Guanidinium Salt Functionalized PEG: An Effective and Recyclable Homo-geneous Catalyst for the Synthesis of Cyclic Carbonates from CO₂ and Epoxides under Solvent-Free Conditions

Abstract

A guanidinium bromide covalently bound to CO₂-philic polyethylene glycol (PEG) is proved to be a highly effective homogeneous catalyst for the eco-friendly synthesis of cyclic carbonates from carbon dioxide and epoxides under mild conditions, which requires no additional organic solvents or co-catalyst. Notably, it has been found that there is a pronouncedly cooperative effect between the catalyst part and the support part. Moreover, the catalyst is able to be reused with retention of high catalytic activity and selectivity. This process looks promising as a strategy for homogeneous catalyst recycling.

guanidinium bromide - functionalized polyethylene glycol - carbon dioxide - epoxide - cycloaddition

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13

The pentaalkylguanidine 3 was synthesized from 1,3-dimethylimidazolidin-2-one(1), POCl₃ and BuNH₂ by the methods reported in the literature;^12b colorless liquid; yield: 71% (lit.^12b 85%). ¹H NMR (300 MHz, CDCl₃): δ = 0.91 (t, J = 14.4 Hz, 3 H, Me), 1.33-1.41 (m, 2 H, CH₂), 1.48-1.58 (m, 2 H, CH₂), 2.79 (s, 6 H, Me), 3.14 (s, 4 H, NCH₂), 3.34 (s, 2 H, CH₂).

14

Procedure for the Synthesis of PEG-Supported Guanidinium Bromide 4: To a solution of polyethylene glycol bromide (12 g, 0.002 mol) in toluene (150 mL), pentaalkylguanidine 3 (3.38 g, 0.02 mol) was added, and the resulting solution was stirred at 65 °C for 72 h. After the reaction was completed, the solvent was removed under reduced pressure, and then anhyd Et₂O (40 mL) was added. The product was precipitated and isolated by filtration, then washed by anhyd Et₂O and dried to obtain the product 4 (94%); white powder; mp 53-55 °C. IR: 1638 (C=N) cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 0.89 (t, J = 14.7 Hz, 6 H, 2 × Me), 1.29-1.36 (m, 4 H, 2 × CH₂), 1.62-1.72 (m, 4 H, 2 × CH₂), 3.11 (s, 6 H, 2 × Me), 3.60 (m, 4 H, OCH₂CH₂O). ¹³C NMR (75 MHz, CDCl₃): δ = 13.08, 19.13, 31.69, 34.85, 43.08, 49.02, 69.90, 158.72.

