Solid-supported dendrimers

Despite the advantage of their easy separation, the use of conventional insoluble polymer-supported catalysts often suffered from a reduced catalytic activity and stereoselectivity, due either to diffusion problems or to a change of the preferred conformations within the chiral pocket created by the ligand around the metal center. In order to circumvent these problems, a new class of crosslinked macromolecule-namely dendronized polymers-has been developed and employed as catalyst supports. In general, two types of such solid-supported dendrimer have been reported (i) with the dendrimer as a hnker of the polymer support and (ii) with dendrons attached to the polymer support [12, 113]. 

Soluble dendrimers bearing catalytic centers located at the periphery can be covalently attached onto the surface of conventional solid supports (such as polymer beads or silica gels), leading to another type of solid-supported dendrimer catalyst. It is expected that this type of immobihzed catalysts would combine the advantages of both the traditional supported catalysts and the dendrimer catalysts. First, the catalytically active species at the dendrimer surface are more easily solvated, which makes the catalytic sites more available in the reaction solutions (relative to cross-hnked polymers). Second, the insoluble supported dendrimers are easily removed from the reaction mixtures as precipitates or via filtration (relative to soluble dendrimers). These solid-supported peripheraUy functionalized chiral dendrimer catalysts have attracted much attention over the past few years [12, 113], but their number of applications in asymmetric catalysis is very limited. 

The field of synthetic enzyme models encompasses attempts to prepare enzymelike functional macromolecules by chemical synthesis [30]. One particularly relevant approach to such enzyme mimics concerns dendrimers, which are treelike synthetic macromolecules with a globular shape similar to a folded protein, and useful in a range of applications including catalysis [31]. Peptide dendrimers, which, like proteins, are composed of amino acids, are particularly well suited as mimics for proteins and enzymes [32]. These dendrimers can be prepared using combinatorial chemistry methods on solid support [33], similar to those used in the context of catalyst and ligand discovery programs in chemistry [34]. Peptide dendrimers used multivalency effects at the dendrimer surface to trigger cooperativity between amino acids, as has been observed in various esterase enzyme models [35]. 

Benters, R. Niemeyer, C. M. Wohrle, D., Dendrimer activated solid supports for nucleic acid and protein microarrays, Chembiochem. 2001, 2, 686 694... 

Although beyond the scope of this book, a vast amount of work has been directed to supporting homogeneous catalysts on solid supports including silica, alumina and zeolites, and functionalized dendrimers and polymers [19]. These give rise to so-called solid-liquid biphasic catalysis and in cases where the substrate and product are both liquids or gases then co-solvents are not always required. In many ways solvent-free synthesis represents the ideal method but currently solvent-free methods can only be applied to a limited number of reactions [20],... 

L. Sun and R. M. Crooks, Dendrimer-mediated immobilization of catalytic nanoparticles on flat, solid supports, Langmuir 18, 8231-8236 (2002). 

A very similar approach to the same solid-phase supported dendrimers was reported by Portnoy and coworkers by means of iterative Mitsunobu/reduction steps starting with a Wang resin [56]. They used [Gl] (loading 0.62 mmol g ) to [G3] (loading 0.32 mmol g ) dendrimers for synthetic applications. The synthesis of a tripeptide was carried out without previous linker introduction to yield a dendri-... 

Physical or electrochemical adsorption uses non-covalent forces to affix the nucleic acid to the solid support and represents a relatively simple mechanism for attachment that is easy to automate. Adsorption was favoured and described in some chapters as suitable immobilization technique when multisite attachment of DNA is needed to exploit the intrinsic DNA oxidation signal in hybridization reactions. Dendrimers such as polyamidoamine with a high density of terminal amino groups have been reported to increase the surface coverage of physically adsorbed DNA to the surface. Furthermore, electrochemical adsorption is described as a useful immobihzation strategy for electrochemical genosensor fabrication. 

A hyperbranched polymerization from solid support was developed as a one-pot synthesis for branched polymers that mimic dendrimers (Scheme... 

In another example of the dendronization of solid supports, Rhee et al. described the design of silica-supported chiral dendritic catalysts for the en-antioselective addition of diethylzinc to benzaldehyde (Fig. 28) [60-62], The immobilized dendritic systems were formed in two different ways one by stepwise propagation of dendrimers and the other by direct immobilization... 

Alternatively, initiator cores have been tethered to solid support (Merrifield resins [83]). Using protein synthesis procedures that avoid the workup difficulties involved in handling large excesses of reagents, one can readily control dendrimer growth. Unfortunately, only noncleavable linkers have been examined to date with these PAMAM dendrimers. The use of initiator cores possessing cleavable linkers should make dendrimer synthesis and isolation very facile. This was demonstrated for mono-dendrons derived from poly(lysines) as reported by Tam et al. [105-107] (Fig. 26). 

Another application of the rearrangement in dendrimers focuses on the development of novel polythioether dendrons on a solid support for supported catalysis applications [94]. 

Dendrimer A polymer having a very highly branched structure. Dendrimets con be used in place of solid supports for attachment of synthetic targets, and then they can be separated by using size cxclu.sion chromatography. 

This section has focused on liquid—liquid biphasic catalysis in which catalysts are supported in different solvents to the substrates and products. Considerable efforts have also been directed to supporting homogeneous catalysts on solid supports including silica, alumina, and zeolites as well as functionalized dendrimers and polymers.33 It has also been found that synergic effects sometimes prevail between particles embedded in the support and the tethered molecular catalyst, increasing the activity of the catalyst.34... 

Figu re 4.1 Commonly encountered chiral catalyst immobilization on dendritic polymer supports (a) core-functionalized chiral dendrimers (b) peripherally modified chiral dendrimers (c) solid-supported dendritic chiral catalysts. 

Solid-Supported Chiral Dendrimer Catalysts for Asymmetric Catalysis... 

Solid-Supported Internally Functionalized Chiral Dendrimer Catalysts... 

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