Concept
Lithium bis(trimethylsilyl)amide is a lithiated organosilicon compound with the formula . It is commonly abbreviated as LiHMDS or Li(HMDS) (lithium hexamethyldisilazide - a reference to its conjugate acid 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. LiHMDS is commercially available, but it can also be prepared by the deprotonation of bis(trimethylsilyl)amine with n-butyllithium. This reaction can be performed in situ. Once formed, the compound can be purified by sublimation or distillation. LiHMDS is often used in organic chemistry as a strong non-nucleophilic base. Its conjugate acid has a pKa of ~26, making it is less basic than other lithium bases, such as LDA (pKa of conjugate acid ~36), but it is more sterically hindered and hence less nucleophilic. It can be used to form various organolithium compounds, including acetylides or lithium enolates. where Me = . 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. An alternative synthesis of tetrasulfur tetranitride entails the use of as a precursor with pre-formed S–N bonds. is prepared by the reaction of lithium bis(trimethylsilyl)amide and sulfur dichloride (). The reacts with the combination of and sulfuryl chloride () to form , trimethylsilyl chloride, and sulfur dioxide: Li(HMDS) can react with a wide range of metal halides, by a salt metathesis reaction, to give metal bis(trimethylsilyl)amides. where 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.
