Ligand-Free and Recyclable Palladium(II) Acetate Catalyzes the Decarboxylative Cross-Coupling of Alkynyl Carboxylic Acids with Arylboronic Acids in Aqueous PEG-400

Abstract

A novel and ligand-free method was developed for the decarboxylative cross-coupling of alkynylcarboxylic acids with arylboronic acids. By using an environmentally friendly H₂O–poly(ethylene glycol) (PEG-400) system as the reaction medium, a series of internal alkynes were synthesized in good yields and with remarkable selectivity. The Pd(OAc)₂–H₂O–PEG-400 catalytic system could be used for up to three cycles without any loss of activity, demonstrating the robustness of the approach.

Key words

palladium catalysis - alkynylcarboxylic acids - arylboronic acids - decarboxylative cross-coupling - alkynes

• References and Notes

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• 28 Arylarynes 3; General Procedure A Schlenk tube was charged with the appropriate alkynylcarboxylic acid 1 (0.3 mmol), arylboronic acid 2 (0.5 mmol), Pd(OAc)₂ (0.015 mmol), Ag₂O (0.6 mmol), pyridine (0.6 mmol), H₂O (0.1 g), and PEG-400 (0.9 g), and the reaction was allowed to proceed under air at 80 °C (oil-bath temperature) for 4 h. When the reaction was complete, the mixture was cooled to r.t., diluted with sat. aq NaCl (10 mL) and extracted with Et₂O (3 × 20 mL). The combined organic layer was dried (Na₂SO₄) and transferred to a round-bottomed flask. The combined organic extracts were concentrated in vacuum, and the resulting residue was purified by column chromatography (silica gel, PE–EtOAc). 1-Methoxy-4-(phenylethynyl)benzene (3a). White solid; yield: 50 mg (80%); mp 93.8–94.7 °C. ¹H NMR (400 MHz, CDCl₃): δ = 7.52–7.49 (m, 2 H), 7.46 (d, J = 8.8 Hz, 2 H), 7.34–7.27 (m, 3 H), 6.86 (d, J = 8.8 Hz, 2 H), 3.80 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 159.6, 133.0, 131.4, 128.3, 127.9, 123.6, 115.3, 114.0, 89.4, 88.0, 55.2.