An Active and Recyclable Polystyrene-Supported N-Heterocyclic Carbene-Palladium Catalyst for the Suzuki Reaction of Arylbromides with Arylboronic Acids under Mild Conditions

Abstract

A novel polymer-supported imidazolium salt has been prepared from a bisimidazolium salt and cross-linked chloromethyl polystyrene. The polymer-supported N-heterocyclic carbene-palladium complex, generated from the supported bisimidazolium salt and Pd(OAc)₂, efficiently catalyzed the Suzuki coupling reaction of aryl bromides and arylboronic acids under air in aqueous DMF ­solution at room temperature, giving biaryls in good to excellent yields. The catalyst could be reused several times still retaining high activity.

Key words

Suzuki reaction - N-heterocyclic carbene-palladium complex - polymer-supported catalyst - recyclable catalyst

References

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8

Procedure for the Preparation of the Bisimidazolium Salt 1.
In a dry nitrogen atmosphere, a mixture of 1,3-dichloro-2-propanol (1.29 g, 10 mmol) and 1-methylimidazole (1.72 g, 21 mmol) was stirred in MeCN (20 mL) at 80 °C for 4 d in an oil bath. The solvent was removed completely under vacuum. Then, CH₂Cl₂ was added to the residue to give a white solid, which was filtered and dried under vacuum affording the salt 1 in 60% yield. ¹H NMR (600 MHz, DMSO-d ₆): δ = 3.88 (s, 6 H, NCH ₃), 4.20 (m, 1 H, CHOH), 4.22-4.51 (br, 4 H, NCH ₂ ), 7.79 (s, 2 H, NCH), 7.87 (s, 2 H, NCH), 9.36 (s, 2 H, NCHN). ¹³C NMR (150 MHz, DMSO-d ₆): δ = 36.23 (NCH₃), 52.17 (CHOH), 68.21 (NCH₂), 123.44 (NCH), 123.77 (NCH), 137.74 (NCHN). HRMS (ESI): m/z calcd for C₁₁H₁₈Cl₂N₄O [M - 2Cl]²⁺: 111.0741; found: 111.0729.

9

Procedure for the Preparation of Polystyrene-Supported Bisimidazolium Salt 2.
A mixture of Merrifield polymer (200 mg 2.7 mmol Cl/g), KI (30 mg, 0.18 mmol), tributylamine (0.3 g, 1.62 mmol) and the bisimidazolium salt 1 (158 mg, 0.54 mmol) dissolved in DMF (10 mL) was stirred for 4 d at r.t. The polymer was filtered off and washed with DMF (6 × 10 mL), MeOH (6 × 10 mL), H₂O (6 × 10 mL), and then dried under vacuum. The loading of the imidazolium groups was determined by means of the nitrogen content obtained from elemental analysis (N, 1.4%, 0.5 mmol imidazolium/g).

10

Procedure for the Preparation of Polystyrene-Supported NHC-Pd Complex 3. In a dry nitrogen atmosphere, a mixture of the imidazolium loaded polymeric support 2 (200 mg, 0.5 mmol imidazolium/g) and Pd(OAc)₂ (22.5 mg, 0.1 mmol) was suspended in DMF (2 mL). To this suspension was added an aqueous solution (2 mL) of Na₂CO₃ (106 mg, 1.0 mmol). The mixture was stirred at r.t. for 30 min, and then agitated for 4 h at 50 °C in an oil bath. After filtration, the polymeric support was washed vigorously with distilled H₂O (10 × 10 mL), MeOH (10 × 10 mL), and dried under reduced pressure to give 3. ICP-AES: 100 µmol Pd/g.

11

Procedure for Measuring the Swelling Properties of the Polystyrene-Supported Bisimidazolium Salt 2 and the Catalyst 3.
The swelling volumes of polymeric supports in various solvents were measured in a fritted column (ID 0.8 cm, length 20 cm). The polymeric support (1.0 g) was swollen in a solvent at r.t. for 2 h, and then washed with a 10-fold volume of each solvent. After filtration, the volume of the polymeric support was measured.

12

Representative Procedure for Suzuki Reaction (Table 2, Entry 14).
The catalyst 3 (20 mg, 2 µmol Pd) was suspended with DMF (2 mL). After a mixture of 4-bromoanisole (63 µL, 0.5 mmol), phenylboronic acid (75 mg, 0.6 mmol), and Na₂CO₃ (265 mg, 2.5 mmol) in distilled H₂O (2 mL) was added, the reaction mixture was stirred at r.t. for 24 h without the protection of inert gas. The catalyst was filtered and washed with distilled H₂O (5 × 4 mL) and Et₂O (5 × 4 mL), and dried in vacuo for use in the next run. The organic portion was dried over MgSO₄ and evaporated under reduced pressure. The biphenyl product (91 mg, yield 99%) was isolated by column chromatography (eluent: n-hexane).