Fluorinated Groups in Silanes and Silicones

There are two possible ways to introduce fluorinated groups into silanes and silicones through the use of fluorinated organometallic reactants (mainly orga-nomagnesians and organolithians) or by hydrosilylation of fluorinated alkenes. 

The reactivity series ofRF depends on the nature ofX and is I Br Cl. In addition, the longer the RF group and the higher the steric hindrance, the lower the yield of l.23 Further, the more hindered RF, the more dimer produced. In order to prepare tetrafluoroalkyl silanes, such fluorinated magnesians must react with alkyl silane halides as follows  

The reactivity of these fluorinated magnesian reactants depends on (a) the chain length and the steric hindrance of the RF group (b) the nature of the silane  

HSiCl3 SiCl4 JsiCl2 =SiCl and (c) the nature of the halogen  

SiF SiCl SiOR. However, it is not possible to synthesize tetrafluoroalkyl silanes as the maximum number of substitutions when X = Cl or F is three. 

---

Previously, trifluorosilyl groups have been bound to phosphorus (40) and silicon via the SiF (g), fluorine-bond insertion-mechanism (41). The new compound HgCSiFs) is readily hydrolyzed, but it can be stored for long periods of time in an inert atmosphere. It is a volatile, white solid that is stable up to at least 80°C. The preparation of bis(trifluoro-silyDmercury, of course, raises the possibility of (a) synthesis of the complete series of trifluorosilyl, "silametallic compounds, as had previously been done for bis(trifluoromethyl)mercury by using conventional syntheses, and (b) transfer reactions similar to those in Section II, as well as (c) further exploration of the metal-vapor approach. The compound Hg(SiF.,)j appears also to be a convenient source of difluoro-silane upon thermal decomposition, analogous to bis(trifluoromethyl)-mercury ... 

However, with aryl(trialkyl)silanes, sometimes for electronic reasons, the reaction does not take place. In order to reduce the electron density on the silicon atom, alkyl groups were replaced by fluorine. The fluorine effect could be explained as follows i) the van der Waals radius of fluorine is roughly comparable to that of hydrogen and hence the fluorine-substituted silyl group is not so bulky than the trimethylsilyl one ii) electronegativity of fluorine favours the formation of the reactive pentacoordinated silicate, enhances the Lewis acidity of the silicate and... 

---