Palladium-Catalysed Heck Alkynylation of Aryl Bromides in an Imidazolium Ionic Liquid: An Unexpected Subsequent Alkyne Hydrogenation Reaction

Abstract

The copper-free palladium-catalysed alkynylation of aryl bromides with phenylacetylene in the imidazolium ionic liquid [BMIM][BF₄], in the presence of triphenylphosphine ligand and pyrrolidine as a base, was found effective and significantly more chemoselective employing deactivated substrates. When activated aryl substrates were used, unexpected side reactions were observed, especially the subsequent hydrogenation of the alkyne function in some coupling products. In other cases, amine arylation reactions occurred, as illustrated by the formation of pyrrolidinyl-4-nitrobenzene, for which an X-ray diffraction structure is reported.

Key words

catalysis - palladium - alkynes - arylation - cross-coupling

References and Notes

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Extensive studies with optimised results will be reported in due course.

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Typical Procedure.
The catalyst was prepared as a solid mixture of [Pd(allyl)Cl]₂ (6.3 mg, 0.03416 mmol of Pd), PPh₃ (26.9 mg, 0.10256 mmol) degassed for 15 min in a 20-mL Schlenk tube equipped with a magnetic stirrer bar and a reflux condenser. Under argon were added the aryl halide (either solid or liquid, 3.416 mmol) and 3 mL of [BMIM][BF₄]. The mixture was then degassed under reduced pressure for another 10 min. The Schlenk tube was heated in an oil bath at 110 °C to give a coloured solution. To the ionic liquid solution was added, out of the oil bath, 0.35 mL pyrrolidine (292 mg, 4.099 mmol, d = 0.87) and then the terminal alkyne (4.099 mmol). The resulting mixture was heated at 130 °C for 2-48 h under argon. The product was extracted from the ionic liquid phase by the addition of Et₂O (six portions of 5-10 mL) and decanting off the Et₂O from the IL phase. After evaporation the residue was purified by silica gel chroma-tography (Et₂O-hexane, 1:9) to give the enyne compound. [BMIM][BF₄] is commercially available (see for instance SOLVIONIC.com).

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The crystal structure has been deposited at the Cambridge Crystallographic Data Centre (deposition no. CCDC 603906)