Silanes fluoromethyl

Fluoromethyl)phosphonate ester anions react with CO2 or COS to yield esters of the type (161 Y=0 or S) which may be dealkylated to the free acid through the use of bromotrimethyl-silane, but the course of the reaction with CS2 depends on the individual ester thus, (162) yields (163), but (165) is formed from (164). 22... 

Multinuclear NMR data for homologous senes of fluoromethylated malo-nates [72] and trimethylsilanes [97] are compiled in Table 11. In both senes, fluoromethyl attachment is to a quaternary site These compounds are readily synthesized using fluorohalomethanes to incorporate the final fluoromethyl moiety All the malonates, except diethyl methyltnfluoromethylmalonate (4) [93], are isopropyl-substituted diethyl esters [72]. The silane data, with the exception of trimethyltrifluoromethylsilane (5) [95], are from reference 97 Chemical shift data are very comparable, with the malonates having higher proton and fluorine chemical shifts but slightly lower carbon values. The magnitudes of cf and 2J jp coupling are similar for both sets of compounds. 

Similarly, a primary — CH2F fluorine adjacent to silicon is shielded by about 50 ppm compared to the respective hydrocarbon, with similar tin or germanium compounds being shielded slightly less. The value of -277 ppm observed for tetrakis(fluoromethyl)silane has the largest chemical shift known for a single carbon-bound fluorine (Scheme 3.40). 

Bell and co-workers 24, 25) have investigated the generation of tri-fluoromethyl radicals from photolysis of HFA in the presence of silanes. Abstraction of the proton is observed in the case of trichlorosilane (24), while methyl(fluoro)silanes lead to the formation of CFjH, CjF, and CF CH 2(25). 

Phosphane 9 is a useful intermediate which can be converted easily into trifluoromethyl anion equivalents, e. g. trimclhyl(trifluoromethyl)silane. Similarly, 1,2-bis(dichlorophosphi-iio)cthanc (10) and bromotrifluoromclhane react in the presence of 8 to form l,2-bis[bis(tri-fluoromethyl)phosphino]ethane (11). ... 

There are several structural investigations of (fluoromethyl)silanes by microwave, electron diffraction and single crystal X-Ray diffraction methods [16-22]. Some results are shown in Table 1. 

While most of the mentioned (fluoromethyl)silanes are stable compounds at ambient temperature, the partially Si-chlorinated silanes F2HCSiH2Cl and FClHCSiH Cl3. , n = 1, 2, are dangerously shock-sensitive. In line with intermolecular interaction in the liquid phase of these compounds is a large phase... 

In order to get more insight into the factors that determine thermal stability, which is crucial for the safe manipulation of (fluoromethyl)silanes, we have investigated the conventional thermal [24] and laser-induced decomposition [25, 26] of the parent silanes F H3 CSiH3. These compounds are stable gases at ambient temperature, and they may be conveniently studied by IR and multinuclear NMR spectroscopy. Therefore these appear to be particularly suited for gas phase investigations. 

The decrease of thermal stability of (fluoromethyl)silanes in comparison to the non-fluorinated methylsilanes is evident from their decomposition temperature under static conditions. This drops significantly in the series H3CSiH3 (> 340 °C), F3CSiH3, and F2HCSiH3 (> 180 °C) to FH2CSiH3... 

C). While H3CS1H3 decomposes under these conditions primarily by geminal elimination of molecular H2 from the silicon moiety and secondarily by a slow surface-catalysed reaction yielding H-and CH3SiH2 radicals [27], the significantly lower decomposition temperatures of (fluoromethyl)silanes require different low energy decomposition pathways. 

The CO2 laser-induced multiphoton decomposition of silanes, known to be a really homogeneous reaction, was utilized for the chemical vapour deposition of fluorine containing SiC films from the parent (fluoromethyl)silanes [25, 26]. In contrast to work with H3CS1H3, irradiation of (fluoromethyl)silanes with a single unfocused CO2 laser pulse at fluence of S 0.9 Jcm" tuned to absorption bands of either the SiH bending or the CF stretching vibrations results in an explosive reaction. This is accompanied by an intense chemoluminescence when the sample pressure exceeds a certain limit in the range of 0.1-6.7 kPa (Fig. 1). 

Fig. 1. Explosion limit for the IR-multiphoton decomposition of (fluoromethyl)silanes, irradiating wavelengths in the given order are 916.6, 1025.3, 927.0, and 922.9 cm empty and fall circles relate to explosive and no reaction, respectively (reprinted from reference [25])...

The explosion limit dropped to lower laser fluence upon increasing the pressure of the (fluoromethyl)silanes. Below the explosion limit, chemical changes of the (fluoromethyl)silanes could only be detected after irradiation with as many as 10 laser pulses. The reaction products revealed an almost quantitative reduction of the CF bond and afforded the gaseous fluorosilanes Sip4 and Sip3H, the hydrocarbons CH4 and C2H2, and different solid Si/C/F/H materials. 

It was supposed that, because of a rapid intramolecular energy pooling, infrared laser multiphoton decomposition at low laser fluence usually proceeds via the energetically most favourable reaction channel [26], From the material balance of gaseous decomposition products (Fig. 2) it was inferred that there are common primary steps for both the thermal and the explosive laser-induced decomposition of (fluoromethyl)silanes. 

The different decomposition pathways of the various (fluoromethyl)silanes do not only determine the composition of the gas phase products but also the composition and the texture of the deposited Si/C/H/F materials. They were shown to consist of carbon and a blend of silicon-carbon based frameworks containing some Si-H and C-H bonds by XPS, SEM, IR, and Raman spectroscopy [26]. 

Trimethyl(perfluorophenyl)silane undergoes the same characteristic reactions as trimethylftri-fluoromethyl)silane. Thus, perfluorophenyl groups can be introduced at multiple bonds (C = C, C = N, etc.) with active halogen atoms, e.g. formation of 17 and 18.63... 

Analogously, the gas-phase thermal decomposition of trifluoro(l,l,2,2-tetrafluoroethyl)silane (at 140-200 C) or trimethyl(l,l,2,2-tetrafluoroethyI)silane (at 300-370°C) generated di-fluoromethyl(fluoro)carbene which underwent addition to alkenes to give 1-difluoromethyl-l-fluorocyclopropane 8. ° (Z)- and ( )-But-2-ene were cyclopropanated stereospecifically, and allylic C-H insertion was not observed. With dimethyl(vinyl)silane or allyl(dimeth-yl)silane, however, the carbene underwent both double bond addition and Si-H insertion. ... 

The cleavage of silane into alkylfluorocarbene requires temperatures of at least 140°C. A competitive reaction is the rearrangement of carbene to fluoroalkene. 1-Fluoro-l-fluoromethyl-2,2-dimethylcyclopropane was prepared from trifluoro(l,l,2-trifluoroethyl)silane and 2-methyl-propene see Houben-Weyl, Vol. E19b, pp 1466-1467. Similarly, fluoro(methyl)carbene was generated and underwent addition to 2-methylpropene to give 2. ... 

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