Silanes, fluorination

Table XII. Effect of Organic Additives (Urea, Silanes, Fluorinated Alkyl esters) on Grafting of Styrene to Polypropylene Initiated by UVa...

Perfluoropyridine is often subjected to synthetic studies in comparison with perfluorotoluene. Both substrates react very well with replacement of the fluorine in position 4, but yields from perfluoropyridine are generally higher. Car-banions, enamines, and silanes react readily with perfluoropyridine [101, 102, 103] (equation 52). 

Table 1 shows the kinetic data available for the (TMSjsSiH, which was chosen because the majority of radical reactions using silanes in organic synthesis deal with this particular silane (see Sections III and IV). Furthermore, the monohydride terminal surface of H-Si(lll) resembles (TMSjsSiH and shows similar reactivity for the organic modification of silicon surfaces (see Section V). Rate constants for the reaction of primary, secondary, and tertiary alkyl radicals with (TMSIsSiH are very similar in the range of temperatures that are useful for chemical transformations in the liquid phase. This is due to compensation of entropic and enthalpic effects through this series of alkyl radicals. Phenyl and fluorinated alkyl radicals show rate constants two to three orders of magnitude... 

The reaction of tetramethylsilane with fluorine led to the isolation of several, partially fluorine-substituted tetramethylsilanes (see Tables VII-IX), and preservation of over 80% of the silicon-carbon bonds in the initial, tetramethylsilane reactant. The stability of many of the partially fluorinated germanes and silanes (some are stable to over 100°C) is very surprising, for the possibility of elimination of hydrogen fluoride is obvious. Indeed, before the first reported synthesis (12) of... 

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 ... 

Silicon, like carbon, is relatively inactive at ordinary temperatures. But, when heated, it reacts vigorously with the halogens (fluorine, chlorine, bromine, cmd iodine) to form halides and with certain metals to form silicides. It is unaffected by all acids except hydrofluoric. At red heat, silicon is attacked by water vapor or by oxygen, forming a surface layer of silicon dioxide. When silicon and carbon are combined at electric furnace temperatures of 2,000 to 2,600 °C (3,600 to 4700 °F), they form silicon carbide (Carborundum = SiC), which is an Importeint abrasive. When reacted with hydrogen, silicon forms a series of hydrides, the silanes. Silicon also forms a series of organic silicon compounds called silicones, when reacted with various organic compounds. 

Whether bound directly to the silicon or on a carbon bound to the silicon, a fluorine substituent within a silane is highly shielded compared to that in a hydrocarbon. For example, the fluorine of TMS fluoride absorbs more than 25-ppm upfield from that in t-butyl fluoride (Scheme 3.28). (For additional data on Si—F compounds, see Chapter 7, which deals with compounds that have heteroatom-fluorine bonds.)... 

Likewise, a primary —CH2F fluorine adjacent to silicon is shielded by more than 50 ppm compared to the respective hydrocarbon (Scheme 3.29), with the value of -277ppm observed for fluoromethyltrimethyl-silane being the largest chemical shift known for a single carbon-bound fluorine. 

As was the case with silanes bearing a CH2F group, the fluorines of those bearing a CF2H group are also considerably shielded by the attached Si substituent (Scheme 4.30). 

Fluorine chemical shift data are also given in Scheme 4.31 for silanes bearing a CF2-halogen group. 

In conclusion, there is a strong fluorine response in the XPS data set, which indicates the presence of a mold release. The binding energy of the silicon best matches that for a silicone, not silicon dioxide. Analysis of the actual filler (silica) used in the epoxy could eliminate it as a possibility since silane-based surface treatments are common. 

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 ... 

Such fluorinated organolithians have also been shown to be precursors of tetraalkyl silanes ... 

However, the reactivity of such compounds is still low e.g., when R, = R2 = R3 = CH and X = Cl, the silanes produced from CF3C2H4Li and C6F13C2H4Li were obtained in 40 and 20% yields, respectively. As a result of this poor reactivity, this method was abandoned and efforts now center around hydrosilylation reactions involving fluorinated olefins. 

Various hydrosilylation reactions of hydrogenosilanes were carried out in the presence of Speirs catalyst for the preparation of fluorinated silanes ... 

Similarly, fluorinated nonconjugated dienes could undergo hydrosilylation with hydrogenochloromethyl silanes ... 

Diisobutylaluminum hydride Dimethyldichlorosilane Diphenyldichlorosilane Dipropylaluminum hydride Ethylaluminum dichloride Ethylaluminum sesquichloride Ethyldichloro silane Ethyltrichlorosilane Fluorine... 

---