Formation of Cyclic Carbosilanes Through Hydrosilylation

The synthesis of cyclic carbosilanes and related compounds through Si—H addition has already been reviewed [60]. Si—H addition catalysed by H2PtCl5 can be used in cyclization, as shown by the following example [71]  

The catalytic Si—H addition of 1,3,5-trisilacyclohexanes to monosilylethynes discussed in this section occurs in polycyclic carbosilane synthesis. The scheme below illustrates the principles of synthesis, whereby only the 1,1-disilylolefme is shown as product, since the 1,2-disilylolefine does not lead to ring formation. 

While only 1,2-disilaalkanes emerge from the catalytic addition of Si—H groups to Si-substituted C=C double bonds [72], 1,1- as well as 1,2-disubstituted products arise from the analogous addition to alkynes [73]. The corresponding reaction of 1,1,3,3,5-pentamethyl-l,3,5-trisilacyclohexane with HC — C—SiMe2—CH2CI proceeds without difficulty to 

The emerging addition product is not applicable to further ring closure, because no Si-functional groups exist in the compound. Reactions of derivatives containing several Si—H groups, for example (MeHSi— 112)3 HC = C— 

If the reaction mixture is run over an AI2O3 chromatographic column, H2PtCl6 is sufficiently removed [74], and formation of highly viscous polymeric reaction products is prevented. Yet, even under these conditions and with a mole ratio of mono-silylethyne to l,3,5-trimethyl-l,3,5-trisilacyclohexane of 1 1, it was not possible to prevent completely the formation of bis- and trissubstituted 1,3,5-trisilacyclohexanes [76]. 

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