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Synlett 2005(13): 2003-2006  
DOI: 10.1055/s-2005-871952
LETTER
© Georg Thieme Verlag Stuttgart · New York

Palladium-Catalyzed Michael Addition of Indoles to α,β-Unsaturated Ketones in an Ionic Liquid

Wei-Jun Li, Xu-Feng Lin, Jun Wang, Guo-Liang Li, Yan-Guang Wang*
Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China
Fax: +86(571)87951512; e-Mail: orgwyg@zju.edu.cn;
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Publication History

Received 8 May 2005
Publication Date:
12 July 2005 (online)

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Abstract

The PdCl2(CH3CN)2-catalyzed Michael addition reaction of indoles with α,β-unsaturated ketones in ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) to afford the corresponding 3-alkylated indoles in good yields is described. The catalyst system PdCl2(CH3CN)2/[bmim][BF4] can be recovered and reused.

Key words

palladium - indoles - Michael reaction - ionic liquid

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Indoles 3; General Procedure. The appropriate indole 1 (1.0 mmol), α,β-unsaturated ketone 2 (1.0 mmol), and PdCl2 (CH3CN)2 (2 mol%) were mixed in [bmim][BF4] (1 mL). The mixture was stirred at 100 °C under N2 for the appropriate time and the reaction was monitored by TLC. After completion, the products were separated by thorough extraction with a mixture of EtOAc-Et2O. The solvent was evaporated and pure 3-substituted indole derivatives 3 were obtained by silica gel column chromatography with hexane-EtOAc (3:1). The ionic liquid containing PdCl2(CH3CN)2 was dried under vacuum for the next run. All products gave satisfactory spectroscopic and analytical data. Representative compound 3a: Yield 94%, pale pink solid, mp 136-138 °C, R f 0.5 (hexane-EtOAc, 3:1). IR (KBr): 3407 (NH), 1675 (C=O) cm-1. 1H NMR (500 MHz, CDCl3): δ = 8.04 (br, 1 H), 7.98 (d, J = 7.5 Hz, 2 H), 7.58 (t, J = 7.4 Hz, 1 H), 7.45-7.50 (m, 3 H), 7.40 (2 H, J = 7.4 Hz, 2 H), 7.28-7.34 (m, 3 H), 7.17-7.23 (m, 2 H), 7.07 (t, J = 7.4 Hz, 1 H), 6.99 (s, 1 H), 5.13 (t, J = 7.2 Hz, 1 H), 3.85 (dd, J = 6.8, 6.8 Hz, 1 H), 3.78 (dd, J = 7.6, 7.6 Hz, 1 H). 13C NMR (125 MHz, CDCl3): δ = 198.9, 144.5, 137.3, 136.8, 133.2, 128.8, 128.6, 128.3, 128.0, 126.8, 126.5, 122.3, 121.7, 119.7, 119.6, 119.4, 111.4, 45.4, 38.4. MS (EI): m/z = 325 [M+]. HRMS (EI): m/z calcd for C23H19NO [M]+: 325.1467, found: 325.1465.