Lithium bis(trimethylsilyl)amide

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).

Uses

Lithium Hexamethyldisilazide is widely used as strong nonnucleophilic base

Uses

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.

Uses

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.

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