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Alkylation of boron trifluoride with a 3,5-bis(trifluoromethyl)phenyl Grignard reagent; Tris[3,5-bis(trifluoromethyl)phenyl]borane

SyntheticPage 805
DOI: 10.1039/SP805
Submitted Dec 11, 2015, published Dec 16, 2015
Elliot Lawrence (elliot.lawrence@uea.ac.uk), Gregory Wildgose (g.wildgoose@uea.ac.uk), Andrew Ashley (a.ashley@imperial.ac.uk)
A contribution from Wildgoose Group, University of East Anglia


			Reaction Scheme: <IMG src="/images/empty.gif"><IMG src="/images/empty.gif">Alkylation of&nbsp;<SPAN id=csm1450264954038 class=csm-chemical-name title="boron trifluoride" grpid="1">boron trifluoride</SPAN>&nbsp;with a <SPAN id=csm1450264984208 class="csm-chemical-name csm-not-validated" title="3,5-bis(trifluoromethyl)phenyl Grignard" grpid="2">3,5-bis(trifluoromethyl)phenyl Grignard</SPAN> reagent<IMG src="/images/empty.gif"><IMG src="/images/empty.gif">

Chemicals Used

3,5-Bis(trifluoromethyl)bromobenzene (Fluorochem)
2.0 M iPrMgCl solution in diethyl ether (Sigma-Aldrich)
BF3OEt2 (Sigma-Aldrich)
Tetrahydrofuran (THF) - dried over Na / benzophenone
 

Procedure

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 BF3OEt2 (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.

Author's Comments

  • 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 Et2OB{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.

Data

B{C6H3(3,5-CF3)2}3 

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).


Lead Reference

1. T. J. Herrington, A. J. W. Thom, A. J. P. White and A. E. Ashley, Dalton Trans., 2012, 41, 9019.
2. E. L. Kolychev, T. Bannenberg, M. Freytag, C. G. Daniliuc, P. G. Jones and M. Tamm, Chem. Eur. J., 2012, 18, 16938–16946.

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Keywords: aromatics/arenes, borane, boron, inorganic compounds, substitution, triarylborane

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