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

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

B(OH)A3+3ROH⟶B(OR)A3+3HA2O

3B(OH)A3+3ROH⟶BA3OA3(OR)A3+6HA2O

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, 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(OCHA3)A3⟶MgBrOCHA3+ArB(OCHA3)A2

ArB(OCHA3)A2+2HA2O⟶ArB(OH)A2+2HOCHA3

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

## References

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
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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