Lithium organyls

Owing to their stability and low nucleophilicity, metal acetylides are less reactive toward Cjq than other lithium organyls or Grignard reagents [11]. Though the reaction is slower and higher reaction temperatures are necessary, various acetylene derivatives of Cjq could be obtained. The first acetylene Cjq hybrids were (trimethyl-silyl)ethynyl- and phenylethynyl-dihydro[60]fullerene, synthesized simultaneously... 

Two-dimensional 6Li/Y correlation spectra of organolithium reagents have been studied for some time and although in previous reports a variety of elements such as 31P, 29Si, and 7Li11,82 have been employed as the heteronucleus Y, recent studies focused essentially on the application of 6Li/13C and 6Li/15N correlations for the structural investigation of lithium amides and lithium organyls, respectively. Methods employed include... 

Addition reactions of lithium organyls have been investigated for 9763-201. The reaction is not a nucleophilic substitution of Br by R in 97-LiBr, but rather a two-step mechanism with an initial dissociation via 97 (equation 124). This was proven by the addition of the very efficient silene trap t-Bu2MeSiN3, that competes with the lithium organyl for the silene. 

The reaction of SiF-coupled six-membered rings with lithium organyls first produces ring contraction and subsequently eliminates LiF. As discussed in Section II, we have now obtained an intermediate ylide that contains silicon atoms with the coordination numbers three and five. This gives rise to a nucleophilic migration of a methanide ion. A fused bicyclic compound is formed (Scheme 24). This compound has also been characterized by an X-ray analysis and the structure is shown in Fig. 13. Both the four-membered rings are planar.42,46,50... 

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. 

With regard to the rate of insertion of the standard silaethene into the CLi bond of lithium organyls, the following can be learnt from Fig. 1 ... 

For steric reasons the reactivity of lithium organyl decreases with increasing bulkiness of the latter, that is in the direction methyl, butyl, phenyl, /butyl, trisyl lithium. 

The source Me2SiBr-CLi(SiMe3)2 of standard silaethene reacts ca. 100 times faster with the standard silaethene than analogously crowded trisyl lithium Me3Si-CLi(SiMc3)2. The reason for this electronic acceleration appears to be different structures of the lithium organyls. 

Recently, we were able to show that lithium organyls are also able to interact with CO [6] in a newly developed LXe cell constructed from one piece of single crystal silicon [7]. In a first step carbon monoxide is complexed by back-bonding to BuLi (n(CO) 2047 cm ) and inserts in a second step at higher temperature into the lithium-carbon bond (n(CO) 1635 cm ). Further... 

Besides protons a series of heteroligands in the a-position of phosphonium ylides can also be substituted, giving rise to the formation of new alkylidenephosphoranes. Halogen atoms have been substituted by carbon groups (with lithium organyls or acyl chlorides) or another halogen. Reaction of a-lithiated ylides (see equation 35) or ylide anions with electrophiles may be considered as substitution of an alkali metal substituent at the ylide carbon atom. 

TTie most used heteronuclear X,Y correlation technique for the characterization of lithium organyls seems to be ID- and 2D- li, C HMQC experiments which rely on V(C,Ii) couplings and allow the detection of directly connected Li, C spin pairs. TTie two-dimensional version is particularly useful for the characterization of compounds containing several distinguishable different Li sites and allowed to prove for the first time the existence of a dilithiated carbon atom carrying two different lithium atoms. ... 

A different approach to locate the metal in a lithium organyl, which exploits dipolar rather than scalar interactions, was presented by the group of Berger who appUed both 2D- C, Li HOESY and lD- C Li difference spectroscopy to measure C, Li heteronuclear Overhauser effects and demonstrated that the resulting data can be used for C-Li distance calculation. Ihe major drawback of this technique is that the use of doubly [ Li, C]-labelled samples is mandatory to ensure reliable measurement of very small NOE effects of some 1%. 

Scheme 2.193 Homologation of fluorinated polyenes by nucleophilic replacement of fluorine by lithium organyls [7, 19].

The lithium organyl LiMe does not add to trimethylsilylmethyldiazene, Me3Si—N=N—Me (29), but acts as a catalyst for the isomerization of this diazene to hydrazone 11 (for the mechanism, cf. Section IV,B) as well as for the decomposition to tetramethylsilane and nitrogen [for mechanism, cf. Eq. (117b,c)]. With an increasing amount of LiMe, the rate of catalytic... 

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