[10043-35-3] · BH3O3 · Boric Acid · (MW 61.83)
Physical Data: mp 169 °C; d 1.435 g cm-3. Heating boric acid above 100 °C gradually produces metaboric acid, HBO2; at higher temperatures all water is lost and boron oxide, B2O3, results.
Solubility: sol cold water (1 g in 18 mL), boiling water (1 g in 4 mL), cold alcohol (1 g in 18 mL), boiling alcohol (1 g in 6 mL), glycerol (1 g in 6 mL), acetone (1 g in 15 mL).
Form Supplied in: white solid, widely available (see also Sodium Tetraborate).
Purification: recrystallize three times from water (3 mL g-1) with filtering. Dry over metaboric acid in a desiccator.
Handling, Storage, and Precautions: boric acid is hygroscopic. It is an irritant to eyes, skin, and mucous membranes, and should be handled with the appropriate precautions to eliminate contact with these areas. Death has resulted from ingestion of 5 to 20 g in adults. Use in a fume hood.
Trigonal borate esters are readily formed by condensing alcohols with boric acid; the reaction is driven by azeotropic removal of water. Borate esters are stable under a variety of anhydrous reaction conditions and can serve as a method of protecting alcohols.1 The reactivity of carbonyl compounds can be enhanced by intramolecular coordination with an adjacent borate ester.7 Borate esters are intermediates in boric acid-catalyzed dehydrations of primary, secondary, and tertiary alcohols.2 Carbocation-derived rearrangements are a potential problem with this method.8
Imines can be hydrolyzed in quantitative yields by using boric acid in refluxing ethanol.3 Imines that are susceptible to intra- and intermolecular attack in the presence of other catalysts have been successfully hydrolyzed using boric acid.9 Conversion of isoxazolines into b-hydroxy ketones and b-hydroxy esters involves hydrogenolysis of the N-O bond and imine hydrolysis in a single step.10 In the presence of boric acid, racemization is inhibited (eq 1).10a
Boric acid has been used to catalyze the decarboxylation of b-keto esters and b-imino esters.4,11 A convenient method for the production of g-keto esters from diethyl a-acylsuccinates in high yield is shown in eq 2.4 The conventional method of saponification, decarboxylation, and reesterification produced low yields.
Boric acid catalyzes the self-condensation of aldehydes and ketones to produce a,b-unsaturated enones.12 Yields were much higher than those reported with other acid or base catalysts. Under similar conditions, aldehydes which are not readily susceptible to aldol condensation, dismutate to form esters (Tischenko reaction).13 A catalytic amount of boric acid/sulfuric acid mixture has been used to synthesize aryl esters (eq 3) in good yields.5 The reaction was unsuccessful using mineral acids or boric acid alone.
Indole can be condensed directly with various carboxylic acids in the presence of boric acid.14 Traditional methods were found to be unsatisfactory due to low yields and the production of 3-acylated and 1,3-diacylated side products.
In alkaline solution, boric acid catalyzes the isomerization of aldoses into ketoses.6 During the synthesis of mono- and diacylglycerides, the use of boric acid to remove acetal15 and trityl16 protecting groups minimizes undesired acyl group migrations.17 The reductive acetylation of azidopyranosides to form N-acetylaminopyranosides is improved in the presence of boric acid.18
Bradley D. Smith & Martin Patrick Hughes
University of Notre Dame, IN, USA