Lithium bis(trimethylsilyl)amide
Encyclopedia
Lithium bisamide (commonly abbreviated as LiHMDS, Lithium HexaMethylDiSilazide) is the organosilicon compound with the formula [(CH3)3Si]2NLi. This colourless solid is a strong non-nucleophilic base
used for deprotonation
reactions and as a ligand
. When solvent-free, this compound is dimeric in solution, and trimeric in solid state.
with n-butyllithium
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The compound can be purified by sublimation.
LiHMDS can often be prepared in situ then reacted, as in the enolate preparation shown below under "reactions".
gives tris(trimethylsilyl)amine, which features a planar, 3-coordinate nitrogen.
In organic chemistry, LiHMDS is often used as a strong base, for example to form lithium acetylide, or to form a lithium enolate, as here with benzylideneacetone
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Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
used for deprotonation
Deprotonation
Deprotonation is the removal of a proton from a molecule, forming the conjugate base.The relative ability of a molecule to give up a proton is measured by its pKa value. A low pKa value indicates that the compound is acidic and will easily give up its proton to a base...
reactions and as a ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
. When solvent-free, this compound is dimeric in solution, and trimeric in solid state.
Preparation
LiHMDS is commercially available but can also be prepared by deprotonation of bis(trimethylsilyl)amineBis(trimethylsilyl)amine
Bisamine is an organosilicon compound with the molecular formula [3Si]2NH. The molecule is a derivative of ammonia with trimethylsilyl groups in place of two hydrogen atoms...
with n-butyllithium
N-Butyllithium
n-Butyllithium is an organolithium reagent. It is widely used as a polymerization initiator in the production of elastomers such as polybutadiene or styrene-butadiene-styrene...
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- [(CH3)3Si]2NH + C4H9Li → [(CH3)3Si]2NLi + C4H10
The compound can be purified by sublimation.
LiHMDS can often be prepared in situ then reacted, as in the enolate preparation shown below under "reactions".
Reactions
LiHMDS is used to generate coordination complexes with low-coordination numbers, taking advantage of the steric bulk of the (tms)2N- ligand. Examples include M[N(tms)2]3 for M = Sc, Ti, V, Fe (tms = (CH3)3Si). Treatment with trimethylsilyl chlorideTrimethylsilyl chloride
Trimethylsilyl chloride, also known as chlorotrimethylsilane is a silyl halide, with a variety of different uses in chemistry. It has the formula 3SiCl, and under standard conditions it is a colourless liquid, which is stable in the absence of water...
gives tris(trimethylsilyl)amine, which features a planar, 3-coordinate nitrogen.
In organic chemistry, LiHMDS is often used as a strong base, for example to form lithium acetylide, or to form a lithium enolate, as here with benzylideneacetone
Benzylideneacetone
Benzylideneacetone is the organic compound described by the formula C6H5CH=CHCCH3. Although both cis- and trans-isomers are possible for the α,β-unsaturated ketone, only the trans isomer is observed...
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