Lithium triethylborohydride
{{Redirect|Superhydride|the class of compounds sometimes known as superhydrides|Polyhydride}}{{chembox
|Watchedfields = changed
|verifiedrevid = 415897994
|Name = Lithium triethylborohydride
|ImageFile = Lithium triethylborohydride Structural Formula V1.svg
|ImageAlt = Skeletal formula of lithium triethylborohydride
|PIN = Lithium triethylboranuide
|OtherNames = Superhydride
LiTEBH
|Section1={{Chembox Identifiers
|CASNo_Ref = {{cascite|correct|??}}
|CASNo = 22560-16-3
|ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
|ChemSpiderID = 2006168
|EC_number = 245-076-8
|PubChem = 23712863
|UNII_Ref = {{fdacite|correct|FDA}}
|UNII = Q1ML638JFD
|StdInChI_Ref = {{stdinchicite|correct|chemspider}}
|StdInChI = 1S/C6H16B.Li/c1-4-7(5-2)6-3;/h7H,4-6H2,1-3H3;/q-1;+1
|StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
|StdInChIKey = WCJAYABJWDIZAJ-UHFFFAOYSA-N
|SMILES = [Li+].CC[BH-](CC)CC
}}
|Section2={{Chembox Properties
|Formula = Li(C2H5)3BH
|MolarMass = 105.95 g/mol
|Appearance = Colorless to yellow liquid
|Density = 0.890 g/cm3, liquid
|Solubility = reactive
|BoilingPtC = 66
|BoilingPt_notes = for THF
}}
|Section3={{Chembox Hazards
|ExternalSDS = [http://www.basf.com/inorganics/pdfs/MSDS/Boranes/CalSelect_LT.pdf External MSDS]
|MainHazards = highly flammable
corrosive
Causes burns
Probable Carcinogen
|NFPA-H = 3
|NFPA-R = 2
|NFPA-F = 2
|NFPA-S = W
|GHSPictograms = {{GHS02}}{{GHS05}}{{GHS07}}
|GHSSignalWord = Danger
|HPhrases = {{H-phrases|250|260|314|335}}
|PPhrases = {{P-phrases|210|222|223|231+232|260|261|264|271|280|301+330+331|302+334|303+361+353|304+340|305+351+338|310|312|321|335+334|363|370+378|402+404|403+233|405|422|501}}
}}
|Section4={{Chembox Related
|OtherFunction_label = hydride
|OtherFunction = 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.{{cite book|author1=Zaidlewicz, M.|author2=Brown, H.C.|author-link2=Herbert C. Brown|chapter=Lithium Triethylborohydride|title=Encyclopedia of Reagents for Organic Synthesis|year=2001|publisher=John Wiley & Sons|doi=10.1002/047084289X.rl148|isbn=0471936235|url=https://doi.org/10.1002/047084289X.rl148.pub2|access-date=2022-02-18}} 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.{{cite journal|title=Lithium Triethylborohydride|author1=Marek Zaidlewicz|author2=Herbert C. Brown|journal=Encyclopedia of Reagents for Organic Synthesis|year=2001|doi=10.1002/047084289X.rl148|isbn=0471936235}}{{cite journal|author1=Brown, H. C.|author2=Kim, S. C.|author3=Krishnamurthy, S.|title=Selective reductions. 27. Reaction of alkyl halides with representative complex metal hydrides and metal hydrides. Comparison of various hydride reducing agents|journal=J. Org. Chem.|date=1980-02-01|volume=45|issue=5|pages=849–856|doi=10.1021/jo01293a018|url=https://pubs.acs.org/doi/10.1021/jo01293a018|access-date=2022-02-18}}
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:
:Image:Reduction of epoxide.png
Acetals and ketals are not reduced by LiBHEt3. It can be used in the reductive cleavage of mesylates and tosylates.{{cite journal|author1=Baer, H.H.|author2=Mekarska-Falicki, M.|title=Stereochemical dependence of the mechanism of deoxygenation, with lithium triethylborohydride, in 4,6-O-benzylidenehexopyranoside p-toluenesulfonates|journal=Canadian Journal of Chemistry|date=November 1985|volume=63|issue=11|page=3043|doi=10.1139/v85-505|url=https://doi.org/10.1139/v85-505|access-date=2022-02-18|doi-access=free}} LiBHEt3 can selectively deprotect tertiary N-acyl groups without affecting secondary amide functionality.{{cite journal|author1=Tanaka, H.|author2=Ogasawara, K.|title=Utilization oh lithium triethylborohydride as a selective N-acyl deprotecting agent|journal=Tetrahedron Lett.|date=2002-06-17|volume=43|issue=25|page=4417|doi=10.1016/S0040-4039(02)00844-4|url=https://doi.org/10.1016/S0040-4039(02)00844-4|access-date=2022-02-18}} 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.{{cite journal|author1=Blough, B.E.|author2=Carroll, F.I.|title=Reduction of isoquinoline and pyridine-containing heterocycles with lithium triethylborohydride (Super-Hydride®)|journal=Tetrahedron Lett.|date=1993-11-05|volume=34|issue=45|page=7239|doi=10.1016/S0040-4039(00)79297-5|url=https://www.sciencedirect.com/science/article/abs/pii/S0040403900792975|access-date=2022-02-18}}
The reduction of β-hydroxysulfinyl imines with catecholborane and LiBHEt3 produces anti-1,3-amino alcohols shown in (8).{{cite journal|author1=Kochi, T.|author2=Tang, T.P.|author3=Ellman, J.A.|title=Asymmetric Synthesis of syn- and anti-1,3-Amino Alcohols|journal=J. Am. Chem. Soc.|date=2002-05-14|volume=124|issue=23|pages=6518–6519|doi=10.1021/ja026292g|pmid=12047156|url=https://doi.org/10.1021/ja026292g|access-date=2022-02-18}}
Precautions
LiBHEt3 reacts exothermically, potentially violently, with water, alcohols, and acids, releasing hydrogen and the pyrophoric triethylborane.