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Synlett 2005(20): 3116-3120  
DOI: 10.1055/s-2005-922754
LETTER
© Georg Thieme Verlag Stuttgart · New York

N-Heterocyclic NCN-Pincer Palladium Complexes: A Source for General, Highly Efficient Catalysts in Heck, Suzuki, and Sonogashira Coupling Reactions

Fátima Churruca, Raul SanMartin*, Imanol Tellitu, Esther Domínguez*
Kimika Organikoa II Saila, Zientzia eta Teknologia Fakultatea, Euskal Herriko Unibertsitatea, P.O. Box 644, 48080 Bilbao, Spain
Fax: +34(94)6012748; e-Mail: qopsafar@lg.ehu.es;
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Publication History

Received 6 October 2005
Publication Date:
28 November 2005 (online)

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Abstract

Readily available NCN-pincer palladium complexes comprising two pyrazole units as the source of both N-donor atoms are successfully employed as catalysts in a range of C-C bond-forming reactions. Good to excellent results are obtained in all cases regardless of the electronic nature of the substrates, along with more convenient procedures and comparatively much lower catalysts loadings in Suzuki and Sonogashira couplings. This paper presents the first report of a Sonogashira coupling reaction by means of a NCN catalyst.

Key words

palladacycles - catalysis - pincer complexes - cross-coupling

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2a: white powder, mp >300 °C (EtOAc). FTIR (neat film): 1714, 1592, 1510, 1412 cm-1. 1H NMR (500 MHz, DMSO): δ = 3.83 (3 H, s, CH3), 5.55 (4 H, s, CH2), 6.46 (2 H, dd, J = 2.0, 1.6 Hz, H-4′), 7.70 (2 H, s, H2, H-6), 7.92 (2 H, d, J = 1.6 Hz, H-5′), 8.14 (2 H, d, J = 2.1 Hz, H-3′). 13C NMR (63 MHz, DMSO): δ = 52.1 (CH3), 56.7 (CH2), 106.6 (C-4′), 125.9 (C-2, C-6), 126.4 (C-1), 133.1 (C-5′), 137.2 (C-3, C-5), 143.2 (C-3′), 150.7 (C-4), 166.2 (CO). Anal. calcd for C16H15ClN4O2Pd: C, 43.96; H, 3.46; N, 12.82. Found: C, 43.93; H, 3.48; N, 12.83.
2b: white powder, mp >300 °C (EtOAc). FTIR (neat film): 1702, 1590, 1549, 1425 cm-1. 1H NMR (500 MHz, CDCl3): δ = 2.34 (6 H, s, C5′-CH3), 2.61 (6 H, s, C3′-CH3), 3.87 (3 H, s, COOCH3), 4.95 (2 H, d, J 14.1 Hz, CHaHb), 5.66 (2 H, d, J = 14.1 Hz, CHaHb), 5.82 (2 H, s, H-4′), 7.58 (2H, s, H-2, H-6). 13C NMR (63 MHz, CDCl3): δ = 11.7 (C-5′CH3), 15.5 (C-3′CH3), 52.0 (COOCH3), 54.1 (CH2), 107.1 (C-4′), 125.5 (C-2, C-6), 126.3 (C-1), 137.3 (C-3, C-5), 140.4 (C-5′), 152.5 (C-3′), 154.6 (C-4), 166.8 (CO). Anal. calcd for C20H23ClN4O2Pd: C, 48.70; H, 4.70; N, 11.36. Found: C, 48.74; H, 4.67; N, 11.35.

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General procedure: A dry 5-mL round-bottom flask was charged with aryl bromide (1 mmol), alkene (1.5 mmol), catalyst 2 (0.001 mmol Pd), and anhyd DMF (1 mL). The mixture was stirred at 140 °C under argon for 18 h. After cooling, H2O (10 mL) was added, and the aqueous layer was extracted with EtOAc (3 × 10 mL). The combined organic extracts were dried over anhyd Na2SO4 and evaporated in vacuo. The residue was dissolved in CDCl3 and analyzed by 1H NMR and 13C NMR spectroscopy [bis(ethylene glycol) dimethyl ether as an internal standard]; the identity of every product was confirmed by comparison with spectroscopic data in the literature.

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General procedure: A 5-mL round-bottom flask was charged with ArBr (1 mmol), ArB(OH)2 (1.5 mmol), catalyst 2 (0.001 mmol Pd), K2CO3 (2 mmol), and H2O (1 mL). The mixture was stirred at 100 °C in air for 2 h. After cooling, Na2CO3 (5 mL of 10% solution in water) was added, and the aqueous layer was extracted with CH2Cl2 (2 × 5 mL). The combined organic extracts were dried over anhyd Na2SO4 and evaporated in vacuo. The residue was dissolved in CDCl3 and analyzed by 1H NMR spectroscopy [using bis(ethylene glycol) dimethyl ether as an internal standard]; the identity of every product was confirmed by comparison with spectroscopic data in the literature.

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A 5-mL round-bottom flask was charged with ArI (1 mmol), alkyne (1.5 mmol), catalyst 2 (0.001 mmol Pd), and pyrrolidine (2 mL). The mixture was stirred at 100 °C in air for 6 h. After cooling, the solvent was evaporated in vacuo. The residue was dissolved in CDCl3 and analyzed by 1H NMR spectroscopy [using bis(ethylene glycol) dimethyl ether as an internal standard]; the identity of every product was confirmed by comparison with spectroscopic data in the literature.

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The highest value (80,000) was achieved by reaction of 4-chlorobenzene and 1-octyne in the presence of catalyst 2b (Table [3] , entry 14). When 0.01 mol% Pd was employed, 100% conversion and 80% yield for the corresponding 1-phenyloctyne were obtained; TOF = 4444.