Borate esters

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, B3O3(OR)3. Metaborates contain 6-membered boroxine rings.

B ( OH ) 3 + 3 ROH ⟶ B ( OR ) 3 + 3 H 2 O {\displaystyle {\ce {B(OH)3 + 3 ROH -> B(OR)3 + 3 H2O}}}

3 B ( OH ) 3 + 3 ROH ⟶ B 3 O 3 ( OR ) 3 + 6 H 2 O {\displaystyle {\ce {3 B(OH)3 + 3 ROH -> B3O3(OR)3 + 6 H2O}}}

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(OCH3)3, is used as a precursor to boronic esters for Suzuki couplings:[5] Unsymmetrical borate esters are prepared from alkylation of trimethyl borate:[6]

ArMgBr + B ( OCH 3 ) 3 ⟶ MgBrOCH 3 + ArB ( OCH 3 ) 2 {\displaystyle {\ce {ArMgBr + B(OCH3)3 -> MgBrOCH3 + ArB(OCH3)2}}}

ArB ( OCH 3 ) 2 + 2 H 2 O ⟶ ArB ( OH ) 2 + 2 HOCH 3 {\displaystyle {\ce {ArB(OCH3)2 + 2H2O -> ArB(OH)2 + 2HOCH3}}}

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

## References

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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. ^ Vogel, Arthur I.; Svehla, G. (1979), Vogel's Textbook of Macro and Semimicro Qualitative Inorganic Analysis (5th ed.), London: Longman, ISBN 0-582-44367-9 – via the Internet Archive
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. Vol. 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; Collected Volumes, vol. 10, p. 80.