Recycling dendrimer catalysts

Here we review recent progress and breakthroughs in research with promising, novel transition metal-functionalized dendrimer catalysts and discuss aspects of catalyst recycling and unique dendritic effects in catalysis. 

Liming successfully attached a concave arrangement of pyridine units to Fre-chet-type dendrimers in homogeneous phase. A remarkable selectivity was thus achieved in base-catalysed addition of ketenes to alcohols and polyols (e.g. monosaccharides). The functionalised dendrimer catalysts exhibit a greater molar mass than conventional non-dendritic catalysts, thus permitting subsequent recycling of the catalyst by nanofiltration. These dendrimers are thus suitable as reagents for selective acylation of polyols [4]. 

Figure 4.2 Schematic representation of dendrimer catalyst recycling via (a) solvent precipitation (b) membrane filtration and (c) phase separation (latent biphasic system [24]).

In order to facilitate recycling of the multiple TsDPEN-functionalized dendrimer catalysts, the same group recently reported the synthesis of a novel form of hybrid dendrimer ligands by coupling polyether dendrons with peripherally TsDPEN-functionahzed Newkome-type poly(ether-amide) dendrimer (Figure 4.28) [90]. The solubility of these hybrid dendrimers was found to be affected by the generation of the polyether dendron. The ruthenium complexes produced were applied in the asymmetric transfer hydrogenation of ketones, enones, imines and activated... 

The field of catalytic metallodendrimers has seen many advances, particularly in enantioselective catalysis. " Thanks to the early work of Seebach and coworkers, the synthesis of core-functionalised TADDOL dendrimers 40 has been achieved by using four Frechet dendrons (Figure 7.2). These dendrimers were used as ligands in the synthesis of titanium TADDOLates, which were evaluated as catalysts in the asymmetric addition of diethylzinc to benzaldehyde. In a general manner, the good stereoselectivities (89-97% enantiomeric excesses) detected were found to be comparable to those observed with the monomeric TADDOL ligand (98% enantiomeric excess). No attempt to recycle the catalyst or the ligand was mentioned by the authors. 

To recycle the catalyst, we have immobilized CP17 on a series of den-drimers. The dendrimer-immobilized catalyst CP44 (Figure 2.13) afforded... 

J.P.K. Reynhardt, Y. Yang, A. Sayari, H. Apler, Periodic Mesoporous SUica-Sup-ported Recyclable Rhodium-Complexed Dendrimer Catalysts, Chemistry of Materials 16, 4095, 2004. 

Catalyst recycling has been achieved using membrane reactors, biphasic solvent systems, as well as catalyst precipitation and subsequent filtration, although frequently with deteriorating catalyst performance over time. This utilitarian aspect of dendrimer catalysis has provided the motivation for much of the work on chiral dendrimer catalysts. 

As stated previously, the dendrimer catalysts had been developed with the purpose of catalyst recycling based on dialysis, using membrane bags fabricated from a commercially available dialysis membrane (Sigma-Aldrich benzoylated dialysis tubing, MWCO 2000). The general principle and its practical implementation are depicted in Figure 19.7. 

Converting homogeneous catalytic reactions to heterogeneous versions will benefit the pharmaceutical and fine chemical industries because heterogeneous catalysts are physically separated from reactants and products and can therefore be easily recycled. Dendrimers have been employed as a recoverable catalyst platform using specially designed techniques, such as nanofiltration, precipitation, and two-phase catalysis [25]. 

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... 

Three generations of dendritic phosphines have been prepared from 3,5-diaminobenzoylglycine and 9-fluorenylmethoxycarbonyl-L-phenylalanine. The dendrimers were then attached to MBHA resin, treated with CH20 and Ph2PH, and converted to their Rh complexes. The polymer-supported complexes are excellent catalysts for the hydroformylation of alkenes, which could be recycled.283 The bidentate diphosphine A,A-bis-(P-(phosphabicyclo[3.3.1] nonan) methyl)aniline was prepared by phosphanomethylation of aniline. It forms a Rh-complex which is a highly regioselective catalyst in the hydroformylation of citronellene.284... 

The use of heterogeneous catalysts in this reaction has also been achieved palladium-montmorillonite clays [93] or palladium/activated carbon [94] in the presence of dppb transformed 2-allylphenols into lactones, the regiose-lectivity of the reaction being largely dependant on the nature of the support. Very recently, palladium complexes immobilized onto silica-supported (polyaminoamido)dendrimers were used as catalysts in the presence of dppb for the cyclocarbonylation of 2-allylphenols, 2-allylanilines, 2-vinylphenols, and 2-vinylanilines affording five-, six-, or seven-membered lactones and lactams. Good conversions are realized and the catalyst can be recycled 3-5 times [95]. 

To demonstrate further the powerful utiHty of the fluorous/organic biphasic approach to catalyst recycling, a dendrimer-encapsulated catalyst (DEC) with covalently attached perfluorinated polyether chains was synthesized [17] and metallic nanoparticles were introduced into the interior. 

Taken together, these initial findings may eventually lead to other recyclable DECS having either modulated activities or selectivities arising from steric effects. Moreover, other catalytic processes requiring a co-catalyst, such as the Wacker process, may be particularly amenable to dendrimer-based catalytic systems because (as discussed in Sect. 2.4.2) the dendrimer interior can accommodate two or more catalytic moieties for example, a metal particle (e. g., Pd) and a metal ion (e. g., Cu +), two different metal ions, or two different zero-valent metal. 

In the search to develop new materials for immobilization of homogeneous transition metal catalyst to facilitate catalyst-product separation and catalyst recychng, the study of dendrimers and hyperbranched polymers for application in catalysis has become a subject of intense research in the last five years [68], because they have excellent solubility and a high number of easily accessible active sites. Moreover, the pseudo-spherical structure with nanometer dimensions opens the possibility of separation and recycling by nanofiltration methods. Although dendrimers allow for controlled incorporation of transition metal catalysts in the core [69] as well as at the surface [70], a serious drawback of this approach is the tedious preparation of functionalized dendrimers by multi-step synthesis. 

Dendritic catalysts can be recycled by using techniques similar to those applied with their monomeric analogues, such as precipitation, two-phase catalysis, and immobilization on insoluble supports. Furthermore, the large size and the globular structure of the dendrimer can be utilized to facilitate catalyst-product separation by means of nanofiltration. Nanofiltration can be performed batch wise or in a continuous-flow membrane reactor (CFMR). The latter offers significant advantages the conditions such as reactant concentrations and reactant residence time can be controlled accurately. These advantages are especially important in reactions in which the product can react further with the catalytically active center to form side products. 

In the first part of this overview, we focus on the recycling of dendritic catalysts. This part of the review is divided according to the various recycling approaches, and the sections are organized by way of the reactions catalyzed. In the second part, we describe examples in which attachment of the catalyst to the dendrimer framework results in modified performance. (Although we attempted to make a clear division between catalyst recycling and dendritic effects, these two properties cannot always be addressed separately.)... 

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