Oligosilanes—

Reduction of the Si-Si bond of oligosilanes with organo- or silyllithium reagents (Scheme 2.44, A,... 

Linear Tetrasilanes with Internal Substituents Oligosilanes with Optical Activity... 

Recently a number of linear and branched oligosilane derivatives have been synthesized from easily accessible precursors using mainly organometallic routes [1,2]. Now we are able to report on the first successful synthesis of tetrasilanes of the general formula H3SiSiHXSiHXSiH3 (X = Ph, Cl, Br) bearing internal substituents. 

We think two mechanisms take place at the same ime. Besides the P -elimination mechanism, which has been described above as a kind of disproportionation, an a-elimination like the Harrod mechanism seems also to take place. From the experimental results, the polymerization of trisilanes or tetrasilanes yields preferably the dimeric species, hexa- or octa-silanes respectively. In addition, oligosilanes with odd numbers of silicon atoms were formed. We do not yet understand why these hexa-and octasilanes were formed in the iso-forms. 

Another important aspect is the very simple preparation of the silyltriflates. Systematic investigations of the cleavage of the silicon element bond (Si-E) by CF3SO3H have shown that the reaction rate decreases in the sequence (E=) a-naphthyl > phenyl > Cl > H > alkyl [3]. Therefore especially pure silyltriflates result from protodesilylation of arylsilanes with CF3SO3H. On the basis of these general results the synthesis of a large number of variously substituted silyltriflates [4,5] can be planned. This is of particular interest in the chemistry of oligosilanes. 

Numerous synthetic strategies for "preceramics" based on polysilanes are known, but the synthesis of Si-Si chains with different substituents on the silicon skeleton is difficult. In the following the advantages of the triflate method for the synthesis of special substituted oligosilanes are shown by some examples. 

Insertion of isocyanides into Si-Si bonds of oligosilanes has been examined. For instance, 2,6-xylyl isocyanide inserts into every Si-Si bond of decamethyltetrasilane in the presence of palladium(n) acetate, whereas the use of a more bulky 2,6-diisopropylphenyl isocyanide leaves the central Si-Si bond intact (Scheme 30).133,134... 

---