Stereoselective synthesis hexamethyldisilazide

As exemplified with the synthesis of 105 (Scheme 1.2), the latter compounds are obtained via the displacement a-chloroalkylboronates with the hexamethyldisilazide anion. This example also emphasizes the powerful neighboring group effect of boron, which allows selectivity in the addition of Cl2CHLi in the presence of a primary alkyl bromide [323]. Other applications of (a-haloalkyl)boronates in stereoselective synthesis are detailed in Chapter 8. 

The term amidolithium is the unambiguous name for the compounds RR NLi (R, R = alkyl, aryl, silyl, etc.) more often termed lithium amides. They derive their importance from the near-ubiquity of their bulkier members lithium diisopropy-lamide (LDA), lithium tetramethylpiperidide (LTMP), and lithium hexamethyldisilazide (LHMDS) in organic synthesis. Using such powerful but nonnucleophilic bases, many useful reactions may be performed, notably the enolization of ketones and esters, which can proceed both regio- and stereoselectively under kinetic control at low temperatures. ... 

In a study on the electrophilic azide transfer to chiral enolates, Evans found that the use of potassium bis(trimethylsilyl)amide was crucial for this process. The KN(TMS)2 played a dual role in the reaction as a base, it was used for the stereoselective generation of the (Z)-enolate (1). Reaction of this enolate with trisyl azide gave an intermediate triazene species (2) (eq 4). The potassium counterion from the KN(TMS)2 used for enolate formation was important for the decomposition of the triazene to the desired azide. Use of other hindered bases such as Lithium Hexamethyldisilazide allowed preparation of the intermediate triazene however, the lithium ion did not catalyze the decomposition of the triazene to the azide.This methodology has been utilized in the synthesis of cyclic tripeptides. 

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