Carbosilane

ADMET is quite possibly the most flexible transition-metal-catalyzed polymerization route known to date. With the introduction of new, functionality-tolerant robust catalysts, the primary limitation of this chemistry involves the synthesis and cost of the diene monomer that is used. ADMET gives the chemist a powerful tool for the synthesis of polymers not easily accessible via other means, and in this chapter, we designate the key elements of ADMET. We detail the synthetic techniques required to perform this reaction and discuss the wide range of properties observed from the variety of polymers that can be synthesized. For example, branched and functionalized polymers produced by this route provide excellent models (after quantitative hydrogenation) for the study of many large-volume commercial copolymers, and the synthesis of reactive carbosilane polymers provides a flexible route to solvent-resistant elastomers with variable properties. Telechelic oligomers can also be made which offer an excellent means for polymer modification or incorporation into block copolymers. All of these examples illustrate the versatility of ADMET. 

The first type of polycarbosilane synthesized by using ADMET methodology was a poly[carbo(dimethyl)silane].14c Linear poly(carbosilanes) are an important class of silicon-containing polymers due to their thermal, electronic, and optical properties.41 They are also ceramic precursors to silicon carbide after pyrolysis. ADMET opens up a new route to synthesize poly(carbosilanes), one that avoids many of the limitations found in earlier synthetic methods.41... 

Another example of the flexibility of ADMET is the demonstration of successful polymerization of o /v-telechelic diene carbosilane macromonomers.45 The synthesis of macromonomer 30 is achieved using catalyst 23 and copolymerized with a rigid small-molecule diene, 4,4/-di-trans-l-propenylbiphenyl (Fig. 8.17). 

Silene-transition metal complexes were proposed by Pannell121 for some iron and tungsten systems, and such species were observed spectroscopically by Wrighton.122,123 Thus intermediates such as 33 have been proposed in the preparation of carbosilane polymers from hydrosilanes,124 both as intermediates in the isotope scrambling observed to occur in similar ruthenium hydride systems125 126 and in the 5N2 addition of alkyllithium species to chlorovinylsilanes.47... 

Carbosilanes, defined this narrowly, as a class, monomers, cyclic and polycyclic oligomers and linear polymers, with emphasis on the cyclic and polycyclic systems, have been discussed in detail in an excellent recent book by Fritz and Matern [2]. 

Figure 4. Carbosilanes containing five to eight Si atoms, isolated by column chromatography and HPLC.

G. Fritz and E. Matern, "Carbosilanes. Syntheses and Reactions," Springer-Verlag, Berlin, 1986. 

One of the main applications of dendrimers is in catalysis allowing easy recycling of the homogeneous catalyst by means of nanofiltration. Carbosilane dendrimers functionalized with diphenylphosphine groups at the periphery have been synthesized and characterized. Palladium complexes of these dendrimers have been used as catalysts in the allylic alkylation reaction. These dendrimeric catalysts can be used in a continuous process using a membrane reactor.509... 

Carbosilane dendrimers capped with 4,8, and 12 (32) (Fig. 22) perfluoroarylborane Lewis acids have been synthesized for use as Lewis-acid catalysts for the hydrosilation of acetophenone.53 The dendrimers were found to be rather effective, exhibiting only slightly inferior activities in comparison to B(C6F5)3. 

Figure 22 Carbosilane dendrimeric Lewis-acid catalyst (32) capped with perfhioroarylborane Lewis acids. (Adapted from ref. 53.)...

Figure 35 A noncoordinating carbosilane dendrimeric polyanion terminated with BR3 groups (52) used as a cocatalyst in the metallocene-catalyzed olefin polymerization. (Adapted from ref. 80.)...

Seyferth and coworkers [181] introduced ethynyl groups onto the periphery of carbosilane dendrimers by displacement of chloride from the terminal silicon groups. They further treated these ethynyl terminated silicon dendrimers with Co2(CO)8 to afford the corresponding acetylenedicobalt hexacarbonyl dendritic complexes. 

Fig. 75. Carbosilane glycodendrimers with pendant galabiose disaccharides.432...

In summary, results of in vivo experiments showed the effectiveness of carbosilane dendrimers having clustered Pk carbohydrate moieties, and the complete neutralization potency against STL-II was discovered when dumbbell-shaped dendrimers were identified as potent candidate inibitors. Although the precise mechanism of action remains to be elucidated, this type of inhibitor provided a new strategy for the detoxification of SLTs present in circulation. 

Fig. 76. Sialyloligosaccharide-capped carbosilane dendrimers successfully used as ligands against various strains of influenza virus hemagglutinins.433...

K. Matsuoka, M. Terabatake, Y. Esumi, D. Terunuma, and H. Kuzuhara, Synthetic assembly of trisaccharide moieties of globotriaosyl ceramide using carbosilane dendrimers as cores. A new type of functional glyco-material, Tetrahedron Lett., 40 (1999) 7839-7842. 

A. Yamada, K. Hatano, K. Matsuoka, T. Koyama, Y. Esumi, H. Koshino, K. Hino, K. Nishikawa, Y. Natori, and D. Terunuma, Syntheses and Vero toxin-binding activities of carbosilane, Tetrahedron, 62 (2006) 5074—5083. 

H. Oka, T. Onaga, T. Koyama, C.-T. Guo, Y. Suzuki, Y. Esumi, K. Hatano, D. Temnuma, and K. Matsuoka, Sialyl 7(2 3) lactose clusters using carbosilane dendrimer core scaffolds as influenza hemagglutinin blockers, Bioorg. Med. Chem. Lett., 18 (2008) 4405 1408. 

Figure 4.11. Gi carbosilane dendrimer with 12 NCN-pincer-Ni groups...

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