Aryl Silicones

Another striking conversion in the phenyl series of siloxanes is that which produces the cyclic tetramer. Diphenylsilanediol is dissolved in boiling alcohol, and a few drops of aqueous caustic are added. Upon cooling, the distinctive crystals of octaphenyleyclotetrasiloxane separate.37 Apparently strong bases bring about the condensation of diol to tetramer almost exclusively, whereas strong acids favor the formation of almost all trimer. Since both condensation reactions proceed at very moderate temperatures it cannot be said that the molecular form assumed by the product is determined by the temperature. 

Octaphenyleyclotetrasiloxane is one of the products isolated by Kipping38 from the mixed hydrolysis products obtained by boiling diphenyldichlorosilane with water. There was some evidence of polymorphism, but Kipping believed from cryoscopic data that the crystals melting at 200° to 201° were the tetramer. Other determina- 

If diphenyldichlorosilane is hydrolyzed incompletely, as by using only one-fourth the water required for the diol, there is formed a series of a-u dichloropolysiloxanes. Of these, tetraphenyl-l,3-di-chlorodisiloxane and hexaphenyl-l,5-dichlorotrisiloxane have been isolated as crystalline solids.37 These substances hydrolyze very slowly when dissolved in a hydrocarbon-alcohol mixture and stirred with cold water, yielding the corresponding diols. 

The initial hydrolysis product of phenyltrichlorosilane is also a brittle resin, but it is fusible even though it is so highly cross-linked.29 

Modification of the properties of the phenyl silicones is possible through chlorination of the aromatic nucleus.43 One or more chlorine atoms may be substituted for hydrogen in each ring, as by chlorination of the phenylchlorosilane with iron powder as a carrier. The chloro-phenylchlorosilane is then hydrolyzed, and the resulting silicols are condensed by heat, just as is done with unsubstituted phenylchloro-silanes. The product is a brittle fusible resin, but it melts at a higher temperature than phenyl silicone and is less flammable. If an average of three chlorine atoms has been introduced into each phenyl nucleus, the product will not bum at all. 

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Acetyl hypofluorite is very effective m the fluorination of the aryl-metal (Hg, Ge, or Si) bond, but yields are frequently low. With aryl silicon compounds some competition exists for replacement of an aromatic hydrogen [5i, 52, 55, 54] (equations 25-27). Fluoroxytrifluoromethane fluorinates p-methoxypheny 1 mercuric acetate to givep-fluoroanisole in 86% yield [52]... 

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

Finally, oxidative cleavage of the remaining aryl-silicon bond with lead tetrakis(trifluoroacetate), [Pb(OCOCF3)4]19, furnishes ( )-estrone [( )-1 ] in nearly quantitative yield. 

Eaborn, C., Cleavages of aryl-silicon and related bonds by electrophiles, <7. Organomet. Chem. 100, 43 (1975). 

Aryloxytitanium halides, 25 83 2-Arylpyridines, 27 111 Aryl phosphate esters, 79 51 Aryl phosphates, 7 7 493 Aryl phosphonates, 79 37 Arylphosphorus compounds, 79 28 Aryls, palladium, 79 652 Aryl-silicon compounds, 22 553, 554 Arylsulfinic acids, 27 248-249 Arylsulfonylated gelatin, 72 444 Aryltin trihalides, 24 810-811 Arylyl amines, 70 396-399 Asahi Chemical Industries EHD processes, 9 676-677 sebacic acid production, 9 679-680 ASAM (alkaline-sulfite-AQ-methanol) process, 27 30... 

For a review of aryl-silicon and related cleavages, see Eaborn J. Organomel. Chem. 1975, 100, 43-57. 

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). 

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. 

Replacements of one or two chlorine atoms and hydrogen by fluorine were observed in various organosilicon compounds in reactions with xenon difluoride99,135, while aryl-silicon bond cleavage was observed in the presence of potassium fluoride136,137 (Scheme 57). 

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

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. 

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. 

Ionic hydrogenation occurs when alkyl- or aryl-silicon hydrides reduce carbonium ions. " Thus, acetals and ketals can be reduced by trialkylsilanes in the presence of 5-10% of ZnCh. Yields of 50-85% of ether are obtained from acyclic acetals (equation 15), but cyclic acetals give appreciable amounts of a by-product (8 equation 16), which is believed not to be the result of further reduction. 

Oxidative aryl-silicon cleavage [14a, 61a] occurred almost quantitatively on exposure to lead tetralds(trifluoroacetate) to produce dl-estrone (Scheme 13.39) [61b]. 

Aryl silicon compounds can be prepared by metal-catalysed reaction of halides with silanes, as in the rhodium-catalysed reaction below. The mechanistic details of this reaction (probably) differ from the palladium-catalysed borane reaction. (NOTE triethoxy silane is extremely toxic )... 

In addition, the cleavage of aryl-silicon bonds by neutral reagents delineates even more the vast differences between the fields of organosilicon and carbon chemistry. 

Silicon tetraalkyl and tetraaryl derivatives (R4Si), as well as alkyl or aryl silicon halides (R SiCl4 , n = 1-3) can be prepared by reaction types 18.38-18.42. Note that variation in stoichiometry provides flexibility in synthesis, although the product specificity may be influenced by steric requirements of the organic substituents. Reaction 18.38 is used industrially (the Rochow process). 

Alkyl- and Aryl-silicon Halides.77 These compounds are of special importance because of their hydrolytic reactions. They may be obtained by normal Grignard procedures from SiCl4, or, in the case of the methyl derivatives, by the Rochow process in which methyl chloride is passed over a heated, copper-activated silicon ... 

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