C-Chlorinated Carbosilanes

The first reaction carried out in this way was with (Cl3Si)2CH, and no Si—C cleavage was observed [99]  

Carbon-chlorinated compounds are hydrolytically easier to cleave than their corresponding CHj-containing analogues, as is shown in this example  

Through C-chlorination a considerable number of new compounds became accessible. From the molecular skeleton of 1,3-disilapropane the following compoimds arose, differing only in the arrangement of Cl and H atoms in the skeleton. Ignoring for the present those compounds containing partially chlorinated groups, we foimd 

The number of compounds obtained increases if one includes those in which the C and Si atoms are partially chlorinated  

In carbosilanes with three or more silicon atoms just by different arrangement of the C-chlorinated groups a number of additional derivatives emerges, as shown in examples 8-11. Moreover, considering the various possible substitution patterns of hydrogen and chlorine on silicon, this number will considerably increase. 

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As the results of the previous section show. Si- and C-chlorinated carbosilanes initiate their reactions with MeMgCl by attack on a CCl group. In Si-fluorinated, C-chlorinat-ed carbosilanes however, the overall reaction path is governed by the reactivity of the SiF group. Hence, (F3Si)2CCl2 reacts with MeLi to methylate the silicon. 

In connection with previously described investigations concerning C-chlorinated carbosilanes, the synthesis and subsequent reactions of C-fluorinated derivatives were of interest. These compounds became accessible by insertion of a CF2 carbene into the Si—Si bond by the process... 

The C-chlorinated linear carbosilanes are liquids, while (Cl3Si—CCl2)2SiCl2 is a viscous compound which degrades on attempted distillation. 

Reduction of partly C-chlorinated SiCl-containing carbosilanes enables preparation of the corresponding SiH-containing carbosilanes with a partly chlorinated bridging C atom ... 

The formation of C-chlorinated SiF-containing cyclic carbosilanes is possible through reversal of the reaction pathway, namely, via photochlorination of the corresponding SiF-, CH-containing compounds (see Table 36) [89]. 

The Si- and C-chlorinated derivatives differ from SiH-containing C-chlorinated compounds only through the Si—Cl group. Because the Si—Cl bond has a large electron density available for shielding of the Si atom, due to the increased C-chlorination a reduced polarization and reactivity of the Si—Cl bond is observed. Therefore the different behavior of SiH- and CCl-containing carbosilanes, as opposed to SiCl-and CCl-containing carbosilanes, can be understood. 

The increased polarization of the Si—C —Si bonds in the skeleton through C-chlorination readily explains also the ring cleavage and chain shortening occurring in reactions of perchlorinated carbosilanes with LiAlH. ... 

Photochlorination leads to attack of CH groups in the carbosilane and, under carefully controlled conditions, it is possible to restrict radical attack to this group. Degradation of the Si—C—Si structure is thus largely avoided (48, 49, 50, 70). As in the case of the alkyl monosilanes, chlorination proceeds as far as steric considerations will allow. Intermediate chlorinated products can be isolated only in a few cases. Thus the following reactions are observed ... 

The next stage could involve the formation of the phosphino-substituted phosphorus ylide C. Whether or not this reaction actually occurs depends on the difference in reactivity of the CCl group in the original carbosilane A, which in turn depends on the extent of Si-chlorination, compared to the PMe2Cl group in the phosphorus ylide B. As far as the phosphino-substituted ylide C is concerned, it can react with excess starting material A to yield the phosphorus ylide B, or rearrange via a competitive reaction to produce the phosphane D. The extent to which these competitive... 

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