Catalysis dendrimers

Keywords Catalysis, dendrimer, dendritic Jeffamines, Micheal addition, PAMAM... 

Monomers of die type Aa B. are used in step-growth polymerization to produce a variety of polymer architectures, including stars, dendrimers, and hyperbranched polymers.26 28 The unique architecture imparts properties distinctly different from linear polymers of similar compositions. These materials are finding applications in areas such as resin modification, micelles and encapsulation, liquid crystals, pharmaceuticals, catalysis, electroluminescent devices, and analytical chemistry. 

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]. 

Esumi K (2003) Dendrimers for Nanoparticle Synthesis and Dispersion Stabilization. 227 31-52 Famulok M, Jenne A (1999) Catalysis Based on Nucleid Acid Structures. 202 101-131 Fechter MH, see de Raadt A (2001) 215 327-345... 

Cornils B (1999) Modern Solvent Systems in Industrial Homogeneous Catalysis.206 133-152 Crooks RM, Lemon III BI, Yeung LK, Zhao M (2001) Dendrimer-Encapsulated Metals and Semiconductors Synthesis, Characterization, and Applications. 212 81 -135 Croteau R, see Davis EM (2000) 209 53 - 95 CurranDP, see Maul JJ (1999) 206 79-105... 

An important property of dendrimers is the presence of internal cavities in which ions or neutral molecules can be hosted [68]. Such a property can potentially be exploited for a variety of purposes, which include catalysis and drug delivery. 

The recent development of structurally controlled dendrimers has led to the development of a wide range of new functional macromolecules. These dendrimers were first applied in the fields of chemistry, including catalysis, pharmacology, and materials science [23-26]. More recently there have been several reports of dendrimers having electro active, photoactive, and recognition elements [27-34]. Important applications in photonics have recently been exploited, though the number of reports is still limited. 

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... 

Biphasic catalysis in an organic/fluorous solvent system by Pd/dendrimer nanocomposites. 

Polyamidoamine (PAMAM) dendrimers have drawn considerable interest in recent years due to their potential applications in medicine, nanotechnology, and catalysis (6,7). The ability to control dendrimer interior/exterior functionalities and the macromolecular architecture of PAMAM dendrimers... 

The synthesis of chiral dendrimers from various building blocks, their - difficult - structure determinations, and their - potential - use in physiological applications, in bioassays, and in enantioselective catalysis are reviewed. 

Keywords Chiral dendrimers, dendrimers, asymmetric synthesis, asymmetric catalysis. 

Verkade and co-workers have shown the usefulness of their phosphazanes in various stoichiometric as well as catalytic reactions <1999PS(144)101>. Compound 290 was used to promote the cyanohydration of benzaldehyde with trimethylsilyl cyanide (TMSCN). The cyanohydrin was isolated in 95% yield, but no enantioselectivity was noticed <2002JOM(646)161>. Compounds 291 and 292 were attached to dendrimers and shown to be effective in the catalysis of Michael reactions, nitroaldol reactions, and aryl isocyanate trimerizations <2004ASC1093>. 

Most of the supports so far studied are conventional in the field of catalysis. Some new kinds of support have emerged including mesostructured materials,193 dendrimers, organic-inorganic hybrids, and natural polymers such as polysaccharides or polyaminoacids. Nevertheless, at the moment, supported catalysts still suffer from relatively poor stability when compared to classical heterogeneous catalysts, and from limited activity when compared to homogeneous catalysts. The driving force for this research is thus to make up some of these deficits. 

The use of dendrimers as supports to anchor transition metal catalysts has attracted considerable attention over the past decades [48] (see also Chapter 4 of this book). Several groups studied the use of dendrimers immobilised on insoluble supports [49], and this type of material meet the requirements for catalysis in interphases. Alper reported the use of diphosphine functionalised polyamidoamine (PAMAM) dendrimers... 

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 set of core-functionalized dendrimers was synthesized by Van Leeuwen et al. and one compound was applied in continuous catalysis. [45] The dendritic dppf, Xantphos and triphenylphosphine derivatives (Figures 4.22, 4.30 and 4.31) were active in rhodium-catalyzed hydroformylation and hydrogenation reactions (performed batch-wise). Dendritic effects were observed which are discussed in paragraph 4.5. The dendritic rhodium-dppf complex was applied in a continuous hydrogenation reaction of dimethyl itaconate. 

Dendrimers are not only unreactive support molecules for homogeneous catalysts, as discussed in the previous paragraph, but they can also have an important influence on the performance of a catalyst. The dendrons of a dendrimer can form a microenvironment in which catalysis shows different results compared to classical homogeneous catalysis while peripheral functionalized dendrimers can enforce cooperative interactions between catalytic sites because of their relative proximity. These effects are called dendritic effects . Dendritic effects can alter the stability, activity and (enantio)selectivity of the catalyst. In this paragraph, different dendritic effects will be discussed. 

Dendrimers and soluble polymers provide alternative supports to solids, which have the advantage that access to the catalytically active sites is not restricted. The main problem in these cases is not in the catalysis - reactions with high rates and selectivities have been reported - but rather in the separation which relies on nano- or... 

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