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<br />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<br />corrosive<br />Causes burns<br />Probable Carcinogen |NFPA-H = 3 |NFPA-R = 2 |NFPA-F = 2 |NFPA-S = W |GHS_ref=<ref>{{cite web |title=Lithium triethylhydroborate |url=https://pubchem.ncbi.nlm.nih.gov/compound/23712863#section=Safety-and-Hazards |website=pubchem.ncbi.nlm.nih.gov |access-date=19 December 2021 |language=en}}</ref> |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<br/>sodium borohydride<br />sodium hydride<br />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.<ref name=HCB>{{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}}</ref> 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.<ref>{{cite book|chapter=Lithium Triethylborohydride|author1=Marek Zaidlewicz|author2=Herbert C. Brown|title=Encyclopedia of Reagents for Organic Synthesis|year=2001|doi=10.1002/047084289X.rl148|isbn=0471936235}}</ref><ref>{{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|url-access=subscription}}</ref><ref name=HCB/>

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.<ref>{{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}}</ref> LiBHEt₃ can selectively deprotect tertiary N-acyl groups without affecting secondary amide functionality.<ref>{{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|url-access=subscription}}</ref> It has also been shown to reduce aromatic esters to the corresponding alcohols as shown in eq 6 and 7.

:Image:Ester to alcohol.png

LiBHEt₃ also reduces pyridine and isoquinolines to piperidines and tetrahydroisoquinolines respectively.<ref>{{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|url-access=subscription}}</ref> <br />The reduction of β-hydroxysulfinyl imines with catecholborane and LiBHEt₃ produces ''anti-''1,3-amino alcohols shown in (8).<ref>{{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|url-access=subscription}}</ref> :Image:Betaimmine reduction.png

==Precautions== LiBHEt₃ reacts exothermically, potentially violently, with water, alcohols, and acids, releasing hydrogen and the pyrophoric triethylborane.<ref name=HCB/>

==References== <references/>

{{Lithium compounds}}

Category:Borohydrides Category:Lithium compounds Category:Reducing agents Category:Organoboron compounds Category:Ethyl compounds Category:Hydrides