Dimethylsilyl dichloride

A totally different kind of polymer is a poly-silylether. Dimethylsilyl dichloride polymerizes easily on treatment with hydroxide. Silicon is more susceptible to the Sn2 reaction than is carbon and long chains grow quickly. 

The stereoselectivity of the reaction shows interesting dependence on the Lewis acid and the enone. For example, in the reaction with 4-phenylpent-3-en-2-one, dimethylsilyl dichloride and trimethylsilyl chloride give opposite stereoselectivity (entries 2 and 3). Depending on which substrate is used, both syn- and anti-stereoselectivity can be obtained (entries 2 and 4). 

Excess dimethylsilyl dichloride added gradually to vigorously stirred trimethyl-[bis(trimethylsilyl)methylene]phosphorane, and the product isolated after 12 hrs. at 25 product. Y 95%. F. e. s. H. S midbaur and W. Malisch, B. 104, 150 (1971). 

A C2-symmetric ansa metallocene is a racemic mixture of an enantiomeric pair—an example is rac-(dimethylsilyl)bis(l-indenyl)zirconium dichloride (XXXIV), abbreviated as rac-(CH3)2SiInd2ZrCl2. The enantiomers are designated as (R, R) and (S, S) to describe the two coordination sites in each enantiomer. Actually, the synthesis of a C2 ansa metallocene usually produces a mixture of the racemic pair plus the meso compound (R, S). The meso compound, which is a diastereomer of the racemic pair, can be separated from the racemic mixture by physical techniques such as recrystallization. The meso stereoisomer possesses Cs symmetry, and its stereoselectivity is very different from that of the enantiomeric pair (Sec. 8-5a-3). 

A 20-liter reactor was charged with 3.11 liters of triisobutylaluminum isododecane solution (110g/l) and 800 ml of 30 wt% methylalumoxane toluene solution and then stirred at 50°C for 1 hour. This mixture was then treated with dimethylsilyl (2-methyl-l-indenyl)-7-(2,5-dimethylcyclopenta[l,2-b 4,3-b-,]-dithiophene) zirconium dichloride (13.1 mmol) suspended in isododecane (500 g) and stirred 1 hour at 50°C. The reaction mixture was then diluted with 520 ml of isododecane so that the final concentration of the catalyst mixture was 100 g/1. The catalyst was then used immediately. 

In addition to the metallocenes described previously, so-called halfsandwich compounds or constrained-geometry catalysts (Fig. 3) such as dimethylsilyl-t-butylamido cyclopentadienyl titanium dichloride are used. These catalysts are excellent for producing polyethylenes with long-chain branching and can incorporate high amounts of comonomers such as 1-octene... 

Dimethylsilyl chloride and diethylsilyl chloride reacted with lithium ethanetellurolate to give dialkylsilyl ethyltellurium derivatives2. With ethylsilyl dichloride, ethylsilylene bis[ethyl tellurium] was obtained1 and with trichlorosilane, tris ethyltelluro]silane2. 

Figure 5 Structure of substituted dimethylsilyl-bridged bis(iude-nyl)ztrconium dichlorides...

For the production of ethylene/l-octene copolymers, metallocenes in combination with oligomeric methylalumoxanes or other compounds are now used [31, 63]. Half-sandwich transition metal complexes such as [(tetramethyl- / -cyclopentadienyl) (A-/-butylamido)dimethylsilyl]titanium dichloride are applied to synthesize linear low-density copolymers and plastomers, called constrained geometry catalysts [31]. Ethylene and styrene can be copolymerized to products ranging from semicrystalline mbber-like elastomers to highly amorphous rigid materials at room temperature [64]. 

Fig. 3.3 Structure of the constrained geometry catalyst [ (/ert-butylamido-iV-dimethylsilyl -(j/ -cyclopentadienyl) zirconium (IV) dichloride]...

Examples of metallocenes with two dimethylsilyl interannular linkers are more numerous than those composed strictly of C—C and C—H bonds. Royo first reported the synthesis of ansa-metallocenes with a [ (CH3)2Si 2(C5H3)2l ligand array. Relevant metallocene diehloride and dimethyl complexes 4a-d are depicted in Figure 4.5. X-ray erystal struetures have been obtained for dichloride complexes 4a and 4b and dimethyl complex 4c (see Table4.1). In all three struetures, the cyclopentadienyl rings are eclipsed. Despite a wealth of synthetie and crystallographie data,... 

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