LiF elimination

Thermally enforced LiF elimination of LiF adducts of organyl(silyl)ami-noorganyliminosilanes leads to another surprising ring-closure reaction. For example,18,20... 

Cross-dimerization products are formed by thermal LiF elimination from two LiF adducts.36... 

Six-membered rings are either formed by thermal LiF-elimination of lithiated fluorosilanols [10] or by the reaction of siloxandiolates with halosilanes, e.g. ... 

The reaction of equation 14 probably occurs stepwise, and it is a complex process involving substitution reactions and/or homo- and heterofunctional condensations. Numerous cyclodi-, tri- and tetrasilazanes (76) are obtained in the reactions of aminofluorosilanes (74) with lithium organyls via thermal LiF elimination of lithium aminofluorosilane derivatives (75) (equation 18)69-75. The primary products of such condensations in the reaction of fluorosilanes with lithium amide have been synthesized in order to study the mechanisms of their formation. An (R2SiFNLiH) compound was characterized by X-ray diffraction8,76 77. 

Cyclotrisilazanes are obtained in the ammonolysis of dichlorosilanes (equation 14), LiF elimination from lithiated aminofluorosilanes (equation 18) and MejSiHal elimination from trimethylsilylaminohalosilanes (equation 19). In the reaction of the dilithiated tert-butyldimethylsilylamine and SiF4 the only known hexafluorocyclotrisilazane (100) was isolated beside a four-membered ring 99 (equation 26). The cyclotrisilazane is the first Si—N six-membered ring which crystallizes in a twist conformation. 

The mechanism of formation of 121 becomes clear with the result of the X-ray analysis of 9. As mentioned earlier, two tautomers, R3SiNLiNH2 and R3SiNHLiNH, exist in the solid state here. LiF elimination from fluorofunctional units in this case leads to the formation of a five-mem-bered ring.15... 

Another possibility for the preparation of cyclodisilatetrazanes is salt elimination from lithiated N-silyl-iV -fluorosilylhydrazines. The LiF elimination from the lithium derivates of the asymmetrical bis(silyi)hydrazines 23, 24, and 28 leads to the formation of the compounds 125-12715 32 [Eq. (38)]. 

If the bulkiness of the silaethene to be produced is ftuther increased, the formation of dimers finally stops. Accordingly, LiF elimination of the silaethene source shown on the left-hand side in the second row of Scheme 6 leads to the silaethene Me2Si=C(SiMe3)(SiMefBu2). With the latter compound we were able for the first time to isolate a system with a normal Si=C bond and to solve its structure [15]. It stabilizes itself above room temperature by elimination of isobutene and not by dimerization. Obviously, the formation of dimers by [2+2] cycloaddition of two molecules of silenes is not the rule. 

Since the first isolation of stable 1-silaallene synthesized by West and coworkers in 1993, six stable examples of Si=C=C compounds have been reported. Si=C=C compounds were mainly synthesized by two methods, that is, the dehalogenative intramolecular cabometalation-elmination method (route A) and the addition of an organolithium reagent at the triply bonded carbon atom of fluoroalkynylsilanes followed by the LiF elimination (route B) (Scheme 43). 

In contrast with being a mere hazard, aryne formation by LiF elimination has been put to synthetic use in the otherwise very difficult preparation of fluorinated naphthalene derivatives [110, 111] (Scheme 2.48). 

However, the extent ofsolvent coordination (THF) around lithium sometimes governs the stability and reactivity of the lithium species (Scheme 3.6). Lithium species (18) undergoes benzyne formation to form either 3 -fluoro or 3-chlorobenzyne, depending on the concentration of THF in toluene. The less THF-coordinated lithium species (19) favors the additional interaction with fluorine and enhances LiF elimination to produce 3-chlorobenzyne (23), while the fully THF-coordinated lithium species (18) eliminates chloride as a better leaving group to form 3-fluorobenzyne (22) [15]. 

Cyclodisilazanes are obtained in the reactions of aminofluorosilanes with organolithiums via thermal LiF elimination of Li aminofluorosilanes - ... 

The silyl-bridged four-membered rings are not A, B observed in these reactions with aminofluorosilyl-substituted rings. These Li salts react by one route to triply substituted products they also form bicyclo[4.2.0]octanes by an intramolecular reaction with LiF elimination and silyl group and methanide ion migration . Figure 4 shows the structure of compound... 

Summary In contrast to organyl-substituted cyclodisilazanes, only a few halogeno substituted Si-N rings are known. Normally LiF elimination from lithium fluorosilylamides leads to the formation of four-membered Si-N rings. Using THF as solvent, a THF cleavage occurs and by-products are observed. Lithiated aminotrichlorosilanes react with THF under formation of new fourteen-membered heterocycles. An unknown Hit-electron system was isolated after thermal treatment of lithiated 2,6-diisopropylphenylfluorosilylamine. 

A simple way of synthesizing cyclotrisiloxanes is thermal LiF elimination from lithiated fluorosilanols (Eq. 3, for example)... 

Another route for the preparation of cyclotetrasiloxanes is thermal LiF elimination from the lithium salts of l-hydroxy-3-fluorodisiloxanes (Eq. 8) ° ... 

Bulky substituents lead to the formation of small-ring compounds. For example, LiF elimination from l-lithium-amido-3-fluoro-l,3-disiloxanes leads to four-membered (SiNSiO)-ring systems (Eq. 13). 

Lithiated mono(siIyl)hydrazones can be used as precursors for the synthesis of fluoro functional bis(silyl)hydiazones. Lithium derivatives of these bis(silyl)hydrazones react spontaneously via LiF elimination to give l,2-diaza-3-sila-5-cyclopentenenes. Further lithiation with ter/-butyllithium leads to the formation of the tricyclic compound 10 (Eq. 8 and Fig. 4) [9]. 

Another unexpected reaction has been observed in the case of the LiF elimination of lithiated N,N -bis(di-ferf-butylmethylsilyl)-iV-difluorosilylhydrazines. The l,2-diaza-3-silacyclopropane is transformd into a l,2-diaza-3,5-disilacyclopentane via silylmethylene group insertion and subsequent protonation of one of the nitrogen atoms (Scheme 11). 

By alternating lithiation and substitution involving LiF elimination, siloxane units up to... 

X=H,SiMc3), and a range of silylated anions Lin(Me3Si)3 nP7 made and alkylated S. xhe arsasilene Is2Si=AsSiPr-i3 results from LiF elimination and adds Te and Ph2CX)359. 

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