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Synlett 2007(6): 0889-0892  
DOI: 10.1055/s-2007-973860
LETTER
© Georg Thieme Verlag Stuttgart · New York

Palladium-Catalyzed Heck Reaction of Aryl Bromides in Aqueous Media Using Tris(N-Heterocyclic Carbene) Ligands

Ismail Özdemir*a, Serpil Demira, Bekir Çetinkayab
a Inönü University, Faculty Science and Arts, Chemistry Department, 44280 Malatya, Turkey
Fax: +90(422)3410212; e-Mail: iozdemir@inonu.edu.tr;
b Department of Chemistry, Ege University, 35100 Bornova-Izmir, Turkey
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Publikationsverlauf

Received 17 April 2006
Publikationsdatum:
26. März 2007 (online)

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Abstract

A highly effective, easy to handle and environmentally benign process for palladium-mediated Heck reaction was developed. The in situ prepared three-component system composed of palladium(II) acetate, a tris(azolinium) bromide and potassium tert-butoxide, quantitatively catalyzes the Heck reaction of aryl bromides in aqueous media.

Key words

Heck reaction - imidazolinium and benzimidazolium salts - C-C coupling - palladium - N-heterocyclic carbene

13

General Procedure for the Synthesis of Tridentateazolinium Bromides To a solution of 1-alkylimidazoline or 1-alkylbenzimidazole (15 mmol) in DMF (15 mL) was added slowly 2,4,6-tris(bromomethyl)mesitylene (5 mmol) at 25 °C and the resulting mixture was heated for 5 h at 60 °C. Then, Et2O (15 mL) was added to obtain a white crystalline solid, which was filtered off. The solid was washed with Et2O (3 × 15 mL), dried under vacuum. The crude product was crystallized from EtOH-Et2O (2:1).
2,4,6-Tris[3-(2,4,6-trimethylbenzyl)imidazolinium]methyl-mesitylene bromide (2a): yield 4.67 g (93%); mp 86-287 °C. IR: ν( CN) = 1650 cm-1. 1H NMR (CDCl3): δ = 2.16 and 2.22 [s, 27 H, CH2C6H2(CH 3)3-2,4,6], 2.49 [s, 9 H, C6(CH 3)3(CH2)3-2,4,6], 3.64 and 4.29 [t, J = 10.8, 10.4 Hz, 12 H, NCH2CH2N], 4.69 [s, 6 H, C6(CH3)3(CH 2)3-2,4,6], 4.91 [s, 6 H, CH 2C6H2(CH3)3-2,4,6], 6.76 [s, 6 H, CH2C6 H 2(CH3)3-2,4,6], 9.54 (s, 3 H, NCHN). 13C{1H}NMR (CDCl3): δ = 20.6 and 21.3 [CH2C6H2(CH3)3-2,4,6], 18.0 [C6(CH3)3(CH2)3-2,4,6], 46.5 [C6(CH3)3(CH2)3-2,4,6], 47.9 and 48.0 (NCH2 CH2N), 50.3 [CH2C6H2(CH3)3-2,4,6], 126.0, 130.1, 138.2 and 139.0 [CH2 C 6H2(CH3)3-2,4,6], 129.4, 141.4 and 156.8 [C 6(CH3)3(CH2)3-2,4,6], 162.9 (NCHN). Anal. Calcd for C51H69N6Br3: C, 60.90; H, 6.91; N, 8.36. Found: C, 60.92; H, 6.88; N, 8.35.
