Silacyclopentadienide anions

The reaction of 1-chlorosiloles with alkali metals leads to unstable species that react with methyl iodide, ethyl bromide or chlorotrimethylsilane in the manner expected for the 1-silacyclopentadienide anion. Interestingly, X-ray quality crystals were isolated from the reaction with Li in THF and the molecular structure was shown to be that of a [2 + 2] head-to-tail dimer, which is formed by the 1,5-rearrangement of the anion in the silole ring185 (equation 70). 

Treatment of hydrosilanes with potassium hydride is a known route to silyl anions and has been used to generate silacyclopentadienide anions. Reinvestigation of the reaction of KH with C4Ph4SiMeH has shown that the dominant pathway is simple addition of the hydride anion to the silicon centre to give the pentavalent silicate [Ph4C4SiMeH2] K+ (129)189 (equation 75). The Si—H coupling constants in 129 (Jsi-H = 192.5 and... 

In contrast to triorganosilyl anions, functionalized silyl anions have been studied less extensively. Functional groups reported so far are hydrogen, chlorine, amino, alkoxy, and carbonyl groups. In addition to these species, silacyclopentadienide anions are also described in this section. Theoretical studies on the effect of functional groups are described in Section... 

Studies on silacyclopentadienide anions have been developing in recent years (67). The term silole means the same thing as silacyclopentadiene. 

An energy difference for the isodesmic reaction [Eq. (28)] indicates a quantitative measure of the extent of electron delocalization (6-31G ) (62). The energy difference for the C, structure of the silacyclopentadienide anion is only 2.2 kcal/mol, which is much smaller than 73.4 kcal/mol for the carbon analog, a planar cyclopentadienide anion (C2 ). On the basis of these calculations, it is concluded that the ground-state structure of the silacyclopentadienide anion has only ca. 3% of the resonance stabilization exhibited by the cyclopentadienide anion. However, the previous calculation (3-21G//STO-2G) on the 2 planar structure shows that the resonance energy is 23 kcal/mol, which means ca. 25% as aromatic as the carbon analog (65). 

A silaaromatic anion analogous to the fluorenide anion has been prepared recently (equation 146) and found to be stable in solution313. It was characterized by NMR and UV spectroscopy and by trapping reactions with electrophiles. An attempt to prepare a monocyclic derivative of the parent silacyclopentadienide anion in an analogous fashion was not successful313. 

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