Sodium Tetraborate

Na2B4O7

[1330-43-4]  · B4Na2O7  · Sodium Tetraborate  · (MW 201.22) (decahydrate)

[1303-96-4]

(source of boric acid at low concentration in aq soln;1 desilylation of trimethylsilyl alkynones2)

Physical Data: mp 743 °C, 720 °C, 663 °C (a, b, and g crystals); bp 1575 °C; d 2.367 g cm-3.

Solubility: slowly sol water (1 g in 91 mL at 0 °C; 1 g in 11.4 mL at 40 °C); sol methanol, ethylene glycol.

Form Supplied in: powder or glasslike plates that turn opaque on exposure to air. Commonly supplied as the decahydrate (borax). However, borax is insoluble in alcohol.

Handling, Storage, and Precautions: hygroscopic; absorbs water up to 300 °C; dehydrates completely at 700 °C. Slow decomposition will occur above 900 °C. Eye, skin, and upper respiratory tract irritant. Incompatible with strong oxidizing agents.

Introduction.

Na2B4O7 is most commonly available as its decahydrate (borax) and is sometimes referred to as fused borax. Below 10% relative humidity, borax will irreversibly decompose to the dihydrate (metakernite); between 10-50% relative humidity there is reversible decomposition to the pentahydrate (tincalconite).3 In aqueous solution, Na2B4O7 dissociates into a number of different boron species depending upon the pH, temperature, concentration of boron, and concentration of any added salt.1 At low concentration (i.e. [B] < 0.025 M), Na2B4O7 dissociates exclusively into Boric Acid, [B(OH)3], and monoborate ion, [B(OH)4-], to yield an alkaline solution that buffers around pH 9.4 At higher boron concentrations, polyborates predominate.1

Desilylation.

The high yields and low cost make Na2B4O7 an attractive reagent for the desilylation of trimethylsilyl alkynones to yield monosubstituted alkynes (eq 1).2,5

Nef Reaction.

The original Nef reaction involves hydrolysis of the conjugate base of a primary or secondary nitro compound to the corresponding aldehyde or ketone with Sulfuric Acid.6 Many alternatives exist for carrying out this reaction. One approach is to oxidize the conjugate base.6 When an alkaline Potassium Permanganate solution is used, it has been shown that addition of Na2B4O7 will improve yields (eq 2).7

Substitution and Addition.

Na2B4O7 in methanol creates a mild source of methoxide ion for carrying out stereoselective substitutions a to a carbonyl function (eq 3).8,9

Na2B4O7 affects the reactivity of a,b-unsaturated esters in two ways: (i) suppression of 1,2 attack;10 and (ii) enhancement of 1,4 addition especially by thioalkanes.11 The combined effects have been used succesfully as a method to selectively reduce isolated alkenes in the presence of conjugated alkenes (eq 4).12

Protecting Group For Vicinal Diols.

The ability of Na2B4O7 to complex with vicinal diols provides a convenient protecting group that can be easily removed under acidic conditions.13 The selective methylation of methyl gallate illustrates the simplicity of the method (eq 5).14


1. Farmer, J. B. Adv. Inorg. Chem. Radiochem. 1982, 25, Chapter 3.
2. &SSbreve;olaja, B.; Huguet, J.; Karpf, M.; Dreiding, A. S. T 1987, 43, 4875.
3. Kemp, P. H. The Chemistry of Borates; Borax Consolidated: London, 1956; Part I, Chapter 5.
4. Sinton, S. W. Macromolecules 1987, 20, 2430.
5. (a) Manzardo, G. G. G.; Karpf, M.; Dreiding, A. S. HCA 1986, 69, 659. (b) Karpf, M.; Dreiding, A. S. HCA 1981, 64, 1123. (c) Koller, M.; Karpf, M.; Dreiding, A. S. HCA 1986, 69, 560.
6. March, J. Advanced Organic Chemistry, 4th ed.; Wiley: New York, 1992; p 886.
7. (a) Ballini, R.; Petrini, M.; Rosini, G. S 1987, 8, 711. (b) Shechter, H.; Williams, F. T.; Jr. JOC 1962, 27, 3699. (c) Ballini, R.; Petrini, M.; Marotta, E. SC 1987, 17, 543. (d) Wälchli, R.; Bienz, S.; Hesse, M. HCA 1985, 68, 484. (e) Kienzle, F.; Holland, G. W.; Jernow, J. L.; Kwoh, S.; Rosen, P. JOC 1973, 38, 3440.
8. Katano, K.; Atsumi, K.; Nishihata, K.; Kai, F.; Akita, E.; Niida, T. CPB 1982, 30, 3054.
9. Baldwin, J. E.; Urban, F. J.; Cooper, R. D. G.; Jose, F. L. JACS 1973, 95, 2401.
10. Priebe, H. ACS(B) 1987, 41, 640.
11. (a) Hillis, L. R.; Ronald, R. C. JOC 1985, 50, 470. (b) Ohkubo, T.; Tsuchiko, F.; Wakasawa, T.; Nambara, T. CPB 1988, 36, 3519.
12. Kupchan, S. M.; Giacobbe, T. J.; Krull, I. S. TL 1970, 10, 2859.
13. (a) Fanta, W. I.; Erman, W. F. TL 1969, 4155. (b) Fleischhacker, W.; Richter, B.; Urban, E. M 1989, 120, 765 (CA 1990, 113, 78 748w). (c) Hayakawa, K.; Kido, K.; Kanematsu, K. JCS(P1) 1988, 511. (d) Scheline, R. R. ACS 1966, 20, 1182. (e) Jain, A. C.; Pankajamani, K. S.; Seshadri, T. R. J. Sci. Ind. Res. 1953, 12B, 127. (f) Pankajamani, K. S.; Seshadri, T. R. J. Sci. Ind. Res. 1962, 21B, 322.
14. Pettit, G. R.; Singh, S. B. CJC 1987, 65, 2390.

Bradley D. Smith & Gregory T. Morin

University of Notre Dame, IN, USA



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