2,4,6-Tris[3-(methoxyethyl)imidazolinium]methyl-mesitylene bromide (2b): yield 3.16 g (81%); mp 156-157 °C. IR: ν( CN) = 1660 cm-1. 1H NMR (CDCl3): δ = 2.44 [s, 9 H, C6(CH 3)3(CH2)3-2,4,6], 3.28 (s, 9 H, NCH2CH2OCH 3), 3.41 and 3.52 (t, J = 4.8 Hz, 12 H, NCH 2CH 2N), 4.14 (t, J = 11.0 Hz, 6 H, NCH 2CH2OCH3), 4.38 (t, J = 11.2 Hz, 6 H, NCH2CH 2OCH3), 4.65 [s, 6 H, C6(CH3)3(CH 2)3-2,4,6], 9.14 (s, 3 H, NCHN). 13C{1H}NMR (CDCl3): δ = 17.7 [C6(CH3)3(CH2)3-2,4,6], 59.0 (NCH2CH2OCH3), 47.7 (NCH2CH2OCH3), 47.9 (NCH2CH 2OCH3), 50.1 and 50.6 (NCH2 CH2N), 69.9 [C6(CH3)3(CH2)3-2,4,6], 129.1, 141.0 and 157.1 [C 6(CH3)3(CH2)3-2,4,6], 162.6 (NCHN). Anal. Calcd for C30H51N6O3Br3: C, 45.99; H, 6.56; N, 10.73. Found: C, 46.00; H, 6.55; N, 10.75.
2,4,6-Tris[3-(ethoxyethyl)imidazolinium]methylmesitylene bromide (2c): yield 3.54 g (86%); mp 123-124 °C. IR: ν( CN) = 1659 cm-1. 1H NMR (CDCl3): δ = 0.96 [t, J = 6.9 Hz, 9 H, NCH2CH2OCH2CH 3], 2.31 [s, 9 H, C6(CH 3)3(CH2)3-2,4,6], 3.30 (q, J = 6.9 Hz, 6 H, NCH2CH2OCH 2CH3), 3.42 and 3.68 (m, 12 H, NCH 2CH 2N), 4.06 (t, J = 10.4 Hz, 6 H, NCH 2CH2OCH2CH3), 4.25 (t, J = 10.6 Hz, 6 H, NCH2CH 2OCH2CH3), 4.50 [s, 6 H, C6(CH3)3(CH 2)3-2,4,6], 9.03 (s, 3 H, NCHN). 13C{1H}NMR (CDCl3): δ = 17.5 [C6(CH3)3(CH2)3-2,4,6], 15.5 (NCH2CH2OCH2 CH3), 66.7 (NCH2CH2OCH2CH3), 47.9 (NCH2CH2OCH2CH3), 48.4 (NCH2 CH2OCH2CH3), 50.5 and 50.8 (NCH2 CH2N), 68.3 [C6(CH3)3(CH2)3-2,4,6], 129.2, 141.2 and 157.2 [C 6(CH3)3(CH2)3-2,4,6], 162.7 [NCHN]. Anal. Calcd for C33H57N6O3Br3: C, 48.01; H, 6.96; N, 10.18. Found: C, 48.03; H, 6.99; N: 10.15.
2,4,6-Tris[3-(2,4,6-trimethylbenzyl)benzimida-zolium]methylmesitylene bromide (3a): yield 5.00 g (87%); mp 214-215 °C. IR: ν( CN) = 1562 cm-1. 1H NMR (DMSO-d 6): δ = 2.22 and 2.24 [s, 27 H, CH2C6H2(CH 3)3-2,4,6], 2.39 [s, 9 H, C6(CH 3)3(CH2)3-2,4,6], 5.94 and 5.95 [s, 12 H, C6(CH3)3(CH 2)3-2,4,6 and CH 2C6H2(CH3)3-2,4,6 ], 6.93 [s, 6 H, CH2C6 H 2(CH3)3-2,4,6], 7.16 [d, J = 8.0 Hz, 3 H, NC6H4N], 7.56 and 7.67 (t, J = 7.6 Hz, 6 H, NC6H4N), 8.13 (d, J = 8.4 Hz, 3 H, NC6H4N), 10.33 (s, 3 H, NCHN). 13C{1H}NMR (DMSO-d 6): δ = 20.4 and 21.3 [CH2C6H2(CH3)3-2,4,6], 17.2 [C6(CH3)3(CH2)3-2,4,6], 47.1 [C6(CH3)3(CH2)3-2,4,6], 47.4 [CH2C6H2(CH3)3-2,4,6], 126.8, 130.4, 138.6 and 139.1 [CH2 C 6H2(CH3)3-2,4,6], 129.5, 141.9 and 142.9 [C 6(CH3)3(CH2)3-2,4,6], 114.3, 115.2, 127.3, 127.8, 132.1 and 132.4 (NC6H4N), 163.1 (NCHN). Anal. Calcd for C63H69N6Br3: C, 65.80; H, 6.05; N, 7.31. Found: C, 65.83; H, 6.02; N, 7.30.
