February 2018
Alkylimidazole-Based Phosphines as Efficient Ligands for Palladium-Catalyzed
555
Suzuki Reactions
(31.69, 57.13, 119.98, 128.41, 129.67, 133.65, 136.76,
144.76); 31P NMR (160 MHz,CDCl3) δ: À23.43 ppm;
MS (70 eV): m/z = 308 (M+); Anal. Calcd for
C19H21N2P:C 74.01, H 6.86, N 9.08; found C 73.92, H
6.83, N 9.11.
Bis(N-tert-butyl-2-imidazolyl)phenylphosphine (L5).
purified on silica gel (200–300 mesh, petroleum ether
60–90°C) to give biphenyl as the white solid (2.89 g, 94%).
Acknowledgment. We gratefully acknowledge financial support
from Foundation of Hubei Provincial Department of Education
(No. Q20111704 and No. B20111702) and Research and
Innovation Initiatives of WHPU (No. 2016y19).
Synthesized as compound L1, using N-tert-
butylimidazole (12.4 g, 0.1 mol) instead of N-iso-
propylimidazole, dichlorophenylphosphine (8.95 g,
0.05 mol) instead of chlorodiphenylphosphine, white
REFERENCES AND NOTES
1
solid (6.5 g, 37%), mp 165–166°C. H NMR (400 MHz,
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CDCl3) δ: 1.66 (s, 18 H), 7.16 (s, 2H), 7.32–7.77 (m,
7H); 13C NMR (100 MHz, CDCl3) δ: (25.13, 49.37,
118.65, 128.38, 128.96, 131.38, 132.61, 132.85, 142.64);
31P NMR (160 MHz, CDCl3) δ: À46.01 ppm; MS
(70 eV): m/z = 354 (M+), Anal. Calcd for C20H27N4P: C
67.77, H 7.68, N 15.81; found C 67.63, H 7.62, N 15.70.
Tris(N-tert-butyl-2-imidazolyl)phosphine
(L6).
Synthesized as compound L1, using N-tert-
butylimidazole (12.4 g, 0.1 mol) instead of N-iso-
propylimidazole, phosphorus trichloride (4.57 g,
0.03 mol) instead of chlorodiphenylphosphine, white
1
solid (3.7 g, 28%), mp 259–260°C. H NMR (400 MHz,
CDCl3) δ: 1.60 (s, 27 H), 7.16–7.28 (m, 6H); 13C NMR
(100 MHz, CDCl3) δ: (31.07, 56.64, 120.71, 129.84,
141.69); 31P NMR (160 MHz, CDCl3) δ: À51.03 ppm,
MS (70 eV): m/z = 400 (M+), Anal. Calcd for
C21H33N6P: C 62.98, H 8.31, N 20.98; found C 62.89, H
8.19, N 20.88.
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General procedure for the Suzuki coupling reaction.
A
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mixture of aryl halide (2 mmol), phenylboronic acid
(292.6 mg, 2.4 mmol), K3PO4·3H2O (2.13 g, 4 mmol),
PdCl2 (1.77 mg, 0.01 mmol), L4 (6.17 mg, 0.02 mmol),
and ethanol (3 mL) under nitrogen in a pressure tube was
heated to given temperature and maintained for given
time. Then it was cooled and extracted with diethyl ether
(4 × 5.0 mL) and dried with Na2SO4. After evaporation
under reduced pressure, the residue was purified on silica
gel to give the desired product.
Large-scale application for the Suzuki coupling reaction.
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A
mixture of bromobenzene (3.14 g, 20 mmol),
phenylboronic acid (2.93 g, 24 mmol), K3PO4·3H2O
(21.3 g, 40 mmol), PdCl2 (17.7 mg, 0.1 mmol), L4
(61.7 mg, 0.2 mmol), and ethanol (20 mL) under nitrogen
in a 50-mL two-necked flask was heated to 60°C and
monitored using TLC. After completion (~3 h), the
reaction mixture was diluted with brine (15 mL) and
extracted with ether (3 × 15 mL). The combined organic
extract was dried over anhydrous Na2SO4. After
evaporation under reduced pressure, the residue was
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet