Carbosilanes formation

Ethylene coordinates too strongly to the ruthenium center to act as a hydrogen acceptor in this system, but slightly larger olefins do show activity. However, reaction selectivity is reduced compared with f-butyl ethylene. Thus, the use of cis and trans 2-pentene as hydrogen acceptor leads to products consisting of 90% carbosilane and 10% hydrosilylated olefin, and 1-hexene yields a carbosilane/hydrosilylation ratio of 4 6. In comparison, no hydrosilylation products are observed for dehydrocoupling in the presence of cyclohexene, but carbosilane formation is accompanied by disproportionation to benzene and cyclohexane as a side reaction. 

Presumably, carbosilane formation in the catalytic cycle occurs via an analogous intermediate silene complex in which one of the two hydrides is replaced by a SiMe3 group. Subsequent migration of the silyl group to the carbon of the silene would produce a new carbosilyl ligand. 

The reaction mechanism, including the possible incorporation of the triphenylborane, has not yet been understood in detail very well. This complex reaction may compete with the branching process. We think that the donor-acceptor interaction between BPhj and the MeCbSi- groups of the oligomers induces first a carbosilane formation, which is then followed by a conversion into MeClSi< groups. Therefore the modified polymer has more linear structural units resulting in lower branching... 

Van Koten et al. reported on a negative dendritic effect in the Kharasch addition reaction. [3 9,40] A fast deactivation for the carbosilane dendrimer supported NCN pincer catalyst (Figures 4.28 and 4.29) was observed by comparison with a mononuclear analogue. This deactivation is expected to be caused by irreversible formation of inactive Ni(III) sites on the periphery of these dendrimers. 

A stochiometric approach was applied by Van Koten and co-workers [29], who used chiral carbosilane dendrimers as soluble supports in the in situ ester enolate-imine condensation in the synthesis of /Mactams (e.g. 19, Scheme 20). The formation of the /Mactam products proceeded with high trans selectivity, and with the same level of stereoinduction as was earlier established in reactions without the dendritic supports, (i.e. the use of the enantiopure dendritic support did not affect the enantioselectivity of the C-C bond formation). After the reaction, the dendrimer species could be separated from the product by precipitation or GPC techniques and reused again. 

The different carbosilane dendrimer supports (generation 0, 1 R=H, Me) were then used for the synthesis of the / -lactam (13). As shown in Scheme 7.2, the first step was again an immobilization of a carboxylic acid via ester bond formation. Treatment with LDA and ZnCl2 yielded in situ the corresponding zinc ester enolate (11) which reacts with N-(trimethylsilyl)phenylimine (12) to form the final four membered lactam ring (13). The last reaction step includes several intermediates. The last one is a supported /9-amino ester which undergoes spontaneous... 

Scheme 7.2 / -Lactam formation on a dendritic carbosilane support.

Although the chemistry of the initial thermal transformation is obviously quite complex, it was determined that considerable carbon insertion into the Si-Si bonds occurs, resulting in an intermediate carbosilane which can be drawn into fibers. At this point, brief oxidation results in the formation of a surface oxide which imparts dimensional stability, and subsequent heating to 1300°C produces silicon carbide fibers (3,5). 

Hydrosilylation of the protected allyl-glycoside 1 with the carbosilane 2 (by means of Silopren , a platinum-siloxane complex from Bayer AG) led via Si-C bond formation to a glycosidic carbosilane dendrimer (Fig. 4.42) [82]. 

Less usual carbosilane dendrimers were synthesised by reaction of acetyl-protected hydroxyethyl glycosides with chlorosilanes. Introduction of the carbohydrate component proceeds via alcoholysis of the chlorosilane with formation of silicon-oxygen bonds a carbosilane core unit is formed with carbosiloxane side arms [82]. Reverse assembly of carbosilanes with a carbohydrate core is also feasible [54]. 

Fig. 6.29 Dendritic carbosilane-nickel complexes with decreasing catalytic activity owing to mixed complex formation...

The decrease in catalytic activity of the nickel-containing carbosilane dendri-mer shown in Fig. 6.28 was attributed to the formation of mixed complexes with nickel in both oxidation states II and III on the dendrimer surface, which competes with the reaction with substrate radicals occurring in Kharash reactions (Fig. 6.29). 

The synthesis of dendritic carbosilanes functionalized with various diphenylphosphino carboxylic acid ester endgroups has also been reported by the Van Koten group in collaboration with Vogt et al. [40,41], The coupling of carbosilane supports containing benzylic alcohol moieties with phos-phinoxy carboxylic acid chlorides resulted in the formation of Go and Gi phosphine oxides, which subsequently were converted into the phosphino... 

In this system, the catalyst G3-I9 showed a similar reaction rate and turnover number as observed with the parent unsupported NCN-pincer nickel complex under the same conditions. This result is in contrast to the earlier observations for periphery-functionalized Ni-containing carbosilane dendrimers (Fig. 4), which suffer from a negative dendritic effect during catalysis due to the proximity of the peripheral catalytic sites. In G3-I9, the catalytic active center is ensconced in the core of the dendrimer, thus preventing catalyst deactivation by the previous described radical homocoupling formation (Scheme 4). 

It was not possible to separate the mixture by gas chromatography as the four-membered ring 88 decomposes during that procedure. This agrees with the results of investigations on the stability of cyclic carbosilanes. No specifications can so far be given for the formation of l,l,3,3-tetramethyl-l,3-disila-cyclopentane 89. 

Exemplary Procedure of Formation of Pt(0)-[poly(vinylmethyl-co-dimethyl) siloxaneJ-Carbosilane Complexes... 

The formation of tri- and especially tetrasilanes which are already branched (tertiary Si-units) as the first reaction products (described elsewhere [4]) suggests the appearance of intermediate silylene species which could enter in insertion reactions of Si-Si as well as Si-Cl bonds. The tri- and tetrasilanes undergo thermal crosslinking reactions at reaction temperatures of 165-250 °C. In addition dehydrochlorination reactions initiated by acid H-abstraction of methyl groups cause the formation of carbosilane (methylene) units in the polymer framework. Table 1 shows the gross compositions of poly(methylchlorosilanes) which are determined by the reaction temperature. 

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