Borate esters

Short description: Organic compounds of the form B(OR)₃ or B₃O₃(OR)₃

In organic chemistry, borate esters are organoboron compounds which are conveniently prepared by the stoichiometric condensation reaction of boric acid with alcohols. There are two main classes of borate esters: orthoborates, B(OR)
3 and metaborates, B
3O
3(OR)
3. Metaborates contain 6-membered boroxine rings.

[math]\ce{ B(OH)3 + 3 ROH -> B(OR)3 + 3 H2O }[/math]

[math]\ce{ 3 B(OH)3 + 3 ROH -> B3O3(OR)3 + 6 H2O }[/math]

A dehydrating agent, such as concentrated sulfuric acid is typically added.^[1] Borate esters are volatile and can be purified by distillation. This procedure is used for analysis of trace amounts of borate and for analysis of boron in steel.^[2] Like all boron compounds, alkyl borates burn with a characteristic green flame. This property is used to determine the presence of boron in qualitative analysis.^[3]

Trimethyl borate is a popular borate ester used in organic synthesis.

Borate esters form spontaneously when treated with diols such as sugars and the reaction with mannitol forms the basis of a titrimetric analytical method for boric acid.

Metaborate esters show considerable Lewis acidity and can initiate epoxide polymerization reactions.^[4] The Lewis acidity of orthoborate esters, as determined by the Gutmann-Beckett method, is relatively low.

Trimethyl borate, B(OCH
3)
3, is used as a precursor to boronic esters for Suzuki couplings:^[5] Unsymmetrical borate esters are prepared from alkylation of trimethyl borate:^[6]

[math]\ce{ ArMgBr + B(OCH3)3 -> MgBrOCH3 + ArB(OCH3)2 }[/math]

[math]\ce{ ArB(OCH3)2 + 2H2O -> ArB(OH)2 + 2HOCH3 }[/math]

These esters hydrolyze to boronic acids, which are used in Suzuki couplings.

References

↑ Brown, Herbert C.; Mead, Edward J.; Shoaf, Charles J. (1956). "Convenient Procedures for the Preparation of Alkyl Borate Esters". J. Am. Chem. Soc. 78 (15): 3613–3614. doi:10.1021/ja01596a015.
↑ Mendham, J.; Denney, R. C.; Barnes, J. D.; Thomas, M. J. K. (2000), Vogel's Quantitative Chemical Analysis (6th ed.), New York: Prentice Hall, p. 666, ISBN 0-582-22628-7
↑ Template:VogelQualitative5th
↑ M.A. Beckett, G.C. Strickland, J.R. Holland, and K.S. Varma, "A convenient NMR method for the measurement of Lewis acidity at boron centres: correlation of reaction rates of Lewis acid initiated epoxide polymerizations with Lewis acidity", Polymer, 1996, 37, 4629–4631. doi: 10.1016/0032-3861(96)00323-0
↑ Li, W.; Nelson, D. P.; Jensen, M. S.; Hoerrner, R. S.; Cai, D.; Larsen, R. D.; Reider, P. J. (2002). "An Improved Protocol for the Preparation of 3-Pyridyl- and Some Arylboronic Acids". J. Org. Chem. 67: p. 5394. doi:10.1021/jo025792p.
↑ R. L. Kidwell; M. Murphy; S. D. Darling (1969). "Phenols: 6-Methoxy-2-naphthol". Organic Syntheses 49: 90. http://www.orgsyn.org/demo.aspx?prep=CV5P0918. ; Collective Volume, 10, pp. 80

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[1] Brown, Herbert C.; Mead, Edward J.; Shoaf, Charles J. (1956). "Convenient Procedures for the Preparation of Alkyl Borate Esters". J. Am. Chem. Soc. 78 (15): 3613–3614. doi:10.1021/ja01596a015.

[2] Mendham, J.; Denney, R. C.; Barnes, J. D.; Thomas, M. J. K. (2000), Vogel's Quantitative Chemical Analysis (6th ed.), New York: Prentice Hall, p. 666, ISBN 0-582-22628-7

[3] Template:VogelQualitative5th

[Beckett-4] M.A. Beckett, G.C. Strickland, J.R. Holland, and K.S. Varma, "A convenient NMR method for the measurement of Lewis acidity at boron centres: correlation of reaction rates of Lewis acid initiated epoxide polymerizations with Lewis acidity", Polymer, 1996, 37, 4629–4631. doi: 10.1016/0032-3861(96)00323-0

[5] Li, W.; Nelson, D. P.; Jensen, M. S.; Hoerrner, R. S.; Cai, D.; Larsen, R. D.; Reider, P. J. (2002). "An Improved Protocol for the Preparation of 3-Pyridyl- and Some Arylboronic Acids". J. Org. Chem. 67: p. 5394. doi:10.1021/jo025792p.

[6] R. L. Kidwell; M. Murphy; S. D. Darling (1969). "Phenols: 6-Methoxy-2-naphthol". Organic Syntheses 49: 90. http://www.orgsyn.org/demo.aspx?prep=CV5P0918. ; Collective Volume, 10, pp. 80

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Borate esters

Topic: Chemistry

References