Synthesis
1. Dissolve ethyl 1,5-dimethyl-1H-pyrazole-3-carboxylate (30.8 g, 200 mmol) in 200 mL of anhydrous tetrahydrofuran. Under cooling in an ice bath, sodium hydride (4.8 g, 200 mmol) was added slowly, ensuring that the sodium hydride was completely incorporated. The reaction mixture was warmed to 50 °C, stirred for 1 h and cooled to room temperature.
2. Methyl iodide (28.2 g, 200 mmol) was dissolved in 100 mL of tetrahydrofuran and slowly added dropwise to the above reaction mixture. After the dropwise addition was completed, the reaction continued to be heated at 50°C for 2 hours.
3. After completion of the reaction, cool to room temperature and remove the tetrahydrofuran by distillation under reduced pressure. 100 mL of water was added and extracted with ethyl acetate (100 mL x 3), the organic layers were combined, dried and the solvent was removed under reduced pressure to give 26.6 g of ethyl 1,5-dimethyl-1H-pyrazole-3-carboxylate in 85% yield.
4. To a dry three-necked flask was added lithium aluminum hydride (3.8 g, 100 mmol) and 200 mL of anhydrous tetrahydrofuran. Ethyl 1,5-dimethyl-1H-pyrazole-3-carboxylate (16.8 g, 100 mmol) was dissolved in 100 mL of dry tetrahydrofuran and slowly added dropwise to the three-necked flask. After the dropwise addition, stirring was continued for 4 hours.
5. Upon completion of the reduction reaction, anhydrous ethanol was added slowly and dropwise to quench the remaining lithium tetrahydroaluminum. Tetrahydrofuran was removed by distillation under reduced pressure, 500 mL of methanol was added, the pH was adjusted to neutral, heated to reflux for 6 hours and filtered. The filtrate was concentrated, dissolved in 100 mL of dichloromethane, washed twice with 50 mL of saturated aqueous sodium chloride, the organic layer was dried and concentrated to give 1,5-dimethyl-1H-pyrazole-3-methanol 8.8 g in 70% yield.
6. 1,5-Dimethyl-1H-pyrazole-3-methanol (6.3 g, 50 mmol) was dissolved in 20 mL of dichloromethane and thionyl chloride (6 g, 50 mmol) was added dropwise slowly. After dropwise addition, stirring was continued for 2 hours.
7. After completion of the reaction, slowly add saturated sodium bicarbonate aqueous solution to adjust the pH to neutral, extracted with 200 mL of dichloromethane. The organic layers were combined, dried and concentrated to give 3-chloromethyl-1,5-dimethyl-1H-pyrazole (6.5 g, 90% yield).
8. To a 50 mL round bottom flask was added 3-chloromethyl-1,5-dimethyl-1H-pyrazole (1.44 g, 10 mmol), N-(2,4,6-trimethylphenyl)imidazole (1.86 g, 10 mmol) and 20 mL of acetonitrile, and heated to reflux for 6 hours. After cooling to room temperature, the solvent was removed by distillation under reduced pressure.
9. The resulting solid was dissolved in water and filtered. The filtrate was saturated with aqueous ammonium hexafluorophosphate, the solid was precipitated and dried to give 3.8 g of imidazolium salt ligand (HL1PF6) in 88% yield.
References
[1] Journal of Medicinal Chemistry, 2015, vol. 58, # 5, p. 2180 - 2194
[2] Anales de Quimica, 1996, vol. 92, # 2, p. 70 - 78
[3] Organic Letters, 2011, vol. 13, # 6, p. 1436 - 1439
[4] Patent: CN106478734, 2017, A. Location in patent: Paragraph 0027; 0028; 0029
[5] Medicinal Chemistry Research, 2012, vol. 21, # 10, p. 2772 - 2778,7
