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Synlett 2011(8): 1081-1084  
DOI: 10.1055/s-0030-1259927
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

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

B. Sreedhar*, A. Suresh Kumar, Divya Yada
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology (Council of Scientific and Industrial Research), Hyderabad 500007, India
Fax: +91(40)27160921; e-Mail: sreedharb@iict.res.in;
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Publication History

Received 2 December 2010
Publication Date:
29 March 2011 (online)

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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.

Key words

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

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Synthesis of the Fe 3 O 4 nanoparticles: FeSO4˙7H2O (13.9 g) and Fe2 (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]+.