Lithium triethylborohydride

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Lithium triethylborohydride

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Template:Longitem	Li(C2H5)3BH
Molar mass	105.95 g/mol
Appearance	Colorless to yellow liquid
Density	0.890 g/cm3, liquid
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Template:Longitem	Lithium borohydride sodium borohydride sodium hydride lithium aluminium hydride

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 liquid but is typically marketed and used as a THF solution. The related reducing agent sodium triethylborohydride is commercially available as toluene solutions.

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

The resulting THF complex is stable indefinitely in the absence of moisture and air.

Reactions

Alkyl halides are reduced to the alkanes by LiBHEt₃.

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:

File:Reduction of epoxide.png

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

File:Ester to alcohol.png

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

File:Betaimmine reduction.png

Precautions

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