View source for Chemistry:Borate esters

{{short description|Organic compounds of the form B(OR)₃ or B₃O₃(OR)₃}}
In [[Chemistry:Organic chemistry|organic chemistry]], '''borate esters''' are organoboron compounds which are conveniently prepared by the stoichiometric [[Chemistry:Condensation reaction|condensation reaction]] of [[Chemistry:Boric acid|boric acid]] with [[Chemistry:Alcohol|alcohols]]. There are two main classes of borate [[Chemistry:Ester|ester]]s: [[Chemistry:Orthoborate|orthoborates]], {{chem2|B(OR)3}} and [[Chemistry:Metaborate|metaborates]], {{chem2|B3O3(OR)3}}.  Metaborates contain 6-membered [[Chemistry:Boroxine|boroxine]] rings.

:<chem>B(OH)3 + 3 ROH -> B(OR)3 + 3 H2O</chem>

:<chem>3 B(OH)3 + 3 ROH -> B3O3(OR)3 + 6 H2O</chem>

A dehydrating agent, such as concentrated [[Chemistry:Sulfuric acid|sulfuric acid]] is typically added.<ref>{{cite journal|last=Brown|first=Herbert C.|author2=Mead, Edward J. |author3=Shoaf, Charles J. |year=1956|title=Convenient Procedures for the Preparation of Alkyl Borate Esters|journal=J. Am. Chem. Soc.|volume=78|issue=15|pages=3613–3614|doi=10.1021/ja01596a015}}</ref> 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.<ref>{{VogelQuantitative6th|page=666}}</ref> 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.<ref>{{VogelQualitative5th}}</ref>
[[File:Structural formula of trimethyl borate.svg|124px|thumb|right|Trimethyl borate is a popular borate ester used in organic synthesis.]]

Borate esters form spontaneously when treated with [[Chemistry:Diol|diol]]s 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.<ref name="Beckett">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</ref>  The Lewis acidity of orthoborate esters, as determined by the Gutmann-Beckett method, is relatively low.

[[Chemistry:Trimethyl borate|Trimethyl borate]], {{chem2|B(OCH3)3}}, is used as a precursor to boronic esters for Suzuki couplings:<ref>{{cite news|author1=Li, W. |author2=Nelson, D. P. |author3=Jensen, M. S. |author4=Hoerrner, R. S. |author5=Cai, D. |author6=Larsen, R. D. |author7=Reider, P. J. |journal=J. Org. Chem. |year=2002 |volume=67 |page=5394 |title=An Improved Protocol for the Preparation of 3-Pyridyl- and Some Arylboronic Acids |doi=10.1021/jo025792p }}</ref> Unsymmetrical borate esters are prepared from alkylation of trimethyl borate:<ref>{{OrgSynth|author=R. L. Kidwell|author2=M. Murphy|author3=S. D. Darling|name-list-style=amp |year=1969 |title=Phenols: 6-Methoxy-2-naphthol |volume=49|pages=90|collvol=10|collvolpages=80|prep=CV5P0918}}</ref>

:<chem>ArMgBr + B(OCH3)3 -> MgBrOCH3 + ArB(OCH3)2</chem>

:<chem>ArB(OCH3)2 + 2H2O -> ArB(OH)2 + 2HOCH3</chem>

These esters hydrolyze to [[Chemistry:Boronic acid|boronic acid]]s, which are used in Suzuki couplings.

== References ==
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[[Category:Borate esters| ]]

{{Sourceattribution|Borate esters}}