Nanocrystalline Magnesium Oxide Stabilized Palladium(0): An Efficient Heterogeneous Catalyst for Heck and Sonogashira Coupling of Aryl Halides [1]

Abstract

Nanocrystalline magnesium oxide supported palladium(0) catalyst [NAP-Mg-Pd(0)], prepared by the counter-ion stabilization of PdCl₄ ^2- followed by reduction with hydrazine hydrate, was effectively used in the Heck olefination of haloarenes (chloro, bromo, and iodo) to afford good to excellent yields of substituted olefins. This ligand-free heterogeneous NAP-Mg-Pd(0) catalyst was quantitatively recovered from the reaction by a simple filtration and reused for a number of cycles with almost no loss of activity. Moreover, NAP-Mg-Pd(0) efficiently catalyzed the Sonogashira coupling of haloarenes with terminal alkynes without any copper co-catalyst under milder conditions to afford excellent yields of substituted alkynes.

Keywords

nanocrystalline magnesium oxide - palladium - heterogeneous reaction - Heck reaction - Sonogashira reaction

1

IICT communication No. 060818.

References and Notes

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1

IICT communication No. 060818.

9

Preparation of NAP-Mg-Pd(0) Catalyst: [5c] NAP-MgO (1 g, BET 600 m²/g, purchased from NanoScale Materials Inc., Manhattan, USA) was treated with Na₂PdCl₄ (294 mg, 1 mmol) dissolved in decarbonated H₂O (100 mL) and the resulting mixture was stirred for 12 h under a nitrogen atmosphere to afford the brown colored NAP-Mg-PdCl₄. Then the catalyst was filtered, washed with deionized H₂O and acetone, and dried. Then NAP-Mg-PdCl_{4 (}1g) was reduced with hydrazine hydrate (1 g, 20 mmol) in anhyd EtOH (20 mL) for 3 h under a nitrogen atmosphere to give the black colored air-stable NAP-Mg-Pd(0) (Pd 0.99
mmol/g).
Heck Coupling of Iodo and Bromoarene: 4-Bromoanisole (2 mmol), styrene (2.2 mol), NaOAc (6 mmol), dimethyl­-acetamide (DMAc, 5 mL), and NAP-Mg-Pd(0) (0.1 mol%) were taken in a 20-mL reaction vessel and stirred under a nitrogen atmosphere at 135 °C (monitored by GC). After completion of the reaction the catalyst was separated by filtration, washed with H₂O and acetone, and dried in air. The filtrate was diluted with Et₂O and washed with H₂O. The organic layer was concentrated to give the crude product, which was purified by column chromatography (hexane-EtOAc, 9:1) to give pure 4-methoxy-trans-stilbene in 94% yield (395 mg); mp 135-137 °C. ¹H NMR (300 MHz, CDCl₃): δ = 3.84 (s, 3 H), 6.81-7.07 (m, 4 H), 7.17-7.53 (m, 7 H). EI-MS: m/z (%) = 210 (M⁺), 195, 167, 165, 152. Anal. Calcd for C₁₅H₁₄O: C, 85.68; H, 6.71. Found: C, 85.45; H, 6.83.
Heck Coupling of Chloroarenes: 4-Chloroacetophenone (2 mmol), styrene (2.2 mol), NaOAc (6 mmol), TBAB (2 mmol), DMAc (5 mL), and NAP-Mg-Pd(0) (1.0 mol%) were taken in a 20-mL reaction vessel and stirred under a nitrogen atmosphere at 135 °C (monitored by GC). After completion of the reaction the catalyst was separated by filtration, washed with H₂O and acetone, and dried in air. The filtrate was diluted with Et₂O and washed with a solution of 1% HCl (to remove the amine formed from decomposition of TBAB) and then a sat. solution of NaCl. Then organic layer was dried over Na₂SO₄ and concentrated to get the crude product, which was purified by column chromatography (hexane-EtOAc, 8:2) to give pure 4-acetyl-trans-stilbene in 92% yield (408 mg); mp 134-136 °C. ¹H NMR (300 MHz, CDCl₃): δ = 2.6 (s, 3 H), 7.12 (d, J = 16.6 Hz, 1 H), 7.22 (d, J = 16.6 Hz, 1 H), 7.28-7.41 (m, 3 H), 7.53 (d, J = 6.8 Hz, 2 H), 7.59 (d, J = 8.3 Hz, 2 H), 7.95 (d, J = 8.3 Hz, 2 H). EI-MS: m/z = 222 (M⁺), 207, 178, 149, 77, 51, 43. Anal. Calcd for C₁₆H₁₄O: C, 86.45; H, 6.35. Found: C, 86.28; H, 6.47.
Sonogashira Reaction: 4-Bromoanisole (0.5 mmol), phenylacetylene (0.6 mol), Et₃N (1 mmol), DMF (3 mL), and NAP-Mg-Pd(0) (1.0 mol%) were taken in a 10-mL reaction vessel and stirred under a nitrogen atmosphere at 75 °C (monitored by GC). After completion of the reaction, the catalyst was separated by filtration, washed with H₂O and acetone, and dried in air. The filtrate was diluted with Et₂O and washed with a sat. solution of NaCl. The organic layer was concentrated to give the crude product, which was purified by column chromatography(hexane) to give pure 4-methoxydiphenylacetylene in 92% yield (96 mg); mp 55-57 °C. ¹H NMR (300 MHz, CDCl₃): δ = 3.8 (s, 3 H), 6.83 (d, J = 8.3 Hz, 2 H), 7.25-7.35 (m, 3 H), 7.39-7.51 (m, 4 H). EI-MS: m/z = 208 (M⁺), 194, 167, 166, 140. Anal. Calcd for C₁₅H₁₂O: C, 86.51; H, 5.81. Found: C, 86.39; H, 5.96.