Layered Double Hydroxide-Supported Nanopalladium: An Efficient and Reusable Catalyst for Suzuki Coupling of Aryl Iodides and Bromides at Room Temperature

Abstract

Layered double hydroxide-supported nanopalladium (LDH-Pd⁰) is an effective catalyst for Suzuki cross-coupling of a wide range of aryl iodides and bromides with arylboronic acids at room temperature. The catalyst is reused for four cycles with consistent activity.

Key words

layered double hydroxides - nanopalladium - room temperature - Suzuki cross coupling - aryl iodides - arylboronic acids

References and Notes

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• For preparation of LDH-Cl, see:
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• 3b

  Typical Procedure. A mixture of MgCl₂·6H₂O (30.49 g, 0.15 mmol) and AlCl₃·6H₂O (12.07 g, 0.05 mmol) was dissolved in 200 mL of deionized H₂O. To this solution was slowly added 100 mL of NaOH (2 M) solution at 25 °C, and a further amount of 2 M NaOH solution was added to maintain a pH of 10 under nitrogen flow. The resulting suspension was stirred overnight at 70 °C. The solid product was isolated by filtration, washed thoroughly with deionized H₂O, and dried at 80 °C. All the synthetic steps were carried out using decarbonated H₂O.
  
  Preparation of LDH-Pd ⁰ Catalyst. [2c]
  LDH-Cl (Mg: Al = 3:1, 1.5 g) was suspended in 150 mL of aq Na₂PdCl₄ (0.441 g, 1.5 mmol) solution and stirred at 25 °C for 12 h under a nitrogen atmosphere. The solid catalyst was filtered, washed thoroughly with 500 mL of H₂O, and vacuum-dried to obtain 1.752 g of LDH-PdCl₄ (0.86 mmol of Pd/g). Subsequently, LDH-PdCl₄ (1 g) was reduced with hydrazine hydrate (1 g, 20 mmol) in EtOH (10 mL) for 3 h at r.t., filtered, and washed with EtOH to give LDH-Pd⁰ as an air-stable black powder (0.95 mmol of Pd/g).
  Typical Procedure for Suzuki Coupling Using LDH-Pd ⁰ as Catalyst.
  In an oven-dried 10 mL round-bottom flask 4-methylbromo-benzene 1a (1 mmol), phenylboronic acid 2a (1.5 mmol), Na₂CO₃ (3 mmol), LDH-Pd⁰ (0.5 mol%), and EtOH-H₂O (1:1, 2 mL) are taken and stirred at r.t. under nitrogen atmosphere and the reaction is monitored by TLC. After the completion of the reaction, the catalyst is filtered and reused. The filtrate is diluted with EtOAc and washed with 10% aq NaOH solution and finally with sat. aq NaCl solution. The organic layer is dried with Na₂SO₄ and concentrated to get the crude product. Then it is subjected to column chromatography to get the pure product. Isolated yield of 4-methylbiphenyl 3a was 90%. H¹ NMR: δ = 2.39 (s, 3 H), 7.16-7.21 (m, 2 H),7.24-7.30 (m, 1 H), 7.33-7.40 (m, 2 H), 7.40-7.45 (m, 2 H), 7.48-7.54 (m, 2 H).