SiH-Bromination of Carbosilanes

The bromination of SiH4 P5] and SiH-containing alkylsilanes is known [96] and can be used in the quantitative determination of Si—H groups present [97]. Partial bromination of carbosilanes, whereby Si—H groups are still contained in the compounds, is of interest in a number of aspects. Partial chlorination with CI2 could be achieved by careful manipulation of the reaction. Reaction with Br2 offers more favorable possibilities [64]. 

Reaction of (H3Si)2CH2 in the absence of solvent at —70 °C by careful dropwise addition of bromine, proceeds quantitatively. Depending upon which molar ratio is selected, a variety of Si-brominated derivatives is obtained. Reaction of the Si—H group with HBr can be excluded under the conditions just mentioned. 

All brominated derivatives of (H3Si)2CH2 form colorless liquids. The viscosity of the bromine derivatives increases as the number of bromine atoms in the compound increases (Br3Si)2CH2 itself is an oily liquid [64]. 

Reaction of (H Si—CH2)3 with bromine allows selective formation of partially brominated derivatives, because formation of an SiBr group reduces the reactivity of the remaining Si—H bond in the SiHBr group. If appropriate amounts of bromine, dissolved in methylcyclohexane are added dropwise at —50 °C to a 0.2 molar solution of (H2Si—CH2)3 in methylcyclohexane, three bromination products are obtained  

Using a mole ratio of 1 1 results in synthesis of 63% monobrominated plus 10% dibrominated compounds a mole ratio of 1 2 yields 6.5% mt nobrominated, 65% dibrominated and 6.5% tribrominated compounds, a mole ratio of 1 3 produces the tribrominated compound in a 93 % yield. 

---