Lithium bis(trimethylsilyl)amide

Description

Lithium bis(trimethylsilyl)amide (LiHMDS) is a sterically hindered strong base. It is commercially available as a solid and as a solution in a variety of solvents (ex. THF, 2Me-THF, toluene, hexane, and MTBE). Similar reagents include sodium bis(trimethylsilyl)amide (NaHMDS) and potassium bis(trimethylsilyl)amide (KHMDS).

Chemical properties

light yellow to yellow crystalline powder

Physical properties

distillable low-melting solid; mp 70–72 °C, bp 115 °C/1mmHg. LHMDS is a cyclic trimer in the solid state,3 whereas in benzene solution it exists in a monomer–dimer equilibrium. LHMDS exists as a tetramer-dimer mixture in hydrocarbons and as a dimer-monomer mixture in THF and ether. Treatment of LHMDS with trialkylamines increases the monomer concentration, whereas the use of diamines affords exclusively the corresponding chelated monomer. LHMDS is less soluble, less basic, more stable, and much less sensitive to air compared to lithium diisopropylamide. pKa 29.5 (THF, 27 °C).

The Uses of Lithium bis(trimethylsilyl)amide

Lithium bis(trimethylsilyl)amide is used as nonnucleophilic base to generate kinetic ketone and ester enolates. It is considerably more selective than LDA and undesired reductions (e.g., of nonenolizable ketones observed in the case of LDA) can be avoided by using LHMDS.

The Uses of Lithium bis(trimethylsilyl)amide

To a suspension of methyl triphenylphosphonium bromide (2.4 g, 6.6 mmol) in THF was added tBuOK (1M in THF, 7.1 mmol) at RT. After stirring for 90 min, a solution of the SM (1.25 g, 3.35 mmol) in dry THF was added. The resulting mixture was stirred at RT for 18 h, after which time it was partitioned between saturated NH4Cl and MTBE. The org layer was dried (Na2SO4), concentrated, and purified by flash chromatography (25-30% EtOAc/hexane) to provide the product as a colorless oil. [0.3 g, 24%] [UK Pat App GB2463151A, page 141]

The Uses of Lithium bis(trimethylsilyl)amide

Lithium Hexamethyldisilazide is widely used as strong nonnucleophilic base

The Uses of Lithium bis(trimethylsilyl)amide

Lithium bis(trimethylsilyl)amide is a base used in preparation of dienes and enolates. It is used to catalyze the addition of phosphine P-H bonds to carbodiimides leading to phosphaguanidines. Lithium bis(trimethylsilyl)amide is also used in a novel three-step synthesis of disubstituted 1,2,5-thiadiazoles.

What are the applications of Application

Lithium bis(trimethylsilyl)amide is a base used in generating enolates

Preparation

It is conveniently prepared by the reaction of hexamethyldisilazane with n-butyllithium in hexane. For most uses the hexane is then evaporated and replaced with THF.

General Description

This product, 0.5 M in 2-methyltetrahydrofuran aligns with Safer Solvents and Auxiliaries, Use of Renewable Feedstocks and Inherently Safer Chemistry for Accident Prevention.

Flammability and Explosibility

Highly flammable