Aryl silanes, fluorination

The SiH2 group. There is a small further fall in the SiH stretching frequency when a second alkyl group is introduced. Dialkyl silanes absorb between 2138—2117 cm", and diaryl silanes between 2147—2130 cm" [17, 25, 29, 32, 33]. The frequency rises sharply if halogens are substituted for the alkyl groups. Difluoro-silane absorbs at 2246 cm" and methyl chlorosilane [17] at 2200 cm". The deformation frequency is less affected by the substituents, and in alkyl or aryl silanes it falls in the 940—928 cm" range close to the SiHs deformations. Replacement of one methyl by fluorine raises this to 975 cm". Methyl chlorosilane absorbs at 960 cm". ... 

Nucleophilic fluorine-aryl substitution in iodine pentafluoride with tetrakis(pentafluoro-phcnyl)silane or tris(pentafluorophenyl)bismuthane produces high-purity (pentafluorophen-yl)iodine tetrafluoride in good yield.133,134... 

Although trimethyl(vinyl)silane undergoes the coupling reaction with aryl halides in the presence of TASF and a palladium catalyst as described in Section 1 (Eq. 3), those having an aliphatic substituent on vinyl fail to couple with aryl iodides under similar conditions, probably because they are not capable of affording pentacoordinate silicates efficiently owing to the electron-donating nature of the substituent. To assist the formation of the pentacoordinated intermediates, the methyl on silicon was replaced by fluorine [13]. The cou-... 

Palladium-catalyzed cross couplings of organofluorosilanes with vinyl [37] and aryl [38] halides and triflates are known, where the activation of the Si—C bond by fluoride ion plays an essential role in the coupling. The pentacoordinated fluoroorganic silanes promote the transmetallation to the aryl halide-palladium complexes strongly. At least one fluorine... 

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... 

As indicated with a strong electron-withdrawing group such as CF3, the reaction gave lower yields of products. When the coupling reaction of aryl(aIkyl)difluoro-silanes and aryl iodides is carried out under an atmospheric pressure of carbon monoxide, unsymmetrical fluorinated diaryl ketones were obtained (refs. 35, 36) ... 

Some halides on silicon are activating groups. Generally two fluorine atoms are required for aryl-aryl coupling. For example, coupling of ethyl(2-thienyl)difluoro-silane (9) with 3-iodothiophene 10 afforded the bisthiophene 11 using a ligandless Pd catalyst in the presence of KF [87]. 

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