Dendritic carbosilane

Schematically, model regular star polymers are obtained directly from living anionic polymers where (Si-Cl p is a multifunctional carbosilane coupling agent, MeSiCl3, SiCl4, Cl3SiCH2CH2SiCl3, etc. including dendritic carbosilanes...

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

For carbosilane dendimers using a repetitive alkenylation/hydroboration/oxidation sequence, see Dendritic Carbosilanes Containing Hydroxy Groups on the Periphery, C. Kim, S. Son, B. Kim,/. Orga-nometallic Chem. 1999, 588, 1-8. 

Fig. 4.43 Dendritic carbosilane dumbbells with galabiose branches (according to Terunuma et a. )...

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

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... 

Frey and coworkers have prepared dendrimers-based on the carbosilane backbone with 4 (GO), 12 (Gl), 36 (G2), and 108 (G3) cholesteryl end-groups via esterification of dendritic carbosilane polyols with cholesteryl chlorofor-mate (Fig. 14) [132-134]. The fast increase in the number of terminal groups... 

Some selected examples of hyperbranched polymer-supported catalysis are summarised in Table 4. Dendritic carbosilane structures are well suited for catalysis because they are relatively inert to common organometaUic reagents and their structures can be easily modified. For example, Frey and van Koten reported on the synthesis of a hyperbranched carbosilane, its fmctionaUsation with NCN moieties and the introduction of paUadium(II) sites into the structure [ 82 ]. This catalyst was introduced in aldol reactions and showed similar activity as the low molecular analogue. 

Gycloplatination of benzylamine derivatives produces complexes having a five-membered azaplatinacycle structure. Complex 456 is able to be bonded to dendritic carbosilanes to form the multinuclear complex 457, ... 

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. 

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). 

Attachment of dendritic wedges of either the carbosilane or benzylphenyl ether type to the para-hydroxy aryl site in [2,6-(ArN=CMe)2C5H3N (1 R = Me, Ar = 2-Me-4-OHC6H3), has been shown to proceed in good yield [162], Complexation with iron(II) chloride allows access to dendrimer-supported precatalyst 42 (Scheme 13). Using MAO as a co-catalyst, it was shown that 42 are active in the oligomerisation of ethylene the activity of these new catalysts is not, however, related to the type of dendritic wedge employed. 

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. 

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. 

Pincer ligand Carbosilane Den- Ni Kharasch reaction 0.03 mol% 1-79 Ultrafiltration Negative dendritic 68, 70,... 

PPh2 Carbosilane Pd Hydrovinylation 0.01-0.05 mol% 68-99.9 Continuous mem- Negative dendritic 27... 

When the catalyst is located in the core of a dendrimer, its stability can also be increased by site-isolation effects. Core-functionalized dendritic catalysts supported on a carbosilane backbone were reported by Oosterom et al. 19). A novel route was developed to synthesize dendritic wedges with arylbromide as the focal point. These wedges were divergently coupled to a ferrocenyl diphosphine core to form dppf-like ligands (5). Other core-functionalized phosphine dendritic ligands have also been prepared by the same strategy 20). 

An early example of a dendritic catalyst was reported by Knapen et al. 24), who functionalized GO and G1 carbosilane dendrimers with up to 12 NCN pincer-nickel(II) groups (7a) and applied them as catalysts in the Kharasch addition of organic halides to alkenes (Scheme 3). 

Rhodium complexes of the phosphine-functionalized carbosilane dendrimers are active for the hydroformylation of alkenes. The influence of the flexibility of the dendritic backbone on the catalytic performance was characterized by comparing dendritic ligands 84a-84c (conditions toluene, 80°C, 20 bar CO/H2) 49). 

Dendrimers, among other applications, are generating interest as soluble supports thanks to the following intrinsic characteristics (i) the well-defined molecular composition of a dendrimer provides a support with a precisely defined arrangement of the reactive sites, (ii) a high loading of reactive sites is achieved on the dendrimer surface and (iii) nanofiltration techniques are available to separate the dendritic support from products. Dendrimer 143, based on a carbosilane core, possesses 12 ester functionalities on... 

Recently, dendrimers, which are hyperbranched macromolecules, were found to be an appropriate support for polymer catalysts, because chiral sites can be designed at the peripheral region of the dendrimers (Scheme 5). Seebach synthesized chiral dendrimer 14, which has TADDOLs on its periphery and used an efficient chiral ligand in the Ti(IV)-promoted enantioselective alkylation [21]. We developed chiral hyperbranched hydrocarbon chain 15 which has six p-ami-no alcohols [22], It catalyzes the enantioselective addition of diethylzinc to aldehydes. We also reported dendritic chiral catalysts with flexible carbosilane backbones [23]. 

In recent years cationic dendritic structures such as carbosilane-based [215], triazine-based [216], polyester [217] and amphiphilic dendrimers [218] have been investigated for transfection purposes with promising results, thus offering novel alternatives to the exploration of dendrimer-based nucleic acid delivery. 

Fig. 6.27 Nickel-loaded carbosilane dendrimers 1-3 of increasing generation number (GO—C2) and a corresponding non-dendritic reference substance 4...

Kleij, A.W., Gossage, R.A., Gebbink, R.J.M.K., Brinkmann, N., Reijerse, E.J., Kragl, U., Lutz, M., Spek, A.L. and van Koten, G. (2000) A dendritic effect in homogeneous catalysis with carbosilane-supported arylnickel(II) catalysts observation of active-site proximity effects in atom-transfer radical addition. J. Am. Chem. Soc., 122, 12, 112. 

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