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.
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| Names | |
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| Preferred IUPAC name
Lithium triethylboranuide | |
| Other names
Superhydride
LiTEBH | |
| Identifiers | |
3D model (JSmol)
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| ChemSpider | |
| ECHA InfoCard | 100.040.963 |
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PubChem CID
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| UNII | |
CompTox Dashboard (EPA)
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| 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
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highly flammable corrosive Causes burns Probable Carcinogen |
| GHS labelling:[1] | |
  
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| 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) | |
| Safety data sheet (SDS) | External MSDS |
| Related compounds | |
Related hydride
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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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LiBHEt3 is a stronger reducing agent than lithium borohydride and lithium aluminium hydride.
Preparation
editLiBHEt3 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
editAlkyl 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:
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.
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]
Precautions
editLiBHEt3 reacts exothermically, potentially violently, with water, alcohols, and acids, releasing hydrogen and the pyrophoric triethylborane.[2]
References
edit- ^ "Lithium triethylhydroborate". pubchem.ncbi.nlm.nih.gov. Retrieved 19 December 2021.
- ^ 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.
- ^ Marek Zaidlewicz; Herbert C. Brown (2001). "Lithium Triethylborohydride". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rl148. ISBN 0471936235.
- ^ 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.
- ^ 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.
- ^ 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.
- ^ 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.
- ^ 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.