Lithium triethylborohydride
Skeletal formula of lithium triethylborohydride
Names
Preferred IUPAC name
Lithium triethylboranuide
Other names
Superhydride
LiTEBH
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.040.963 Edit this at Wikidata
EC Number
  • 245-076-8
UNII
  • InChI=1S/C6H16B.Li/c1-4-7(5-2)6-3;/h7H,4-6H2,1-3H3;/q-1;+1 checkY
    Key: WCJAYABJWDIZAJ-UHFFFAOYSA-N checkY
  • [Li+].CC[BH-](CC)CC
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
Occupational safety and health (OHS/OSH):
Main hazards
highly flammable
corrosive
Causes burns
Probable Carcinogen
GHS labelling:[1]
GHS02: Flammable
GHS05: Corrosive
GHS07: Exclamation mark
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)
3
2
2
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).
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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.[2] 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

Its THF solutions are 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 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]

References

  1. ^ "Lithium triethylhydroborate". pubchem.ncbi.nlm.nih.gov. Retrieved 19 December 2021.
  2. ^ a b c Zaidlewicz, M.; Brown, H.C. (2001). "Lithium Triethylborohydride". Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons. doi:10.1002/047084289X.rl148. ISBN 0471936235. Retrieved 2022-02-18.
  3. ^ Marek Zaidlewicz; Herbert C. Brown (2001). "Lithium Triethylborohydride". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rl148. ISBN 0471936235.
  4. ^ Brown, H.C.; Kim, S.C.; Krishnamurthy, S. (1980-02-01). "Selective reductions. 27. Reaction of alkyl halides with representative complex metal hydrides and metal hydrides. Comparison of various hydride reducing agents". J. Org. Chem. 45 (5): 849–856. doi:10.1021/jo01293a018. Retrieved 2022-02-18.
  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.
  6. ^ Tanaka, H.; Ogasawara, K. (2002-06-17). "Utilization oh lithium triethylborohydride as a selective N-acyl deprotecting agent". Tetrahedron Lett. 43 (25): 4417. doi:10.1016/S0040-4039(02)00844-4. Retrieved 2022-02-18.
  7. ^ Blough, B.E.; Carroll, F.I. (1993-11-05). "Reduction of isoquinoline and pyridine-containing heterocycles with lithium triethylborohydride (Super-Hydride®)". Tetrahedron Lett. 34 (45): 7239. doi:10.1016/S0040-4039(00)79297-5. Retrieved 2022-02-18.
  8. ^ Kochi, T.; Tang, T.P.; Ellman, J.A. (2002-05-14). "Asymmetric Synthesis of syn- and anti-1,3-Amino Alcohols". J. Am. Chem. Soc. 124 (23): 6518–6519. doi:10.1021/ja026292g. PMID 12047156. Retrieved 2022-02-18.