15

Representative Procedure for the Cycloaddition Reaction of Epoxide with CO ₂ : In a 25-mL inner volume stainless-steel autoclave equipped with a magnetic stirrer, isopropyl glycidyl ether (15.8 mmol) and PEG-supported hexaalkylguanidinium bromide (0.5 mol%) were added, and CO₂ (liquid, 3.0 MPa) was charged into the reactor at r.t. The initial pressure was generally adjusted to 4 MPa at 110 °C. The reactor was heated at that temperature for 4 h. After cooling, the products were separated by adding Et₂O and analyzed by a gas chromatograph (Shimadzu GC-2014) equipped with a capillary column (RTX-5, 30 m × 0.25 µm) using a flame ionization detector and the side-products were detected by GC-MS. All of the products were further identified using GC-MS by comparing the retention times and fragmentation patterns with authentic samples. The structures of the isolated products were also characterized by ¹H NMR and ¹³C NMR spectroscopy. Spectral characteristics of cyclic carbonates shown in Table [2] are as follows:
4-Methyl-1,3-dioxolan-2-one (6a): ¹H NMR (400 MHz, CDCl₃): δ = 1.43 (d, J = 6.0 Hz, 3 H, Me), 3.98 (t, J = 8.4 Hz, 1 H, OCH₂), 4.51 (t, J = 8.4 Hz, 1 H, OCH₂), 4.82 (m, 1 H, CHO). ¹³C NMR (100.4 MHz, CDCl₃): δ = 19.15, 70.53, 73.49, 154.95. 1,3-Dioxolan-2-one (6b): ¹H NMR (400 MHz, CDCl₃): δ = 4.50 (s, 4 H, OCH₂). ¹³C NMR (100.4 MHz, CDCl₃): δ = 64.62, 155.55. 4-Phenyl-1,3-dioxolan-2-one (6c): ¹H NMR (400 MHz, CDCl₃): δ = 4.35 (t, J = 8.4 Hz, 1 H, OCH₂), 4.80 (t, J = 8.4 Hz, 1 H, OCH₂), 5.70 (t, J = 8.0 Hz, 1 H, OCH), 7.36 (d, J = 7.6 Hz, 2 H, Ph), 7.44 (d, J = 6.4 Hz, 3 H, Ph). ¹³C NMR (100.4 MHz, CDCl₃): δ = 71.12, 77.95, 125.83, 129.22, 129.71, 135.78, 154.76.
4-Chloromethyl-1,3-dioxolan-2-one (6d): ¹H NMR (400 MHz, CDCl₃): δ = 3.71 (dd, J = 3.2, 12.0 Hz, 1 H, ClCH₂), 3.80 (dd, J = 5.2, 12.0 Hz, 1 H, ClCH₂), 4.39 (dd, J = 6.0, 8.4 Hz, 1 H, OCH₂), 4.58 (t, J = 8.4 Hz, 1 H, OCH₂), 4.98 (m, 1 H, CHO). ¹³C NMR (100.4 MHz, CDCl₃): δ = 43.84, 66.83, 74.29, 154.28. 4-Isopropoxy-1,3-dioxolan-2-one (6e): ¹H NMR (400 MHz, CDCl₃): δ = 1.08 (t, J = 6.4 Hz, 6 H, 2 × Me), 3.51-3.62 (m, 3 H, CHO, CH₂O), 4.30 (dd, J = 8.0, 15.6 Hz, 1 H, OCH₂), 4.42 (dd, J = 8.0, 15.6 Hz, 1 H, OCH₂), 4.74 (m, 1 H, CHO). ¹³C NMR (100.4 MHz, CDCl₃): δ = 21.53, 21.65, 66.16, 66.89, 72.59, 75.18, 155.03. 4-Phenoxymethyl-1,3-dioxolan-2-one (6f): ¹H NMR (400 MHz, CDCl₃): δ = 4.15 (dd, J = 4.4, 10.8 Hz, 1 H, OCH₂), 4.24 (dd, J = 3.6, 10.8 Hz, 1 H, OCH₂), 4.55 (dd, J = 6.0, 8.4 Hz, 1 H, PhOCH₂), 4.62 (t, J = 8.4 Hz, 1 H, PhOCH₂), 5.03 (m, 1 H, OCH), 6.91 (d, J = 8.0 Hz, 2 H, Ph), 7.02 (t, J = 7.4 Hz, 1 H, Ph), 7.31 (t, J = 8.0 Hz, 2 H, Ph). ¹³C NMR (100.4 MHz, CDCl₃): δ = 66.17, 68.84, 74.11, 114.57, 121.92, 129.63, 154.65, 157.71.

16

Extraction Procedure with Et ₂ O: After addition of Et₂O (3 × 4 mL) to the resulting mixture upon completion of the reaction, the PEG-guanidinium bromide was solidified when cooled to -20 °C to -10 °C, followed by simple decantation of the Et₂O phase containing the products, thus allowing the catalyst to be recycled. The combined extracts were dried over MgSO₄ and concentrated in vacuo to give the product cyclic carbonate. We conducted further reaction by the addition of successive portions of the epoxide to the recovered catalyst under identical reaction conditions.