Covalently functionalized dendrimers

A non-covalently functionalized dendrimer was also applied in a continuous allylic amination reaction.[33] PPI dendrimers functionalized with urea adamantyl groups can act as host molecules for phosphorus ligands equipped with acetyl urea groups (Figure 4.18). The so formed supramolecular complex was reacted with a palladium precursor... 

Both the acid and ester were applied in continuous allylic amination. The maximum conversion (ca. 80%) was reached after 1 h in both experiments. Using the acid derivative of the guest, a slight drop in activity was observed ((a) in Figure 4.19), which is probably caused by a slow deactivation of the catalyst and has also been observed for covalently functionalized dendrimers (described above). When using the ester-functionalized guest, the activity dropped faster ((b) in Figure 4.19). This decrease in activity is caused by lack of retention (99.4% for the acid vs. 97% for the ester) as well as by deactivation. 

Figure 4.19. Continuous catalysis with non-covalently functionalized dendrimers a) acid-, b) ester-functionalized guest. (Reprinted with permission from ref. 33. Copyright 2001 American Chemical Society)...

A. 1.1. Covalently Functionalized Dendrimers Applied in a CFMR The palladium-catalyzed allylic substitution reaction has been investigated extensively in the preceding decades and provides an important tool for the formation of carbon—carbon and carbon—heteroatom bonds 14). The product is formed after attack of a nucleophile to an (f/ -allyl)Pd(II) species, formed by oxidative addition of the unsaturated substrate to palladium(0) (Scheme 1). To date several nucleophiles have been used, mostly resulting in the formation of carbon—carbon and... 

The supramolecular guest—Pd—dendrimer complex was found to have a retention of 99.4% in a CFMR and was investigated as a catalyst for the allylic ami-nation reaction. A solution of crotyl acetate and piperidine in dichloromethane was pumped through the reactor. The conversion reached its maximum ca. 80%) after approximately 1.5 h (which is equivalent to 2—3 reactor volumes of substrate solution pumped through the reactor). The conversion remained fairly constant during the course of the experiment (Fig. 8). A small decrease in conversion was observed, which was attributed to the slow deactivation of the catalyst. This experiment, however, clearly demonstrated that the non-covalently functionalized dendrimers are suitable as soluble and recyclable supports for catalysts. 

Fio. 8. Application of a non-covalently functionalized dendrimer (see fig. 7) in a CFMR in the allylic amination of crotyl acetate and piperidine in dichloromethane (Koch MPF-60 NF membrane, molecular weight cut-off = 400 Da) (21). 

Figure 7.10 An NHS-PEG-maleimide compound can be used to functionalize dendrimers to provide a hydrophilic spacer terminating in thiol-reactive groups. Thiol-containing proteins then can be conjugated to this reactive intermediate to form covalent thioether bonds.

Covalent functionalization of fullerenes has also been used to obtain surface-modified fullerenes that are more compatible to polymer matrices in order to fabricate composites. In this context, four basic strategies were developed. The first one allows the fullerenes to react during the monomer polymerization, so that the fullerene can be attached to the polymer chain [111, 112]. Second, an already synthesized polymer is treated using specific conditions that allow the chemical reaction with fullerenes [113,114]. Third, the fullerenes are chemically bonded to a monomer which is polymerized or co-polymerized to obtain the modified monomer [115,116]. Fourth, a dendrimer can be synthesized around a fullerene which then acts as a nucleus [117,118]. 

By covalent linkage of different types of molecules it is possible to obtain materials with novel properties that are different from those of the parent compounds. Examples of such materials are block-copolymers, soaps, or lipids which can self-assemble into periodic geometries with long-range order. Due to their amphiphilic character, these molecules tend to micellize and to phase-separate on the nanometer scale. By this self-assembly process the fabrication of new na-noscopic devices is possible, such as the micellization of diblock-co-polymers for the organization of nanometer-sized particles of metals or semiconductors [72 - 74]. The micelle formation is a dynamic process, which depends on a number of factors like solvent, temperature, and concentration. Synthesis of micelles which are independent of all of these factors via appropriately functionalized dendrimers which form unimolecular micelles is a straightforward strategy. In... 

The oxidative introduction of carboxylic functions to nanotubes provides a large number of CNT-functional exploitations and permits covalent functionalization by the formation of amide and ester linkages and other carboxyl derivatives [24]. Bifunctional molecules (diamines, diols, etc.) are often utilized as linkers. More illustrative examples are nanotubes decorated with amino-functionalized dendrimers, nucleic acids, enzymes, etc., and the formation of bioconjugates of CNTs [96]. 

Dendritic polymers can be covalently functionalized with organometallic complexes to obtain a dendritic catalyst with molecularly defined catalytic sites [5-7]. Moreover, a considerable number of reports on the applicability of functionalized dendrimers in catalysis have led to the idea of a dendritic effect on the catalyst activity/selectivity, which can either be positive or... 

Although this work does not focus on the host properties of dendrimers, it is interesting to mention briefly some work that has been done on the covalent functionalization of cyclodextrins with dendritic structures, to yield what can be appropriately described as dendritic cyclodextrins. Early work on this area was reported by Ahern et al. [18], which prepared P-CD derivatives per-functionalized with hydroxyethylamino chains on the 6-deoxy position (primary face). These CDs, considered as dendrimer precursors, showed a modest degree of binding selectivity with fluorescent guests. 

Dendrimers are attractive nanosize model compounds because of their globular architecture and their highly functionalized surface. These hyperbranched compounds are synthesized in a repetitive reaction sequence of nearly quantitative reactions. The synthetic route can either be divergent, starting from the nucleus toward the surface, or convergent, where dendrons or wedges are covalently linked to a polyfunctional nucleus. The number of metallodendrimers is still limited.493-506... 

Initial efforts gave rise to well-characterized dendritic macromolecules, but applications remained limited because of the lack of specific functionalities. An exponential increase of publication volume observed for about 15 years testified the growing interest for dendrimers and has led to versatile and powerful iterative methodologies for systematically and expeditiously accessing complex dendritic structures. The perfect control of tridimensional parameters (size, shape, geometry) and the covalent introduction of functionalities in the core, the branches, or the high number extremities, or by physical encapsulation in the microenvironment created by cavities confer such desired properties as solubility, and hydrophilic/hydrophobic balance. Thus, creativity has allowed these structures to become integrated with nearly all contemporary scientific disciplines. 

Dendrimer synthesis involves a repetitive building of generations through alternating chemistry steps which approximately double the mass and surface functionality with every generation as discussed earlier [1-4, 18], Random (statistical) hyperbranched polymer synthesis involves the self-condensation of multifunctional monomers, usually in a one-pot single series of covalent formation events [31], Random hyperbranched polymers and dendrimers of comparable molecular mass have the same number of branch points and terminal units, and any application requiring only these two characteristics could be satisfied by either architectural type. Since dendrimer synthesis requires many defined synthetic and process purification steps while hyperbranched synthesis may involve a one-pot synthetic step with no purification, the dendrimers will necessarily be a much more expensive material to produce. 

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