2,4,6-Tris[3-(3,4,5-trimethoxybenzyl)benzimida-zolium]methylmesitylene bromide (3b): yield 5.81 g (90%); mp 195-196 °C. IR ν( CN) = 1596 cm-1. 1H NMR (CDCl3): δ = 2.44 [s, 9 H, C6(CH 3)3(CH2)3-2,4,6], 3.72 and 3.77 [s, 27 H, CH2C6H2(OCH 3)3-3,4,5], 5.84 [s, 6 H, C6(CH3)3(CH 2)3-2,4,6], 5.90 [s, 6 H, CH 2C6H2(OCH3)3-3,4,5], 6.82 [s, 6 H, CH2C6 H 2(OCH3)3-3,4,5], 7.56 (m, 9 H, NC6H4N), 8.25 (d, J = 8.1 Hz, 3 H, NC6H4N), 10.59 (s, 3 H, NCHN). 13C{1H}NMR (CDCl3): δ = 18.5 [C6(CH3)3(CH2)3-2,4,6], 47.5 [C6(CH3)3 ( CH2)3-2,4,6], 52.7 [CH2C6H2(OCH3)3-3,4,5], 57.1 and 61.2 [CH2C6H2(OCH3)3-3,4,5], 106.5, 138.6, 141.5 and 153.9 [CH2 C 6H2(OCH3)3-3,4,5], 129.2, 142.4 and 142.7 [C 6(CH3)3(CH2)3-2,4,6], 114.2, 114.7, 127.5, 127.8, 132.0 and 132.1 (NC6H4N), 190.1 (NCHN). Anal. Calcd for C63H69N6O9Br3: C, 58.48; H, 5.37; N, 6.49. Found: C, 58.45; H, 5.36; N, 6.50. 2,4,6-Tris[3-(ethoxtethyl)benzimidazolium]methyl-mesitylene bromide (3c): yield 4.01 g (83%); mp 282-283 °C. IR ν( CN) = 1563 cm-1. 1H NMR (CDCl3): δ = 0.79 (t, J = 6.8 Hz, 9 H, NCH2CH2OCH2CH 3), 2.48 [s, 9 H, C6 (CH 3)3(CH2)3-2,4,6], 3.76 (quart, J = 7.2 Hz, 6 H, NCH2CH2OCH 2CH3), 3.79 (t, J = 12.0 Hz, 6 H, NCH 2CH2OCH2CH3), 4.85 (t, J = 12.0 Hz, 6 H, NCH2CH 2OCH2CH3), 5.90 [s, 6 H, C6(CH3)3(CH 2)3-2,4,6], 8.12 (d, J = 7.6 Hz, 3 H, NC6H4N), 8.29 (d, J = 8.0 Hz, 3 H, NC6H4N), 7.76 (m, 6 H, NC6H4N), 10.09 (s, 3 H, NCHN). 13C{1H}NMR (CDCl3): δ = 17.1 [C6(CH3)3(CH2)3-2,4,6], 15.4 (NCH2CH2OCH2 CH3), 46.7 (NCH2CH2OCH2CH3), 47.3 (NCH2 CH2OCH2CH3), 66.2 (NCH2CH2OCH2CH3), 68.2 [C6(CH3)3(CH2)3-2,4,6], 129.4, 142.0 and 142.8 [C 6(CH3)3(CH2)3-2,4,6], 114.8, 114.9, 127.4, 127.6, 131.8 and 132.4 (NC6H4N), 171.2 (NCHN). Anal. Calcd for C45H57N6O3Br3: C, 55.74; H, 5.92; N, 8.67. Found: C, 55.75; H, 5.90; N: 8.66. General Procedure for the Heck Coupling Reaction Pd(OAc)2 (1.0 mmol%), 2,4,6-tris[(3-alkyl)azo-linium]methylmesitylene bromide 2 or 3 (0.7 mmol%), aryl bromide (1.0 mmol), styrene (1.5 mmol), t-BuOK (2 mmol), H2O-DMF (6 mL, 1:1) were added to a small Schlenk tube and the mixture was heated at 50 °C for 5 h. Finally, the mixture was cooled, extracted with EtOAc-hexane (1:5), filtered through a pad of silica gel with copious washings, concentrated and purified by flash chromatography on silica gel. Purity of the compounds was checked by NMR and GC, yields are based on aryl bromide.