Magnetically Recoverable Pd/Fe₃O₄-Catalyzed Hiyama Cross-Coupling of Aryl Bromides with Aryl Siloxanes

Abstract

This letter describes a simple and efficient method for the synthesis of biaryls by fluoride-free Hiyama cross-coupling of bromoarenes with aryl siloxanes using magnetically separable Pd/Fe₃O₄ as the catalyst under aqueous conditions. This methodology is applicable to wide range of aryl bromides and aryl siloxanes. High catalytic activity, ease of recovery using an external magnetic field and use of water as the solvent are additional eco-friendly attributes of this catalytic system. The catalyst was recycled five times without significant loss of catalytic activity.

Hiyama cross-coupling - palladium - aryl siloxanes - aryl bromides - biaryls

References and Notes

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19

Synthesis of the Fe ₃ O ₄ nanoparticles: FeSO₄˙7H₂O (13.9 g) and Fe_{2 (}SO₄)₃ (20 g) were dissolved in H₂O (500 mL) in a 1000 mL beaker. NH₄OH (aq, 25%) was added slowly to adjust the pH of the solution to 10. The reaction mixture was then continually stirred for 1 h at 60 ˚C. The precipitated nanoparticles were separated magnetically, washed with water until the pH 7, and then dried under vacuum at 60 ˚C for 2 h. This magnetic nano ferrite (Fe₃O₄) was then used for the preparation of Pd/Fe₃O₄.
Synthesis of the Pd/Fe ₃ O ₄ catalyst: Fe₃O₄ nanoparticles were impregnated with Na₂PdCl₄ (1.0%) aqueous solution and stirred for 1 h. After impregnation, the suspension was adjusted to pH 12 by adding NaOH (1 M) and stirred for 6 h. The solid was washed with distilled H₂O. The catalyst precursors were reduced by adding 0.2 M NaBH₄ solution dropwise under gentle stirring in an ice-water bath for 30 min until no obvious bubbles were observed in the solution. The resulting Pd/Fe₃O₄ was washed thoroughly with distilled H₂O and subsequently with EtOH. The palladium content in the catalyst was measured as 0.023 mmol˙g^-¹ using ICP-AES.
General procedure for the Hiyama reaction: A mixture
of aryl bromide (1 mmol), aryl siloxane (1.2 mmol), NaOH (3 mmol), Pd/Fe₃O₄ catalyst (50 mg, 0.2 mol% of Pd) and distilled H₂O (3 mL) was taken in a round-bottomed flask and stirred at 90 ˚C for 6 h. After completion of the reaction (monitored by TLC) the catalyst was easily separated from the reaction mixture with an external magnet. After removing the solvent, the crude material was purified by chromatography on silica gel to afford the pure product. The spectroscopic data of all known compounds were identical to those reported in the literature. 2′-Methoxy-4-methylbiphenyl (Table  [²] , entry 8): ^¹H NMR (300 MHz, CDCl₃): δ = 2.35 (s, 3 H), 3.78 (s, 3 H), 6.88-6.99 (m, 2 H), 7.22-7.28 (m, 2 H), 7.15 (d, J = 8.0 Hz, 2 H), 7.35 (d, J = 8.0 Hz, 2 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 21.1, 55.4, 111.1, 120.7, 128.3, 128.6, 129.3, 130.7, 131.4, 131.5, 136.4, 156.4. MS (EI): m/z = 198 [M]⁺. 4′-Methoxy-2,4,6-trimethylbiphenyl (Table  [²] , entry 16): ^¹H NMR (300 MHz, CDCl₃): δ = 2.03 (s, 6 H), 2.35 (s, 3 H), 3.86 (s, 3 H), 6.95 (s, 2 H), 7.07 (d, J = 8.6 Hz, 2 H), 7.49 (d, J = 8.6 Hz, 2 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 20.7, 20.9, 55.2, 113.7, 127.9, 130.3, 133.2, 136.4, 138.6, 158.1. MS (EI): m/z = 226 [M]⁺. 4′-tert-Butyl-2,4,6-trimethylbiphenyl (Table  [²] , entry 17): ^¹H NMR (300 MHz, CDCl₃): δ = 1.39 (s, 9 H), 1.98 (s, 6 H), 2.30 (s, 3 H), 6.85 (s, 2 H), 7.01 (d, J = 8.3 Hz, 2 H), 7.41 (d, J = 8.3 Hz, 2 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 20.8, 27.0, 31.3, 34.4, 125.6, 126.6, 127.9, 128.8, 136.2, 138.2, 149.8. MS (EI): m/z = 252 [M]⁺. 1-p-Tolylnaphthalene (Table  [²] , entry 18): ^¹H NMR (300 MHz, CDCl₃): δ = 2.45 (s, 3 H), 7.25 (d, J = 8.3 Hz, 2 H), 7.33-7.38 (m, 3 H), 7.39-7.49 (m, 3 H), 7.77-7.92 (m, 3 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 21.2, 125.3, 125.6, 125.8, 126.0, 126.8, 127.4, 128.2, 128.9, 129.9, 131.6, 133.7, 136.8, 137.7, 140.2. MS (ESI): m/z = 218 [M]⁺.