Dendrimers catalyst recycling

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

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. 

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. 

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.)... 

II. Catalyst Recycling in Applications of Dendrimer-Supported Catalysts... 

Mizugaki et al. 74) have recently utilized thermomorphic properties of Pd(0)-complexed phosphinated dendrimers for dendritic catalyst recycling. Using the method developed by Reetz 16), they prepared dendritic ligands containing, respectively, 2, 8, 16, and 32 chelating diphosphines. Palladium dichloride was com-plexed to the dendrimers, and a reduction in the presence of triphenylphosphine gave the Pd(0)-complexed dendrimers (80—83). The dendritic complexes were active... 

While a number of dendritic catalysts have been described, catalyst recyclization in most cases is an unsolved problem. Diaminopropyl-type dendrimers bearing Pd-phosphine complexes have been retained by ultra- or nanofiltration membranes, and the constructs have been used as catalysts for allylic substitution in a continuously operating chemzyme membrane reactor (CMR) (Brinkmann, 1999). Retention rates were found to be higher than 99.9%, resulting in a sixfold increase in the total turnover number (TTN) for the Pd catalyst. 

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]. 

The aim of catalyst recycling and the exploitation of possible constructive interactions between catalytic sites underlie the rapidly growing field of stereoselective dendrimer catalysis. Since enantioselection is governed by small increments in the free enthalpy of activation, such transformations are particularly suited to assessing dendrimer effects , which result from the immobilization of catalysts. 

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... 

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. 

The performance of several palladium piecatalysts, like palladium(n) acetate, palladium(0)NPs encapsulated into polyfamidoamine) (PAMAM) dendrimers (Pd DENs) and palladium(II)-PAMAM complexes, in the Stille reaction between trichloro(pheityl)stannane and iodoarenes in water was compared [26]. The reactivity of Pd DENs is sirttilar or inferior to that of palladium(II) acetate, although the presence of the derrdrimer suppresses the formation of homocouplirrg products and allows catalyst recycling. It is suggested that the reaction catalyzed by Pd DENs occtrrs via palladirrm species which are leached form the NP but which rerrrain co-ordirrated to the derrdritic macromolecule. 

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. 

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