Lithium triethylborohydride

Lithium triethylborohydride
Skeletal formula of lithium triethylborohydride
Names
IUPAC name
Lithium triethylborohydride
Other names
Superhydride
LiTEBH
Identifiers
22560-16-3 YesY
3D model (Jmol) Interactive image
ChemSpider 2006168 YesY
ECHA InfoCard 100.040.963
PubChem 2723993
Properties
Li(C2H5)3BH
Molar mass 105.95 g/mol
Appearance Colorless to yellow liquid
Density 0.890 g/cm3, liquid
Boiling point 66 °C (151 °F; 339 K) for THF
reactive
Hazards
Main hazards highly flammable
corrosive
Causes burns
Probable Carcinogen
Safety data sheet External MSDS
R-phrases 11-14/15-19-34
S-phrases 16-26-33-36/37/39-43-45
NFPA 704
Flammability code 2: Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur. Flash point between 38 and 93 °C (100 and 200 °F). E.g., diesel fuel Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond
2
3
2
Related compounds
Related hydride
 sodium borohydride
sodium hydride
lithium aluminium hydride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Lithium triethylborohydride is the organoboron compound with the formula LiEt3BH). Commonly referred to as LiTEBH or Superhydride, it is a powerful reducing agent used in organometallic and organic chemistry. LiBHEt3 is far more reducing than lithium borohydride and lithium aluminium hydride. It is a colorless or white solid but is typically marketed as a THF solution.[1] The related reducing agent, sodium triethylborohydride, is commercially available as toluene solutions.

Preparation

LiBHEt3 is prepared by the reaction of lithium hydride (LiH) and triethylborane (Et3B) in tetrahydrofuran (THF):

LiH + Et3B → LiEt3BH

Its THF solutions are stable indefinitely in the absence of moisture and air.

Uses

Carbonyl substrates

LiBHEt3 reduces a wide range of functional groups, not only aldehydes, ketones, acid chlorides, but also esters and even tertiary amides. The immediate product of these reductions are lithium alkoxides, which, upon hydrolytic workup, are converted to the alcohols. The stoichiometry for the reduction of ketones is shown:

R2CO + LiBHEt3 → R2CHOLi + BEt3
R2CHOLi + H2O → R2CHOH + LiOH

Even sterically hindered substrates are reduced, as shown with 2,2,4,4-tetramethyl-3-pentanone.

LiBHEt3 deprotonates carboxylic acids, but does not reduce the resulting lithium carboxylates. For similar reasons, it reduces acid anhydrides to alcohols and the carboxylic acid, not to the diol. Similarly lactones reduce to diols. α,β-Enones undergo 1,4-addition to give lithium enolates. Disulfides reduce to thiols (via thiolates).

Other substrates

Alkyl halides are reduced to the alkanes by LiBHEt3.[2][1]

Epoxides undergo ring-opening upon treatment with LiBHEt3 to give the alcohol. With unsymmetrical epoxides, the reaction can proceed with high regio- and stereo- selectivity, favoring the Markovnikov alcohol.[2] For example, the 1,2-epoxybutane is reduced to 1-methylcyclohexanol:

Acetals and ketals are not reduced by LiBHEt3. It can be used in the reductive cleavage of mesylates and tosylates.[3] LiBHEt3 can selectively deprotect tertiary N-acyl groups without affecting secondary amide functionality.[4] It has also shown to reduce aromatic esters to the corresponding alcohols as shown in eq 6 and 7.

LiBHEt3 also reduces pyridine and isoquinolines to piperidines and tetrahydroisoquinolines respectively.[5]
The reduction of β-hydroxysulfinyl imines with catecholborane and LiBHEt3 produces anti-1,3-amino alcohols shown in (8).[6]

Precautions

LiBHEt3 reacts exothermically, potentially violently, with water, alcohols, or acids, releasing hydrogen and the pyrophoric triethylborane.[1]

References

  1. 1 2 3 Marek Zaidlewicz, Herbert C. Brown "Lithium Triethylborohydride" Encyclopedia of Reagents for Organic Synthesis 2001, John Wiley & Sons. doi:10.1002/047084289X.rl148
  2. ↑ Brown, H.C.; Kim, S.C.; Krishnamurthy, S. "Selective reductions. 27. Reaction of alkyl halides with representative complex metal hydrides and metal hydrides. Comparison of various hydride reducing agents" J. Org. Chem. 1980, 45, 1-12. doi:10.1021/jo01293a018
  3. ↑ Baer, H.H.; Mekarska-Falicki, M. Can. J. Chem. 1985, 63, 3043.
  4. ↑ Tanaka, H.; Ogasawara, K. Tetrahedron Lett. 2002, 43, 4417.
  5. ↑ Blough, B.E.; Carroll, F. I. "Reduction of isoquinoline and pyridine-containing heterocycles with lithium triethylborohydride (Super-Hydride®)" Tetrahedron Lett. 1993, 34, 7239. doi:10.1016/S0040-4039(00)79297-5
  6. ↑ Kochi, T; Tang, T.P.; Ellman, J.A. J. Am. Chem. Soc. 2002, 124, 6518.
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