BORIC ACID 1
Boric Acid Decarboxylation. Boric acid has been used to catalyze the
decarboxylation of ²-keto esters and ²-imino esters.4,11 A con-
venient method for the production of Å‚-keto esters from diethyl
H3BO3
Ä…-acylsuccinates in high yield is shown in eq 2.4 The conven-
tional method of saponification, decarboxylation, and reesterifi-
[10043-35-3] H3BO3 (MW 61.83)
cation produced low yields.
InChI = 1/BH3O3/c2-1(3)4/h2-4H
O O O
InChIKey = KGBXLFKZBHKPEV-UHFFFAOYAI
1. H3BO3
170 °C, 1.5 h
R OEt R
(reacts with alcohols to form borate esters;1 catalyzes
(2)
OEt 2. H2O OEt
dehydration,2 hydrolysis,3 decarboxylation,4 and condensation
80%
reactions;5 useful in carbohydrate chemistry6)
O O
ć%
Physical Data: mp 169 C; d 1.435 g cm-3. Heating boric acid
ć%
above 100 C gradually produces metaboric acid, HBO2; at
Condensation. Boric acid catalyzes the self-condensation
higher temperatures all water is lost and boron oxide, B2O3,
of aldehydes and ketones to produce Ä…,²-unsaturated enones.12
results.
Yields were much higher than those reported with other acid or
Solubility: sol cold water (1 g in 18 mL), boiling water (1 g in
base catalysts. Under similar conditions, aldehydes which are not
4 mL), cold alcohol (1 g in 18 mL), boiling alcohol (1 g in 6
readily susceptible to aldol condensation, dismutate to form esters
mL), glycerol (1 g in 6 mL), acetone (1 g in 15 mL).
(Tischenko reaction).13 A catalytic amount of boric acid/sulfuric
Form Supplied in: white solid, widely available (see also Sodium
acid mixture has been used to synthesize aryl esters (eq 3) in
Tetraborate).
good yields.5 The reaction was unsuccessful using mineral acids
Purification: recrystallize three times from water (3 mL g-1)
or boric acid alone.
with filtering. Dry over metaboric acid in a desiccator.
Handling, Storage, and Precautions: boric acid is hygroscopic. It H2SO4, H3BO3
(1 5 mol%)
is an irritant to eyes, skin, and mucous membranes, and should
RCO2H + ArOH (3)
RCO2Ar
xylene, reflux
be handled with the appropriate precautions to eliminate contact
eight examples
 H2O
with these areas. Death has resulted from ingestion of 5 to 20 g
58 94%
in adults. Use in a fume hood.
Indole can be condensed directly with various carboxylic acids
in the presence of boric acid.14 Traditional methods were found
Borate Esters. Trigonal borate esters are readily formed
to be unsatisfactory due to low yields and the production of 3-
by condensing alcohols with boric acid; the reaction is driven
acylated and 1,3-diacylated side products.
by azeotropic removal of water. Borate esters are stable under
a variety of anhydrous reaction conditions and can serve as a
Carbohydrate Chemistry. In alkaline solution, boric acid cat-
method of protecting alcohols.1 The reactivity of carbonyl com-
alyzes the isomerization of aldoses into ketoses.6 During the syn-
pounds can be enhanced by intramolecular coordination with an
thesis of mono- and diacylglycerides, the use of boric acid to re-
adjacent borate ester.7 Borate esters are intermediates in boric
move acetal15 and trityl16 protecting groups minimizes undesired
acid-catalyzed dehydrations of primary, secondary, and tertiary
acyl group migrations.17 The reductive acetylation of azidopy-
alcohols.2 Carbocation-derived rearrangements are a potential
ranosides to form N-acetylaminopyranosides is improved in the
problem with this method.8
presence of boric acid.18
Imine Hydrolysis. Imines can be hydrolyzed in quantitative
yields by using boric acid in refluxing ethanol.3 Imines that are sus-
1. Fanta, W. I.; Erman, W. F., Tetrahedron Lett. 1969, 4155.
ceptible to intra- and intermolecular attack in the presence of other
catalysts have been successfully hydrolyzed using boric acid.9 2. (a) Majerski, Z.; `
kare, D.; Vulić, L., Synth. Commun. 1986, 16, 51.
