Bu₄N[AuCl₄]-Catalyzed Synthesis of Highly Substituted Furans from 2-(1-Alkynyl)-2-alken-1-ones in Ionic Liquids: An Air-Stable and Recyclable Catalytic System

Abstract

The cyclization of 2-(1-alkynyl)-2-alken-1-ones proceeded smoothly in the presence of various nucleophiles with a ­catalytic amount of Bu₄N[AuCl₄] in [bmim]BF₄, leading to the formation of highly substituted furans. This system is air-stable and ­recyclable. The catalyst is inexpensive and readily available.

Key words

gold-catalyzed - cyclization reaction - ionic liquids - furans - recyclable

References and Notes

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Representative Procedure.
To the appropriate 2-(1-alkynyl)-2-alken-1-one (0.2 mmol) and nucleophile (1.5 equiv) in [bmim]BF₄ (1 mL) was added Bu₄N[AuCl₄] (1 mol%). The mixture was stirred at r.t. for 3 h unless otherwise specified. The mixture was extracted with Et₂O (2 ¥ 5 mL). The isolated ether layer was concentrated in vacuo and the residue was purified by flash chromatography on silica gel.

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Recycling Procedure.
To the 2-phenylethynyl-2-cyclohexen-1-one (1.0 mmol) and MeOH (1.5 equiv) in [bmim]BF₄ (3 mL) was added Bu₄N[AuCl₄] (1 mol%). The mixture was stirred at r.t. for 3 h. The product was extracted from the reaction mixture by addition of Et₂O (2 ¥ 20 mL). The ether layer was separated. The ionic liquid layer containing Au catalyst was pumped for several minutes and reused for the next reaction.

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Spectral Data of the Representative Product 2a.
¹H NMR (300 MHz, CDCl₃): δ = 1.79-1.93 (m, 2 H), 1.94-2.05 (m, 2 H), 2.59-2.75 (m, 2 H), 3.43 (s, 3 H), 4.28 (t, J = 3.9 Hz, 1 H), 6.64 (s, 1 H), 7.18-7.23 (m, 1 H), 7.31-7.36 (m, 2 H), 7.60-7.63 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 18.8, 23.2, 28.3, 56.1, 72.3, 105.3, 119.9, 123.3, 126.8, 128.5, 131.1, 152.2, 152.8.