Lithiation by MeLi of three families of branched polysilanes (Me(3)Si)
(3)Si-X {(1) X = SiMe(2)H, t-BuMe(2)Si, SiMe(2)SiMe(3), t-BuSiMe(2)SiM
e(2); (2) X = SiMe(SiMe(3))(2), Si(SiMe(3))(3), SiMe(2)Si(SiMe(3))(3),
SiMeHSi(SiMe(3))(3); (3) X = Si(SiMe(3))(2)Cl, Si[Si(SiMe(3))(3)]MeCl
, Si[Si(SiMe(3))(3)]HCl} yields, depending on X, one of the following:
(a) novel branched polysilyllithium compounds; (b) the known (Me(3)Si
)(3)SiLi; (c) various mixtures. The cleavage of family 1 can be unders
tood in terms of steric effects, i.e., assuming that MeLi attacks pref
erentially the sterically less hindered Si atom leading to the formati
on of new polysilyllithium compounds, e.g., t-BuMe(2)Si(Me(3)Si)(2)SiL
i . 3THF and t-BuMe(2)SiSiMe(2)(Me(3)Si)(2)SiLi . 3THF, which were iso
lated in a crystalline form. The X-ray structure of the former is pres
ented. In contrast to 1 the lithiation of the branched polysilanes 2 l
eads exclusively (mostly with (Me(3)Si)(3)SiSiMe(SiMe(3))(2)) to the k
nown (Me(3)Si)(3)SiLi. Lithiation of 3 leads to a number of novel poly
silyllithium compounds. We tentatively propose that the lithiation of
3 proceeds by a two step reaction in which a disilene is formed as an
intermediate, and this is followed by fast addition of a second molecu
le of MeLi to produce the final silyllithium product.