Alkyl-Aryl Silicones

It has been pointed out that the alkyl silicones of high R/Si ratio are oily liquids or weak gels when polymerized by heat alone, and that in order to get resinous and adhesive properties the R/Si ratio must be brought down to about 1.5 for a methyl silicone and about 1.0 for a butyl or benzyl silicone. 

At the same time, the aryl silicones of high R/Si ratio are brittle vitreous soluble fusible solids, which actually are supercooled liquids. By themselves they are too brittle and weak for coating or impregnation. 

Fortunately, it is found that some alkyl-aryl copolymers have mechanical strength and toughness exceeding those of the pure alkyl or aryl silicones, so that it becomes possible to improve upon the pure types in this respect. Such improvement in strength is obtained without sacrifice of other desirable properties, and therefore the alkyl-aryl copolymers often are more useful wherever strength and toughness are important. 

Ethyl phenyl silicone is another alkyl-aryl silicone which may be made either from ethylphenyldichlorosilane41 or by cocondensation of mixed ethyl and phenyl chlorosilanes. The cross-linked ethyl phenyl silicone resins have good dielectric and mechanical properties, but their maximum service temperatures in air are somewhat lower than those for methyl phenyl silicone, being limited to about 250° C. for 

Many other alkyl-aryl silicones are possible, and some may prove to have mechanical properties superior to those of the two copolymers which have been described. What is needed is a theory of structure of such copolymers which will explain the development of mechanical properties not present in the alkyl or aryl polysiloxanes separately and which thereby will serve as a guide to the formulation of more useful compositions. 

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It is necessary that the discussion be confined to those organosilicon products which, on the basis of available information, show the greatest promise of widespread use. This would seem to mean the methyl, ethyl, and various alkyl-aryl silicone resins, methyl silicone oils and elastomers, and the methylchlorosilanes for water-repellent films. 

The dicyclopentadienyl metal compounds undergo Friedel-Crafts alkylation and acylation, sulfonation, metalation, arylation, and formyla-tion in the case of ferrocene, dicyclopentadienyl ruthenium, and dicyclopentadienyl osmium, whereas the others are unstable to such reactions ( ). Competition experiments (128) gave the order of electrophilic reactivity as ferrocene > ruthenocene > osmocene and the reverse for nucleophilic substitution of the first two by n-butyl lithium. A similar rate sequence applies to the acid-catalysed cleavage of the cyclopentadienyl silicon bonds in trimethylsilylferrocene and related compounds (129), a process known to occur by electrophilic substitution for aryl-silicon bonds (130). 

Asymmetric reduction of alkyl aryl ketones with trialkoxysilanes is promoted by a catalytic amount of chiral nucleophiles [39]. The reactive species is a transiently prepared hypervalent silicon hydride. 2, 4, 6 -Trimethylacetophenone was reduced with equimolecular amounts of trimethoxysilane in the presence of the monolithio salt of (R)-BINAPHTHOL (substrate Li=20 l) in a 30 1 ether-TMEDA mixed solvent at 0 °C to afford the R product in 90% ee (Scheme 21) [40]. The presence of TMEDA was crucial to achieve high yield and enantiose-lectivity. Reduction of less hindered ketonic substrates preferentially gave the... 

The product types vary as a function of the degree and type of alkyl/aryl substitution at silicon. Thus Ph2t-BuSi—SiPh2(t-Bu) only yields silyl radicals and no Si=C species when irradiated139. 

Since an insertion reaction into alkyl or aryl silicon bonds seems to be difficult, a radical mechanism usually takes place in the pyrolysis of such compounds. On the other hand, at Si—H, Si—Hal, Si—OR and Si—N bonds, a silylene mechanism occurs because the insertion reaction has only a small activation energy. 

The C-M a bonds (where M = Si, Ge, Sn, Pb) in the fragment C-MR3 (R = alkyl, aryl) are powerful a donor orbitals the electrons in the C-M bond are weakly held and easily ionized, and, importantly, the C-M bonding electrons are polarized towards the carbon. As a result, the presence of group 4 metal substituents in organic compounds has long been recognized to have very profound effects on both the ground-state properties and reactivities of the compounds.10-13 The most well known of these effects is the silicon (3-effect,10 12 which is the... 

