Silane dendrimer

Figure 7.25 The multivalent nature of dendrimers can be used to add increased functionality to surfaces. Aminopropyl silane surfaces can be activated with either PDITC or through use of a cyclic anhydride plus DCC/ NHS to give amine-reactive surfaces. These reactive surfaces can be used to couple amine-dendrimers to provide a high density of amine groups on the surface for further bioconjugation.

Figure 18.8. Silane dendrimers via hydrosilylation and Grignard coupling...

Repeating the sequence shown in Figure 18.8 another five or six times (when congestion at the periphery occurs) is not an easy task without the introduction of Si-O-Si links and very few silane dendrimers of such a high generation have been prepared. The reaction is much more facile when HSi(CH3)Cl2 is used. Equipped with phosphine complexes either in the core or at the periphery the dendrimeric systems are used as catalysts [12], Controlling the state of platinum seems to be a key problem. 

For carbosilane dendimers using an iterative alkenylation/hydrosilation sequence, see (a) Silane Dendrimers, A.W. van der Made, P.W.N.M. van Leeuwen,/. 

These dendrimers were then used to achieve a stable coating of amino-silanized glass surfaces with NH2-modified DNA-probes (see Fig. 10). DNA chips which were prepared according to this procedure exhibited several advantages regarding the signal-to-noise ratio and their reliability over commercial chips prepared by conventional linking. 

Scheme 5. Silane-based dendrimers containing at the periphery metal-silicon o-bonds.

Scheme 6. van Koten s construction of a silane-based dendrimer functionalized at the periphery with arylnickel(ll) catalytically active sites. 

Siloxanes, prepared in 1989 as representatives of silicon-based dendritic molecules ( silicodendrimers ), were the first dendrimers to contain heteroatoms other than the usual ones (N, O, S, halogens) [68]. As with the phosphodendri-mers (Section 4.1.10), their readily modifiable architecture and their pronounced thermostability hold promise of applications, for example, in the form of carbo-silanes as liquid-crystalline materials and catalyst supports. They can be subdivided into a number of basic types and their properties are presented below with the aid of characteristic representatives ... 

Independently of each other, Lambert et al. [69] and Suzuki et al. [70] both gained access to low-generation silane dendrimers (Gl, G2) in 1995. The latter prepared a first-generation polysilane dendrimer 3 by a stepped growth polymerisation technique. Coupling of methyl[tris(chlorodimethylsilyl)]silane (1) to tris-(trimethylsilyl)silyllithiurn (2) led to the first-generation branched dendrimer 3 (Fig. 4.38). 

Fig. 4.39 Synthetic route to a second-generation silane dendrimer (according to Sekiguchi et al.)...

Silane dendrimers with longer polysilane chains (e.g. with thirteen silicon atoms in the longest chain) were prepared by Lambert and Wu via a convergent route [73]. These molecules also assume an almost spherical shape. 

Like monodisperse carbosiloxane dendrimers, hyperbranched carbosiloxanes can be produced by hydrosilylation [100]. The first compounds of this kind were synthesised in 1991 by Mathias and Carothers [101] by polymerisation of an al-lyl-tris(dimethylsiloxy)silane monomer with addition of chloroplatinic acid hydrate (Fig. 4.52). The reaction proceeds relatively fast however, addition of more catalyst does not increase the molecular masses (19000 g/mol). The NMR spec-... 

Phenolic substrates are not readily reduced to benzene derivatives, and this fact can be used to effect the transformation of methyl or benzyl ethers of phenols to their silyl ethers with elimination of CH4 or toluene as the only by-products. These studies include detailed mechanistic studies that support and corroborate our experiments and support the silane activation mechanism. The reaction is clean and high yielding enough to be applied towards the functionalization of the periphery of carbosilane dendrimers with perfluoroaryl borane moieties (Scheme 25).196... 

Repetitive silicon-based transformations were then employed for dendritic construction. Palladium-catalyzed silane hydroxylation of the core 71 afforded triol 73, which was then treated with three equivalents of monochloropolysiloxane 72 to generate the hexa-hydrido, first generation dendrimer 74. Further application of the Pd-mediated hydroxylation, followed by attachment of monochloro-monomer 72, led to the second (75) and third (76) generation polysiloxane cascades. 

Scheme 4.21. Construction of polysiloxane dendrimers prepared by an iterative silane hydroxylation and chloride displacement at silicon.

Scheme 4.22. Alkyl silane dendrimer construction by Pt-mediated silane alkenylation and vinylation.

The application of this approach to phosphorus dendrimers is readily applied11111 to the creation of a tetrahedral series via the use of a four-directional silane core (Scheme 4.26). The treatment of tetravinylsilane (78) with phosphine 98 quantitatively gave the desired small dendrimer 99, which can be transformed to the tetrakis(square planar palladium) complex see Chapter 8. 

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