Triisopropyl Borate

B(O-i-Pr)3

[5419-55-6]  · C9H21BO3  · Triisopropyl Borate  · (MW 188.11)

(used for the preparation of boronic acids and esters;1a also used as Lewis acid catalyst or additive in many reactions)

Physical Data: bp 139-141 °C; n20D 1.3764; d 0.815 g cm-3.

Solubility: very sol ether, ethanol, isopropanol, benzene; dec in H2O.2

Form Supplied in: colorless liquid, about 98% pure.

Purification: by distillation.

Handling, Storage, and Precautions: moisture sensitive; must be stored under nitrogen in a cool dry place; irritating to the skin and to the eyes.3

Synthesis of Borinic and Boronic Acids and Esters.

B(O-i-Pr)3 is a useful reagent for the preparation of isopropyl borinates (eq 1)1b or diisopropyl boronates (eq 2)1a,1c-i by reaction with 2 or 1 equiv of the appropriate organolithium or Grignard reagent, followed by addition of just 1 equiv of HCl. B(O-i-Pr)3 is superior to B(OMe)3 for these reactions.

Borinic esters are useful intermediates for the preparation of tertiary alcohols, ketones, a-haloborinic esters, lithium dialkylborohydrides, and new types of dialkylboranes.1b Boronic esters can be easily converted into the corresponding boronic acids by aqueous hydrolysis,1c with or without the presence of an acid (see also Trimethyl Borate). An improved procedure for the preparation of both borinic and boronic isopropyl esters involves the pyrolysis of the intermediate ate complexes.1e Many boronic esters and acids can be prepared by the procedure summarized in eqs 1 and 2; some examples include allyl (substituted or not),1f,g 1-alkynyl,1h and 1-alkenyl1j boronic esters. Usually B(O-i-Pr)3 gives better yields in boronic acids synthesis (especially for methylboronic acid) than B(OMe)3 or other trialkyl borates.1a,1d,1g

Reaction of triisopropyl borate with Chloromethyllithium,1a Bromomethyllithium,4a or a-haloallyllithium4b represents an efficient entry into a-haloboronates, which can in turn undergo various substitution reactions.

Use of Boronic Esters and Acids.

One of the main uses of diisopropoxyboranes is their conversion into other boronates, passing through the corresponding boronic acids. These new boronates are usually cyclic and can be chiral either on the diol residue1f,g used for the esterification, on the R-B moiety,1a or both.1i Allyl and substituted allyl boronates have been used in stereoselective C-C bond formation, by reaction with aldehydes (for a series of examples see Trimethyl Borate).1a,f,g,5 They can also react with C=N electrophiles, such as aldoximes, imines, and sulfenimides,5 although more slowly than with aldehydes.

The reaction of BrCH2Li with simple boronates (achiral or chiral) inserts a CH2 group into the carbon-boron bond, affording homologated boronates.4a Reaction with Cl2CHLi inserts the CHCl group.1a The resulting a-haloboronic esters are useful starting materials for various substitution reactions. The same applies for a-haloallylboronates.4b

Diisopropyl boronates1a and boronic acids have been employed as nucleophilic partners in palladium-catalyzed cross-coupling reactions. However, only few have been prepared from B(O-i-Pr)3 (usually boronic acids are prepared using B(OMe)3 and Catecholborane as boron reagents), including some arylboronic acids used in the synthesis of 5-arylnicotinates.6 (1-Alkynyl)diisopropoxyboranes can be used in Boron Trifluoride Etherate-catalyzed conjugate addition to a,b-unsaturated ketones to provide 3-alkynyl ketones.1h Finally, a regiospecific synthesis of 2-fluoro-3-O-methylestrone (2) was realized by electrophilic displacement of boronic acid (1) with Cesium Fluoroxysulfate (CFS) (eq 3);7 boronic acid formation was regiospecific in position 2 when the metalation [1. n-BuLi; 2. B(O-i-Pr)3] was performed on the h6-arenetricarbonylchromium(0) complex, derived from estrone.

Miscellaneous.

B(O-i-Pr)3 has been used to prepare a boron reagent from lithium (trimethylsilyl)acetonitrile, which was employed for the stereoselective alkenation of aldehydes to give a,b-unsaturated nitriles. Compared to Li, Ti, and Mg reagents, this boron reagent gives the best results in regard to chemical yields and (Z/E) ratios.8 B(O-i-Pr)3, together with other trialkyl borates, has been used to transform unactivated bromides (R1 = alkyl, vinyl, aromatic) into the corresponding homologated ester, via a carbonylation reaction catalyzed by 1,5-hexadienerhodium(I) chloride dimer and Tetrakis(triphenylphosphine)palladium(0);9a the same reaction can also be performed on benzyl chlorides in the presence of iodide ion and the above mentioned RhI catalyst (eq 4).9b

Finally, the reaction of B(O-i-Pr)3 with KH10 gives Potassium Triisopropoxyborohydride, a mild reducing agent for carbonyl compounds, disulfides, and haloboranes.


1. (a) Matteson, D. S. T 1989, 45, 1859. (b) Cole, T. E.; Haly, B. D. OM 1992, 11, 652. (c) Brown, H. C.; Cole, T. E. OM 1985, 4, 816. (d) Brown, H. C.; Cole, T. E. OM 1983, 2, 1316. (e) Brown, H. C.; Srebnik, M.; Cole, T. E. OM 1986, 5, 2300. (f) Hoffmann, R. W. PAC 1988, 60, 123. (g) Roush, W. R.; Grover, P. T. T 1992, 48, 1981 and references therein. (h) Fujishima, H.; Takada, E.-I.; Hara, S.; Suzuki, A. CL 1992, 695. (i) Stürmer, R. AG(E) 1990, 29, 59. (j) Matteson, D. S.; Beedle, E. C. HC 1990, 1, 135.
2. Handbook of Chemistry and Physics, 55th ed.; Weast, R. C., Ed.; CRC: Cleveland, 1974/75; p C-208.
3. The Sigma-Aldrich Library of Chemical Safety Data, 2nd ed.; Lenga, R. E., Ed.; Sigma-Aldrich: Milwaukee, 1988; Vol. 2, p 3470b.
4. (a) Michnick, T. J.; Matteson, D. S. SL 1991, 631. (b) Brown, H. C.; Rangaishenvi, M. V. TL 1990, 31, 7115.
5. Roush, W. R. COS 1991, 2, 1.
6. Thompson, W. J.; Gaudino, J. JOC 1984, 49, 5237.
7. Diorazio, L. J.; Widdowson, D. A.; Clough, J. M. JCS(P1) 1992, 421.
8. Haruta, R.; Ishiguro, M.; Furuta, K.; Mori, A.; Ikeda, N.; Yamamoto, H. CL 1982, 1093.
9. (a) Hashem, K. E.; Woell, J. B.; Alper, H. TL 1984, 25, 4879. (b) Alper, H.; Hamel, N.; Smith, D. J. H.; Woell, J. B. TL 1985, 26, 2273.
10. Brown, H. C.; Cha, J. S.; Nazer, B. IC 1984, 23, 2929.

Luca Banfi, Enrica Narisano & Renata Riva

Università di Genova, Italy



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