Lithium triethylborohydride: Difference between revisions

{{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 [[Lithium|Li]][[Ethyl group|Et₃]][[Borohydride|BH]]. Commonly referred to as '''LiTEBH''' or '''Superhydride''', it is a powerful [[reducing agent]] used in [[organometallic chemistry|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]].

LiBHEt₃ is prepared by the reaction of [[lithium hydride]] (LiH) and [[triethylborane]] (Et₃B) in [[tetrahydrofuran]] (THF):

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 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}}</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}}</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 [[lactone]]s reduce to diols. α,β-Enones undergo 1,4-addition to give lithium [[enolate]]s. [[Disulfide]]s reduce to [[thiol]]s (via thiolates). LiBHEt₃ deprotonates carboxylic acids, but does not reduce the resulting lithium carboxylates. For similar reasons, [[epoxide]]s 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:

[[Acetal]]s and [[ketal]]s are not reduced by LiBHEt₃. It can be used in the reductive cleavage of [[mesylates]] and [[tosylate]]s.<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}}</ref> 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.<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}}</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}}</ref>

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

{{Lithium compounds}}

[[Category:Borohydrides]]

[[Category:Organoboron compounds]]

[[Category:Ethyl compounds]]

[[Category:Hydrides]]