Vinyltrichlorosilane is used as raw stock for various silicone products. For example, hydrolytic cocondensation of alkyl(aryl)chlorosilanes with vinyltrichlorosilane and subsequent polymerisation in the presence of benzoyl or kumyl peroxides gives thermostable polymers. Besides, vinyltrichlorosilane is used for the surface treatment (dressing) of glass cloth and other glass products. 

Ethers of orthosilicon acid and their derivatives, tetraalkoxy(aroxy)silanes and alkyl(aryl)alkoxy(aroxy)silanes are a rather extensive class of silicone compounds. They are independently applied in various spheres of technology, but are particularly valuable as semi-products for preparing important silicone oligomers and polymers. 

The substituted ethers of orthosilicon acid, alkyl(aryl)alkoxysilanes or al-kyl(aryl)aroxysilanes, have the common formula RnSi(OR )4. where n = 1-K3, whereas R and R are any organic radicals the radicals situated at the silicon atom and in the ether group can be the same or different. 

Similarly to triacetoxymethylsilane, acetylation of chlorosilanes with metal acetates can also yield tetraacetoxysilane and other al-kyl(aryl)acetoxysilanes. The physicochemical properties of important acy-loxyorganosilanes are given in Table 13. The practical value of acetoxysi-lanes is that their hydrolysis, unlike the hydrolysis of organochlorosilanes, forms weak acetic acid, rather than hydrogen chloride. That is why acetox-ysilanes can be used to waterproof various materials (textiles, paper, etc.). Alkyl(aryl)acetoxysilanes are also used to obtain some silicone varnishes and as hardeners for low-molecular silicone elastomers. 

Finally, we shall briefly discuss the compounds of the silicon subgroup elements in their divalent state (germylenes, stannylenes and plumbylenes) R2M (R = alkyl, aryl). The chemical bonds in R2M are formed by the px and py orbitals of M. The pz orbital is unoccupied and there is an unshared electron pair in an s-orbital of M. Therefore, the R2M compounds have both electrophilic and nucleophilic properties. The valence angle R—M—R diminishes as the atomic number of M increases. The distances of M—R bonds in R2M are less than in the corresponding tetracoordinate R4M derivatives87. The spatial structure of R2M compounds, the shielding of the reaction centre (M ) as well as the possibility for R2M molecules to transit from a singlet state into a triplet state depend on the steric bulk of the R substituents. If the R substituents are bulky, some R2M ... 

Other aryl silicones have been prepared in considerable variety, and they all resemble phenyl silicone rather than the higher alkyl... 

An in stereoisomer of fluorosilaphane 260 was obtained as depicted in Scheme 44 in overall 0.4% yield. First, tri(o-tolyl)fluorosilane 258 was fluorinated at silicon with AgF and then brominated with NBS to give tris[2-(bromomethyl)phenyl]fluorosilane 259. Condensation of 259 with l,3,5-tris(mercaptomethyl)benzene yielded the final product 260 <1998JA6421, 1999JOC5626>. The inside location of the fluorine atom was established by X-ray crystallographic analysis. The Si-F bond distance is very short (1.591 A) compared to the mean Si-F distance for all tetracoordinate tris[(alkyl(aryl)]fluorosilanes found in the Cambridge Structural Database. The 19F NMR resonance for 260 appears at a = 5.3 ppm, that is, 155 ppm upfield from that of tri(o-tolyl)fluorosilane 258. 

Carbodiimides are a unique class of reactive organic compounds having the heterocumu-lene structure R—N=C=N—R. They can be formally considered to be the diimides of carbon dioxide or the anhydrides of 1,3-substituted ureas, and they are closely related to the monoimides of carbon dioxide, the isocyanates. The substituent R can be alkyl, aryl, acyl, aroyl, imidoyl or sulfonyl, but nitrogen, silicon, phosphorous and metal substituted carbodiimides are also known. The unsubstituted carbodiimide HN=C=NH is isomeric with cyanamide, H2NCN. Mono substituted carbodiimides, generated in the thermolysis of 1-substituted tetrazoles, can be isolated at liquid nitrogen temperature but isomerize to the cyanamides at higher temperatures. ... 

This chapter concentrates on those processes in which oxidative cleavage of a carbon-silicon bond results in production of the alkyl/aryl fragment as an alcohol/phenol. Other cleavage processes are dealt with, but more briefly. 

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