(b) Bubnov, Yu. N.; Grandberg, A. I.; Grigorian, M. Sh.; Kiselev, V. G.;
Conversion of isoxazolines into ²-hydroxy ketones and ²-hydroxy
Struchkova, M. I.; Mikhailov, B. M., J. Organomet. Chem. 1985, 292,
esters involves hydrogenolysis of the N O bond and imine hydrol-
93. (c) Campbell, J. R. B.; Islam, A. M.; Raphael, R. A., J. Chem. Soc
ysis in a single step.10 In the presence of boric acid, racemization 1956, 4096.
is inhibited (eq 1).10a 3. (a) Barton, D. H. R.; Jaszberenyi, J. Cs.; Theodorakis, E. A., J. Am. Chem.
Soc. 1992, 114, 5904. (b) Matsuda, H.; Nagamatsu, H.; Okuyama, T.;
Fueno, T., Bull. Chem. Soc. Jpn. 1984, 57, 500.
4. Wehrli, P. A.; Chu, V., J. Org. Chem. 1973, 38, 3436.
H2 (1 atm)
N O
O OH O OH
Raney Ni
5. Lowrance, W. W., Jr., Tetrahedron Lett. 1971, 3453.
+ (1)
6. Mendicino, J. F., J. Am. Chem. Soc. 1960, 82, 4975
MeOH H2O
(5:1)
7. (a) Takeuchi, I.; Hamada, Y.; Okamura, K., Heterocycles 1989, 29, 2109.
>90%
(1)(2) (b) Morita, S.; Otsubo, K.; Uchida, M.; Kawabata, S.; Tamaoka, H.;
Shimizu, T., Chem. Pharm. Bull. 1990, 38, 2027.
Additive (2 5 equiv) (1) : (2)
8. Chapman, O. L.; Borden, G. W., J. Org. Chem. 1961, 26, 4193.
acetate 91 : 9 9. (a) Ouazzani, F.; Roumestant, M.-L.; Viallefont, P., Tetrahedron:
phosphate 94 : 6
Asymmetry 1991, 2, 913. (b) Trost, B. M.; Li, L.; Guile, S. D., J. Am.
boric acid 100 : 0
Chem. Soc. 1992, 114, 8745.
Avoid Skin Contact with All Reagents
2 BORIC ACID
10. (a) Curran, D. P., J. Am. Chem. Soc. 1983, 105, 5826. (b) Curran, D. P.; 16. (a) Strawn, L. M.; Martell, R. E.; Simpson, R. U.; Leach, K. L.; Counsell,
Fenk, C. J., Tetrahedron Lett. 1986, 4865. (c) Duclos, O.; Mondange, M.; R. E., J. Med. Chem. 1989, 32, 2104. (b) van Boeckel, C. A. A.; van
Duréault, A.; Depezay, J. C., Tetrahedron Lett. 1992, 8061. (d) Calderola, Boom, J. H., Tetrahedron 1985, 41, 4545.
P.; Ciancaglione, M.; De Amici, M.; De Micheli, C., Tetrahedron Lett.
17. Gunstone, F. D. In Comprehensive Organic Chemistry; Barton, D. H. R.;
1986, 4647.
Ollis, W. D., Eds.; Pergamon: Oxford, 1979; Vol. 5, p 648.
11. (a) Ho, T. L., Synth. Commun. 1979, 9, 609. (b) Bacos, D.; Celerier, J.-P.;
18. (a) Broxterman, H. J. G.; van der Marel, G. A.; van Boom, J. H., J.
Lhommet, G., Tetrahedron Lett. 1987, 2353.
Carbohydr. Chem. 1991, 10, 215. (b) Hiroyuki, I.; Ogawa, T., Carbohydr.
12. Offenhauer, R. D.; Nelsen, S. F., J. Org. Chem. 1968, 33, 775. Res. 1989, 186, 107.
13. Stapp, P. R., J. Org. Chem. 1973, 38, 1433.
14. Terashima, M.; Fujioka, M., Heterocycles 1982, 19, 91. Bradley D. Smith & Martin Patrick Hughes
University of Notre Dame, Notre Dame, IN, USA
15. Strawn, L. M.; Martell, R. E.; Simpson, R. U.; Leach, K. L.; Counsell,
R. E., J. Med. Chem. 1989, 32, 643.
A list of General Abbreviations appears on the front Endpapers