Dendrimers, carbosilane

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. 

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. 

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. 

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...

Van Leeuwen et al. used several generations of carbosilane dendrimers with 4, 8, 24, and 36 diphenylphosphine end-groups (Figure 4.15) for the allylic alkylation reaction of allyl trifluoracetate with sodium diethyl 2-methylmalonate.[31]... 

Figure 4.15. Synthesis of phosphine-functionalized carbosilane dendrimers.[31]...

Figure 4.22. Core functionalized dppf-carbosilane dendrimer.[45]...

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. 

The last example of a dendritic effect discussed in this chapter is the use of core-functionalized dendritic mono- and diphosphine rhodium complexes by Van Leeuwen el al. [45] Carbosilane dendrimers were functionalized in the core with Xantphos, bis(diphenylphosphino)ferrocene (dppf) and triphenylphosphine (Figures 4.22, 4.32 and 4.33). 

Spin-coating films of other compositionally different dendritic macromolecules such as carbosilane dendrimers and hyperbranched poly(styrene) were studied by Sheiko et al. [20-22] using tapping mode AFM. 

The research group of Van Leeuwen has focused on catalysis at the core of a carbosilane dendrimer in an effort to be able to control stereoselectivity [10]. To this end, a ferrocenyl diphosphine backbone was functionalized with different generations of carbosilane dendrons producing a series of dendrimer phosphine ligands with an increasing steric demand (see 7 for an example, Scheme 6). In situ... 

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. 

Carbosilane dendrimers with perfluorinated end groups in perfluorohexane were studied by Stark et al. [308]. The significant deviations from simple diffusion that are observed in the NSE data in this case are attributed to shape fluctuations following a procedure that had been developed for the analysis of micro-emulsion droplet fluctuations [309]. 

In contrast to solid dendrimers [71, 84, 88], molecular resolution was not achieved for a closed film of the carbosilane dendrimers. However, one could extract structural information about the molecular conformation from the film thickness and wetting edges on a solid substrate and from surface pressure/area... 

Fig. 8. Carbosilane dendrimers with different branch ends prepared by A. Muzafarov et al. [7]...

The experimental observation is confirmed by molecular dynamics simulation of carbosilane dendrimers [77] on a polar substrate (Fig. 12). The interac-... 

Fig. 10 a-c SFM micrographs demonstrate wetting of HOPG by carbosilane dendrimers [72] a initially, droplets with a contact angle of 8° were observed b after 1 month at 23 °C, the droplets spread and formed islands with a spherical cap providing a reservoir for spreading c after 20 min at 150°C, lamellae, mostly bilayers with a thickness of about 4.5 nm, were formed. Within small areas single layer films are also observed with a thickness of 2.4 nm... 

Fig. 11 a, b. Autophobic wetting of mica by carbosilane dendrimer with OH end groups [73]... 

Complementary information about the interfacial conformation of the carbosilane dendrimers was obtained from u-A isotherms measured during compression of molecular films at the air/water interface [74]. The full reversibility of the isotherm in Fig. 13 a indicated that the experiment was done under equilibrium conditions, and that the data give direct evidence on the phase behavior. Two transitions, marked by I and II in Fig. 13 a, were observed for the OH terminated carbosilane dendrimer (dendrimer 2 in Fig. 8). The first transition (I) was... 

---