Dichlorosilanes, condensation

The complete reaction was performed under Schlenk conditions. Dry diethyl ether (250 mL) was placed in 500 mL Schlenk three-necked flask equipped with a dropping funnel and reflux condenser. The dropping funnel was charged with a mixture of 31.9 g (389 mmol, 2 equiv) 1-methylimidazole and 38.9 g (389 mmol, 2 equiv) 2-methylpent-4-en-2-ol in 50 mL diethyl ether. One quarter of the prepared mixture was added to the diethyl ether. The 19.6 g (194 mmol, 1 equiv) dichlorosilane condensed into a Schlenk tube at -78°C were allowed to slowly diffuse into the stirred reaction mixture at —78°C. Meanwhile the residual mixture from the dropping funnel was slowly added to the reaction mixture which is then stirred for a further 10 h at room temperature. The resulting methylimidazole hydrochloride was removed by filtration through a Schlenk frit and the solvent was evaporated imder reduced pressure (200 mbar). After fractionated condensation in vacuo (0.3 mbar, 60°C) 35.7 g (156 mmol, 81%) bis((2-methylpent-4-en-2-yl)oxy)silane were obtained as a colorless liquid. 

During the aqueous hydrolysis of dichlorosilanes there is always a very important side reaction. It is the self-condensation of silanols which are formed initially during the hydrolysis. These reactions also give rise to the formation of cyclic siloxanes together with the linear oligomers or polymers (Reaction Scheme III). The amount of cyclic products usually depends on the hydrolysis conditions and the degree of the self-condensation attained as well as concentration considerations. 

The currently accepted mechanism of the alkali metal-mediated Wurtz-type condensation of dichlorosilanes is essentially that outlined in COMC II (1995) (chapter Organopolysilanes, p 98) which derived from studies by Gautier and Worsfold,42 and the groups of Matyjaszewski43 and Jones,22,44,45 a modified polymerization scheme of which is included here. The mechanism was deduced from careful observations on the progress of polymerizations in different solvents (such as those which better stabilize anions and those which do not), at different temperatures,44 with additives, and with different alkali metal reductants. Silyl anions, silyl anion radicals,42 and silyl radicals28,46,47 are believed to be involved, as shown in Scheme 3. 

In addition to the unconventional methods of the formation of siloxane bonds such as those discussed earlier, the non-hydrolytic reactions of chlorosilanes in the presence of dimethyl sulfoxide, 7 and the reaction of dichlorosilanes with metal oxides,111 a new method has recently been discovered which involves the condensation of alkoxysilanes with organohydrosilanes with the release of hydrocarbon.112... 

Linear polysiloxanes obtained by the hydrolysis of dichlorosilanes are of relatively low molecular weight they can, however, be condensed further through the terminal OH groups by thermal after-treatment. 

Condensation of diethyl difluorosilane with 100% hydrogen peroxide in the presence of ammonia results in the formation of ethoxysilane, whereas an analogous condensation with diethyl dichlorosilane produces polymeric peroxides containing ethoxy and siloxane units. The initially formed straight chain or cyclic peroxides probably rearrange to products 3 or 4 containing alkoxy and siloxane moieties (equation 5) . 

As an alternative to condensation of dichlorosilanes by alkali metals, it is possible to synthesize poly silanes by the reaction of preformed dilithio compounds with dichlorosilanes. An early synthesis of a copolymer by this route is illustrated in equation (19).10 (see also Section 5.9.2). Finally, poly(phenylmethylsilylene) has been made by thermal decomposition of a silyl-mercury polymer36 (equation (20)). 

Rectification can be conducted in packed towers filled with Raschig rings. The rectification separates three fractions. Fraction I, which consists of trichlorosilane with a small amount of dichlorosilane, is separated at 35 °C at the top of the tower. Fraction II (a mixture of trichlorosilane and silicon tetrachloride) is separated at 35-36 °C at the top of the tower (this fraction can later be sent into the tower tank for repeated rectification. Fraction III (tank residue) mostly consists of silicon tetrachloride. Subsequent rectification of fraction II yields HSiCl3 (95-100%) and SiCl4 (to 5%). For obtaining phenyltrichlorosilane by high-temperature condensation, this condensate can be used without rectification. 

Fig. 19. Production diagram of methyl(chloromethyl)dichlorosilane 1,4 -batch boxes 2 - apparatus for preparing the initiator solution 3 - backflow condenser 5 - chlorinator 6 - rotameter 7, 9 - rectification towers 8, 10 -refluxers 11-13 - receptacles 14- tank 15 - boiler 16-18- containers...

Free carbon thereby is deposited on the reactive mass of silicon, covering it over and serving as a catalyst for further pyrolysis of methyl groups. Furthermore, the methane and hydrogen which appear in the exit gases impair the efficiency of the condensers and represent a waste of organic halide. For these reasons the formation of trichlorosilanes is to be avoided as uneconomical and detrimental to the continued production of dichlorosilanes. 

The R1 values obtained for such phenylethynyl substituted siloxanes are higher then that reported for traditional aromatic-based systems [9] or the phenol modified ones (1.50-1.53) [10]. The synthesis of high refractive index (methyl)(diphenyle thenyl)-dichlorosilane via hydrosilylation was also described [1]. Such monomer was later hydrolyzed and condensed into silicone fluid. Similar process was also presented, applying silylative coupling process in the synthesis of an analogous (methyl)(phenylethenyl)diethoxysilane [11], so the two reactions shall be discussed in the following section. 

Rantala [5] prepared hybrid organic-inorganic polymers by condensing pentafluorophenyl-vinyl-dichlorosilane, (V), and pentafluorotrichlorosilane, (V), with water and then free radically initiating the mixture. 

The sodium condensation reaction of a,co-bis(chlorosilyl)-substituted compounds and the coupling reaction of dilithio derivatives of compounds bearing 7t-electron systems with dichlorosilanes offer a convenient route to various silicon containing polymers. However, the polymers prepared by these methods always contain a small proportion of siloxy units in the polymer backbone, which would interrapt the electron delocalisation. Therefore, new synthetic routes to organosilicon polymers have been developed in which no alkali metal halide condensations are involved [6, 7]. We report syntheses of organosilicon... 

As was previously mentioned, alternative synthetic routes to polycarbosi-lanes are metathesis reactions of carbo anions and silicon halides. For example, Wurtz-Uke condensation of dichlorosilanes and dibromoethane with sodium metal yields [SiH2CH2] . 3... 

The silane-based ligands (249)-(253) (see Table 16) are obtained in highly efficient multi-component syntheses (see Equation (53)) by condensation of the corresponding dichlorosilane... 

The perphenylsilacycles, (Ph2Si) where n = 4,5 and 6, can be made from diphenyl-dichlorosilane with Na metal, but are best obtained by condensing diphenyldichlorosi-lane with Li metaL . With just 2 equiv Li in THF, the kinetic product (PhjSi), obtained in yields up to 75% ... 

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