Lithium triethylborohydride

Last updated
Lithium triethylborohydride
Lithium triethylborohydride Structural Formula V1.svg
Names
Preferred IUPAC name
Lithium triethylboranuide
Other names
Superhydride
LiTEBH
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.040.963 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 245-076-8
PubChem CID
UNII
  • InChI=1S/C6H16B.Li/c1-4-7(5-2)6-3;/h7H,4-6H2,1-3H3;/q-1;+1 Yes check.svgY
    Key: WCJAYABJWDIZAJ-UHFFFAOYSA-N Yes check.svgY
  • [Li+].CC[BH-](CC)CC
Properties
Li(C2H5)3BH
Molar mass 105.95 g/mol
AppearanceColorless to yellow liquid
Density 0.890 g/cm3, liquid
Boiling point 66 °C (151 °F; 339 K) for THF
reactive
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
highly flammable
corrosive
Causes burns
Probable Carcinogen
GHS labelling: [1]
GHS-pictogram-flamme.svg GHS-pictogram-acid.svg GHS-pictogram-exclam.svg
Danger
H250, H260, H314, H335
P210, P222, P223, P231+P232, P260, P261, P264, P271, P280, P301+P330+P331, P302+P334, P303+P361+P353, P304+P340, P305+P351+P338, P310, P312, P321, P335+P334, P363, P370+P378, P402+P404, P403+P233, P405, P422, P501
NFPA 704 (fire diamond)
NFPA 704.svgHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 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 fuelInstability 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
3
2
2
W
Safety data sheet (SDS) External MSDS
Related compounds
Related hydride
Lithium borohydride
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).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

Lithium triethylborohydride is the organoboron compound with the formula Li Et3 BH. 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. [2] The related reducing agent sodium triethylborohydride is commercially available as toluene solutions.

Contents

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. [3] [4] [2]

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:

Reduction of epoxide.png

Acetals and ketals are not reduced by LiBHEt3. It can be used in the reductive cleavage of mesylates and tosylates. [5] LiBHEt3 can selectively deprotect tertiary N-acyl groups without affecting secondary amide functionality. [6] It has also been shown to reduce aromatic esters to the corresponding alcohols as shown in eq 6 and 7.

Ester to alcohol.png

LiBHEt3 also reduces pyridine and isoquinolines to piperidines and tetrahydroisoquinolines respectively. [7]
The reduction of β-hydroxysulfinyl imines with catecholborane and LiBHEt3 produces anti-1,3-amino alcohols shown in (8). [8]

Betaimmine reduction.png

Precautions

LiBHEt3 reacts exothermically, potentially violently, with water, alcohols, and acids, releasing hydrogen and the pyrophoric triethylborane. [2]

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References

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  5. Baer, H.H.; Mekarska-Falicki, M. (November 1985). "Stereochemical dependence of the mechanism of deoxygenation, with lithium triethylborohydride, in 4,6-O-benzylidenehexopyranoside p-toluenesulfonates". Canadian Journal of Chemistry. 63 (11): 3043. doi: 10.1139/v85-505 . Retrieved 2022-02-18.
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