Synthesis
General steps:
1. First reaction step: 55 mL of tetrahydrofuran and 2,2,6,6-tetramethylpiperidine (15.5 g, 0.11 mol) were mixed under nitrogen protection and cooled to -20 °C. The reaction was carried out at a temperature of -25 °C to -20 °C. A tetrahydrofuran solution of 2 M isopropylmagnesium chloride (53 mL, 0.105 mol) was added slowly and the reaction was stirred for 30 minutes over a temperature range of -25°C to -20°C. The reaction mixture was then cooled to -78 °C and controlled in a temperature range of -78 °C to -65 °C. A solution of m-trifluoromethyl chloride benzene (18.1 g, 0.1 mol) was added to this mixture and reacted for 1 hour. Carbon dioxide gas was then passed into the system until the gas was no longer absorbed. After the reaction was completed, the reaction was terminated with 10% hydrochloric acid solution, 150 mL of ethyl acetate was added and the organic layer was evaporated to dryness for the next step.
2. Second reaction step: the product obtained in the first step was dissolved in 110 mL of dioxane and stirred until completely dissolved. Potassium fluoride dihydrate (20.7 g, 0.22 mol) and 18-crown-6 (0.005 mol) were added and heated to reflux. When the reaction was complete, it was cooled to room temperature and water and 350 mL of ethyl acetate were added. The organic layer was washed with saturated brine and evaporated to dryness. The residue was dissolved in a mixed solvent of methanol and heptane (1:4) and recrystallized to give 2-fluoro-4-trifluoromethylbenzoic acid (11.8 g) in a total yield of 57% in two steps. The product was analyzed by HPLC for 98.8% purity and HNMR analysis showed ≥98% purity.
