A new method for the synthesis of branched polysilane anions

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)SiMe(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 understood in terms of steric effects, i.e., assuming that MeLi attacks preferentially the sterically less hindered Si atom leading to the formation of new polysilyllithium compounds, e.g., t-BuMe(2)Si(Me(3)Si)(2)SiLi . 3THF and t-BuMe(2)SiSiMe(2)(Me(3)Si)(2)SiLi . 3THF, which were isolated in a crystalline form. The X-ray structure of the former is presented. In contrast to 1 the lithiation of the branched polysilanes 2 leads exclusively (mostly with (Me(3)Si)(3)SiSiMe(SiMe(3))(2)) to the known (Me(3)Si)(3)SiLi. Lithiation of 3 leads to a number of novel polysilyllithium 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 molecule of MeLi to produce the final silyllithium product.