Lithium bis(trimethylsilyl)amide

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Lithium bis(trimethylsilyl)amide
Li-HMDS.svg
Monomer (does not exist)
LiNtms2Trimer.png
Cyclic trimer
Identifiers
CAS number 4039-32-1 YesY
PubChem 2733832
ChemSpider 21170111 N
Jmol-3D images Image 1
Properties
Molecular formula C6H18LiNSi2
Molar mass 167.326 g/mol
Appearance White solid
Density 0.86 g/cm3 at 25 °C
Melting point 71-72 °C
Boiling point 80 - 84 °C (0.001 mm Hg)
Solubility in water decomposes
Solubility Most aprotic solvents
THF, hexane, toluene
Acidity (pKa) 36
Hazards
Main hazards flammable, corrosive
Related compounds
Related compounds Sodium bis(trimethylsilyl)amide
Potassium bis(trimethylsilyl)amide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

Lithium bis(trimethylsilyl)amide is a lithiated organosilicon compound with the formula LiN(SiMe3)2. It is commonly abbreviated as LiHMDS (Lithium HexaMethylDiSilazane - a reference to its starting material HMDS) and is primarily used as a strong non-nucleophilic base and as a ligand. Like many lithium reagents it has a tendency to aggregate and will form a cyclic trimer in the absence of coordinating species.

Preparation[edit]

LiHMDS is commercially available, but it can also be prepared by the deprotonation of bis(trimethylsilyl)amine with n-butyllithium.[1] This reaction can be performed in situ.[2]

HN(SiMe3)2 + C4H9Li → LiN(SiMe3)2 + C4H10

Once formed, the compound can be purified by sublimation or distillation.

Reactions and applications[edit]

As a base[edit]

In organic chemistry, LiHMDS is often used as a strong non-nucleophilic base. It has a similar pKa (~26)[3] to that of LDA (~36) but it is more sterically hindered and hence less nucleophilic. It can be used to form compounds such as lithium acetylide,[4] or lithium enolates.[2]

LiHMDS EnolateFormation.png

As such it finds use in a range of coupling reactions; particularly carbon-carbon bond forming reactions such as the Fráter–Seebach alkylation and mixed Claisen condensations.

As a ligand[edit]

LiHMDS can react with a wide range of metal halides, via a salt metathesis reaction, to give metal bis(trimethylsilyl)amides.

MXx + x Li(hmds) → M(hmds)x + x LiX
(X = Cl, Br, I and sometimes F)

Metal bis(trimethylsilyl)amide complexes are lipophilic due to the ligand and hence are soluble in a range of nonpolar organic solvents, this often makes them more reactive than the corresponding metal halides, which can be difficult to solubilise. The steric bulk of the ligands causes their complexes to be discrete and monomeric; further increasing their reactivity. Having a built-in base, these compounds conveniently react with protic ligand precursors to give other metal complexes and hence are important precursors to more complex coordination compounds.[5]

Niche uses[edit]

LiHMDS is volatile and has been discussed for use for atomic layer deposition of lithium compounds.

Structure[edit]

Like many organolithium reagents, lithium bis(trimethylsilyl)amide can form aggregates in solution. The extent of aggregation depends on the solvent. In coordinating solvents such as ethers[6] and amines[7] the so-called monomer and dimer are prevalent. In the monomeric and dimeric state, one or two solvent molecules bind to lithium centers. In noncoordinating solvents, such as aromatics or pentane, the complex oligomers predominate, including the trimer.[7] In the solid state lithium bis(trimethylsilyl)amide is trimeric.[8]

LiHMDS aggregation.png
LiHMDS-tmeda complex.png
LiHMDS adduct with TMEDA.
Li2(Sitms2)2(THF)2.png
THF solvated dimer: (LiHMDS)2•THF2
The real "Li N(Sitms2 )2".png
Trimer, solvent free: (LiHMDS)3

See also[edit]

References[edit]

  1. ^ Amonoo-Neizer, E. H.; Shaw, R. A.; Skovlin, D. O.; Smith, B. C. (1966). "Lithium Bis(Trimethylsilyl)Amide and Tris(Trimethylsilyl)Amine". Inorg. Synth. Inorganic Syntheses 8: 19–22. doi:10.1002/9780470132395.ch6. ISBN 978-0-470-13239-5. 
  2. ^ a b Danheiser, R. L.; Miller, R. F.; Brisbois, R. G. (1990), Detrifluoroacetylative Diazo Group Transfer: (E)-1-Diazo-4-phenyl-3-buten-2-one, Org. Synth. 73: 134 ; Coll. Vol. 9: 197 
  3. ^ J. Org. Chem. 1985, 50, 3232-3234
  4. ^ Reich, Melanie (Aug 24, 2001). "Addition of a lithium acetylide to an aldehyde; 1-(2-pentyn-4-ol)-cyclopent-2-en-1-ol". ChemSpider Synthetic Pages. p. 137. Retrieved 5 September 2010. 
  5. ^ Michael Lappert, Andrey Protchenko, Philip Power, Alexandra Seeber (2009). Metal Amide Chemistry. Weinheim: Wiley-VCH. doi:10.1002/9780470740385. ISBN 0-470-72184-7. 
  6. ^ Lucht, Brett L.; Collum, David B. (1995). "Ethereal Solvation of Lithium Hexamethyldisilazide: Unexpected Relationships of Solvation Number, Solvation Energy, and Aggregation State". Journal of the American Chemical Society 117 (39): 9863–9874. doi:10.1021/ja00144a012. 
  7. ^ a b Lucht, Brett L.; Collum, David B. (1996). "Lithium Ion Solvation: Amine and Unsaturated Hydrocarbon Solvates of Lithium Hexamethyldisilazide (LiHMDS)". Journal of the American Chemical Society 118 (9): 2217–2225. doi:10.1021/ja953029p. 
  8. ^ Rogers, Robin D.; Atwood, Jerry L.; Grüning, Rainer (1978). "The crystal structure of N-lithiohexamethyldisilazane, [LiN(SiMe3)2]3". J. Organomet. Chem. 157 (2): 229–237. doi:10.1016/S0022-328X(00)92291-5.