Lithium triethylborohydride

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. It is a colorless or white solid but is typically marketed and used as a THF solution.[1] The related reducing agent sodium triethylborohydride is commercially available as toluene solutions.

Lithium triethylborohydride
Names
IUPAC name
Lithium triethylborohydride
Other names
Superhydride
LiTEBH
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.040.963
UNII
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R-phrases (outdated) 11-14/15-19-34
S-phrases (outdated) 16-26-33-36/37/39-43-45
NFPA 704 (fire diamond)
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 fuelHealth code 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasReactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acid
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

LiBHEt3 is a stronger reducing agent than lithium borohydride and lithium aluminium hydride.

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.

Reactions

Alkyl halides are reduced to the alkanes by LiBHEt3.[2][3][1]

LiBHEt3 reduces a wide range of functional groups, but so do many other hydride reagents. Instead, LiBHEt3 is reserved for difficult substrates, such as sterically hindered carbonyls, as illustrated by reduction of 2,2,4,4-tetramethyl-3-pentanone. Otherwise, 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). LiBHEt3 deprotonates carboxylic acids, but does not reduce the resulting lithium carboxylates. For similar reasons, 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 attack at the least hindered position:[2]

Acetals and ketals are not reduced by LiBHEt3. It can be used in the reductive cleavage of mesylates and tosylates.[4] LiBHEt3 can selectively deprotect tertiary N-acyl groups without affecting secondary amide functionality.[5] 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.[6]
The reduction of β-hydroxysulfinyl imines with catecholborane and LiBHEt3 produces anti-1,3-amino alcohols shown in (8).[7]

Precautions

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

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References
  1. Marek Zaidlewicz, Herbert C. Brown "Lithium Triethylborohydride" Encyclopedia of Reagents for Organic Synthesis 2001, John Wiley & Sons. doi:10.1002/047084289X.rl148
  2. Marek Zaidlewicz, Herbert C. Brown (2001). "Lithium Triethylborohydride". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rl148.CS1 maint: uses authors parameter (link)
  3. 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
  4. Baer, H.H.; Mekarska-Falicki, M. Can. J. Chem. 1985, 63, 3043.
  5. Tanaka, H.; Ogasawara, K. Tetrahedron Lett. 2002, 43, 4417.
  6. 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
  7. Kochi, T; Tang, T.P.; Ellman, J.A. J. Am. Chem. Soc. 2002, 124, 6518.
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