To a stirred 0°C solution of 3,5-bis(trifluoromethyl)bromobenzene (3.5 mL, 6.00 g, 20.5 mmol) in THF (100 mL) was slowly added a 2.0 M solution of ⁱPrMgCl in Et₂O (10.7 mL, 21.4 mmol). The reaction mixture was left to stir under N₂ at 0°C for 30 minutes, before BF₃∙OEt₂ (0.84 mL, 0.97 g, 6.8 mmol) was added dropwise. The reaction mixture was then immediately warmed to room temperature and the volatiles were removed under vacuum to give the crude product as an off-white solid foam. The foam was left to dry under vacuum for an hour before being broken up using a spatula. The crude product was then sublimed twice (160°C, 0.5 mbar) to give B{C₆H₃(3,5-CF₃)₂}₃ (2.5 g, 3.8 mmol, 56%) as a white powder.

• The preparation should be performed under N₂ or Ar using standard Schlenk techniques.
• 2.0 M ⁱPrMgCl solution in diethyl ether was used as a reagent because it was available in our laboratory; ⁱPrMgCl solutions in THF should also work.
• It has been found that significantly poorer yields are obtained when the reaction mixture is left to stir in THF for any longer than is necessary. It is important that the solvent is immediately removed from the reaction mixture after the complete addition of BF₃.OEt₂.
• Although recrystallization using a minimum of hot toluene (100 °C) is possible, this results in a poorer yield – presumably due to thermal decomposition of the borane as indicated by darkening of the toluene solution. In our hands, sublimation proved to be the best option.
• Sometimes during the sublimation step, a small quantity of EtOB{C₆H₃(3,5-CF₃)₂}₂ (ca 100 mg) is collected at lower temperatures as large needle-like crystals. This is presumably due to the thermal decomposition of unreacted Et₂O∙B{C₆H₃(3,5-CF₃)₂}₂F – an intermediate in the formation of B{C₆H₃(3,5-CF₃)₂}₃.
• Note that the commonly-encountered hydroxide impurity, HOB{C₆H₃(3,5-CF₃)₂}₂, is more soluble in CH₂Cl₂ than B{C₆H₃(3,5-CF₃)₂}₃. If the product is found to be contaminated with HOB{C₆H₃(3,5-CF₃)₂}₂, it may be purified by washing 2-3 times with a small quantity (ca 10 mL) of CH₂Cl₂. This has a negligible impact on the overall product yield.

¹H NMR (CD₂Cl₂, 500 MHz): δ 8.24 (s, 3H, para-H), 8.02 (s, 6H, ortho-H). ¹⁹F NMR (CD₂Cl₂, 471 MHz): δ −63.4 (s, CF₃). ¹¹B NMR (CD₂Cl₂, 160 MHz): δ 67.9 (s, br).

EtOB{C₆H₃(3,5-CF₃)₂}₂

¹H NMR (CD₂Cl₂, 500 MHz) δ 8.04 (s, 6H), 4.22 (q, J = 7.0 Hz, 2H, CH₂), 1.39 (t, J = 7.0 Hz, 3H, CH₃). ¹⁹F NMR (CD₂Cl₂, 471 MHz): δ −63.3 (s, CF₃). ¹¹B NMR (CD₂Cl₂, 160 MHz): δ 43.5 (s, br).

HOB{C₆H₃(3,5-CF₃)₂}₂

¹H NMR (CD₂Cl₂, 500 MHz): δ 8.21 (s, 4H, ortho-H), 8.10 (s, 2H, para-H), 6.61 (s, 1H, OH). ¹⁹F NMR (CD₂Cl₂, 471 MHz): δ −63.4 (s, CF₃). ¹¹B NMR (CD₂Cl₂, 160 MHz): δ 44.4 (s, br).