed vessel was placed into the microwave. The mixture was irradiated in a sealed tube at 150 °C (initial power 100 W). After 60 min the reaction was cooled to r.t. Then H2O was added and the aqueous layer was extracted with Et2O (3 ×). After drying (Na2SO4) and evaporation of the solvent, the crude product was purified by column chromatography (silica gel, hexanes-EtOAc).
4-Nitrobiphenyl (8a): yellow solid; mp 112 °C. 1H NMR (500 Hz, CDCl3): δ = 8.30 (d, J = 8.9 Hz, 2 H), 7.74 (d, J = 8.9 Hz, 2 H), 7.63 (d, J = 7.1 Hz, 2 H), 7.50 (dd, J = 7.1, 7.5 Hz, 2 H), 7.45 (t, J = 7.5 Hz, 1 H). 13C NMR (500 Hz, CDCl3): δ = 147.6, 147.1, 138.7, 129.1, 128.9, 127.8, 127.4, 124.1.
2-Hydroxybiphenyl (8b): white solid; mp 57 °C. 1H NMR (500 Hz, CDCl3): δ = 7.52-7.55 (m, 4 H), 7.44 (m, 1 H), 7.28-7.33 (m, 2 H), 7.03-7.06 (m, 2 H), 5.32 (br s, 1 H). 13C NMR (500 Hz, CDCl3): δ = 152.4, 137.1, 130.2, 129.2, 129.1, 129.1, 128.1, 127.8, 120.8, 115.8.
4-Hydroxybiphenyl (8c): white solid; mp 57 °C. 1H NMR (500 Hz, CDCl3): δ = 7.55 (d, J = 7.5 Hz, 2 H), 7.49 (d, J = 8.6 Hz, 2 H), 7.42 (dd, J = 7.4, 7.5 Hz, 2 H), 7.31 (t, J = 7.4 Hz, 1 H), 6.91 (d, J = 8.6 Hz, 2 H), 4.81 (br s, 1 H).13C NMR (500 Hz, CDCl3): δ = 155.0, 140.7, 134.1, 128.7, 128.4, 126.7, 115.6.
Biphenyl-4-carboxylic Acid (8d): white solid; mp 215 °C. 1H NMR (500 Hz, DMSO-d 6): δ = 13.00 (br s, 1 H), 8.03 (d, J = 8.4 Hz, 2 H), 7.79 (d, J = 8.4 Hz, 2 H), 7.73 (d, J = 7.4 Hz, 2 H), 7.50 (dd, J = 7.4, 7.6 Hz, 2 H), 7.42 (t, J = 7.6 Hz, 1 H), 3.87 (s, 3 H). 13C NMR (500 Hz, DMSO-d 6): δ = 167.1, 144.2, 138.9, 131.6, 129.9, 129.0, 128.2, 126.9, 126.7.
Biphenyl-4-ylamine (8e): orange-brown solid (162 mg, 0.96 mmol, 96%); mp 53 °C. 1H NMR (500 Hz, CDCl3): δ = 7.57 (dd, J = 1.2, 8.2 Hz, 2 H), 7.45 (d, J = 8.4 Hz, 2 H), 7.41 (dd, J = 7.3, 8.2 Hz, 2 H), 7.30 (dt, J = 1.2, 7.3 Hz, 1 H), 6.77 (d, J = 8.4 Hz, 2 H), 3.73 (br s, 2 H). 13C NMR (500 Hz, CDCl3): δ = 145.8, 141.1, 131.5, 128.6, 128.0, 126.4, 126.2, 115.3.
Biphenyl-3-carbaldehyde (8f): colorless liquid. 1H NMR (500 Hz, CDCl3): δ = 10.09 (s, 1 H), 8.11 (dd, J = 1.6, 1.6 Hz, 1 H), 7.86 (dd, J = 1.6, 7.7 Hz, 2 H), 7.63 (d, J = 7.7 Hz, 2 H), 7.61 (dd, J = 7.7, 7.7 Hz, 1 H), 7.48 (dd, J = 7.3, 7.7 Hz, 2 H), 7.41 (t, J = 7.3 Hz, 1 H). 13C NMR (500 Hz, CDCl3): δ = 192.2, 142.1, 139.6, 136.9, 133.0, 129.4, 128.9, 128.6, 128.1, 128.0, 127.1.
4-Methoxybiphenyl (8g): white solid; mp 88 °C. 1H NMR (500 Hz, CDCl3): δ = 7.54-7.58 (sh, 4 H), 7.48 (dd, J = 7.5, 7.9 Hz, 2 H), 7.32 (t, J = 7.5 Hz, 1 H), 7.00 (d, J = 8.8 Hz, 2 H), 3.87 (s, 3 H). 13C NMR (500 Hz, CDCl3): δ = 159.1, 140.8, 133.8, 128.7, 128.1, 126.7, 126.6, 114.2, 55.3.
5-Phenylpyrimidine (8h): white solid; mp 41 °C. 1H NMR (500 Hz, CDCl3): δ = 9.21 (s, 1 H), 8.95 (s, 2 H), 7.58 (d, J = 7.5 Hz, 2 H), 7.52 (dd, J = 7.2, 7.5 Hz, 2 H), 7.47 (t, J = 7.2 Hz, 1 H). 13C NMR (500 Hz, CDCl3): δ = 157.5, 154.9, 134.3, 134.3, 129.4, 129.0, 127.0.
Biphenyl-4-carbaldehyde (8i): white solid; mp 59 °C. 1H NMR (500 Hz, CDCl3): δ = 10.06 (s, 1 H), 7.95 (d, J = 8.3 Hz, 2 H), 7.75 (d, J = 8.3 Hz, 2 H), 7.64 (d, J = 7.1 Hz, 2 H), 7.50 (dd, J = 7.1, 7.5 Hz, 2 H), 7.42 (t, J = 7.5 Hz, 1 H). 13C NMR (500 Hz, CDCl3): δ = 191.8, 147.1, 139.6, 135.1, 130.2, 128.9, 128.4, 127.6, 127.3.

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Determination of Pd and La Concentrations in Solution with ICP-MS: ICP-MS was conducted with a VG Elemental Plasma Quad 3 instrument. The examined isotopes were 105Pd and 139La. Calibration was carried out with external standards of 10 ppt, 100 ppt, 1 ppb, 2 ppb, 5 ppb, 10 ppb and 100 ppb palladium and lanthanum. The freshly irradiated (100 W, 150 °C, 1 h) and filtered solution of catalyst C was diluted to 1:20, 1:30, 1:100 and 1:200. A 3% nitric acid solution was added to all samples. The average of the Pd and La concentrations in solution was calculated from these acquired concentrations.