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