Non-Cross-Linked Polystyrene-Supported Triphenylphosphine-­Microencapsulated Palladium: An Efficient and Recyclable Catalyst for Suzuki-Miyaura Reactions

Abstract

A recyclable, all-in-one, polystyrene-microencapsulated palladium catalyst has been developed and used as a heterogeneous catalyst in Suzuki-Miyaura coupling reactions. This catalyst can be recovered and reused with only minimal palladium leaching observed.

polystyrene - microencapsulation - palladium - Suzuki-Miyaura reaction - heterogeneous catalysis

References and Notes

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21

Preparation of Catalyst 2.
Polymer 1 (3.96 g, 4.04 mmol, 1.02 mmol PPh₃/g) was dissolved in THF (40 mL) at 50 °C and to this solution was added Pd(OAc)₂ (0.182 g, 0.81 mmol). The mixture was stirred at this temperature for 4 h and it became dark red in color. After the solution was cooled to 0 °C, the mixture was added dropwise into cooled hexane (80 mL). The resulting orange suspension was stirred at r.t. for 12 h and then filtered and washed with MeOH. Polymer 2 was isolated as an orange powder (3.98 g, ca 100%). ¹H NMR (400 MHz, CDCl₃): δ = 0.86-2.25 (bm, 24 H), 6.48-7.04 (bm, 35 H), 7.51-7.87 (bm, 14 H). The Pd content of 2 was determined to be 2.08% (theoretical value: 2.13%), which corresponds to a loading level of 0.20 mmol Pd/g, by X-ray fluorescence analysis.

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General Procedure for Suzuki-Miyaura Reactions.
A vial was charged with the arylboronic acid (0.7 mmol), Cs₂CO₃ (0.27 g, 0.8 mmol), 2 (0.012 g, 0.5 mol%), the aryl halide (0.45 mmol), and i-PrOH (4 mL). The mixture was heated at 70 °C until the reaction was determined to be complete by TLC analysis (generally 1 h), and then allowed to cool to r.t. Catalyst 2 was filtered off and washed with MeOH. The filtrate was concentrated and the resulting crude product was purified by silica gel chromatography (5-10% EtOAc in hexane) and characterized by ¹H NMR and ¹³C NMR spectroscopy and MS analysis. The characterization data obtained agreed with previously reported data or that of commercial samples.

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Water was added to these reactions since it seemed to make 2 easier to recover and handle. However, it had the effect of lengthening the reaction times. When recycling is not an issue, i-PrOH alone is the preferred solvent due to the solubility of the reactants in it, and thus shorter reaction times.