Dendrimer modified catalysts

In Kharasch addition to acrylic acid ester that is catalyzed by nickel complexes with silicon-containing dendrimers modified with a chelate aryldiamide ligand, the catalyst activity is lower than that of a low molecular weight analogue, van Koten and coworkers assumed that this is due to the small distance between nickel atoms on the dendrimer surface and to the formation of mixed-valence binuclear complexes [114, 126, 127]. 

As we have commented earlier, there are some reports in where polymers are used to act as interface between the metal NPs and the G support increasing the adherence of the metal NPs to Gs. In one of these examples polypropylene imine dendrimer has been used to increase the stability of Pd-Co alloy NPs on r-GO (Scheme 3.42). This dendrimer modified r-GO catalyst is able to promote the Sonogashira coupling of alkynes and aryl halides using K COj at 25 °C. Importantly metal leaching was not observed and as a reflection of the catalyst stability, the material could be reused six times with the catalytic activity decreasing only from 99 to 93 % after the sixth cycles. 

A pentaerythritol-based dendrimer modified with bis-terpyridyl Ru(II) was shown to be effective as a catalyst for the electrochemical oxidation of methionine (L-Met) and cystine (L-Cys) in aqueous solution or the mixed solvent AN-water (12% AN) [100]. In this case, the dendrimer was mixed with carhon powder and, using a sol-gel hinder, the carhon electrode doped with the [Ru(tpy)2] " -functionalized dendrimer was prepared. The oxidation peak of [Ru(tpy)2] was enhanced by the addition of L-Met, indicating the electro-catalytic effect of the dendrimer. Using the composite electrode doped with the dendrimer as an amperometric detector for flow-injection analysis, a linear calibration curve was obtained over the range 1-lOpM of L-Met in phosphate buffer (pH 7.0). A similar cahbration curve was obtained for L-Cys over the range 1-10 pM in phosphate buffer (pH 2.3). 

Another way of retaining the catalyst is to create dendrimer-supported ligands, thereby allowing separation of the product and catalyst by membranes. Based on the readily modified BICOL backbone, two dendrimer-Hgands 43 were prepared that had performance comparable to that of MonoPhos 29 a in the hydrogenation of methyl N-acyl dehydrophenylalanine [81]. 

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.)... 

Fig. 13). The cross-linked scandium-modified dendrimer was tested in a number of Lewis acid-catalyzed reactions, including Mukaiyama aldol additions to aldehydes and aldimines, Diels-Alder reactions, and Friedel-Crafts acylations. The dendritic catalyst was recovered by a simple filtration. The Mukaiyama aldol... 

Siloxanes, prepared in 1989 as representatives of silicon-based dendritic molecules ( silicodendrimers ), were the first dendrimers to contain heteroatoms other than the usual ones (N, O, S, halogens) [68]. As with the phosphodendri-mers (Section 4.1.10), their readily modifiable architecture and their pronounced thermostability hold promise of applications, for example, in the form of carbo-silanes as liquid-crystalline materials and catalyst supports. They can be subdivided into a number of basic types and their properties are presented below with the aid of characteristic representatives ... 

Completely aromatic, hyperbranched polyphenylenes were synthesized as monodendrons from AB2 type monomers by Kim and Webster [111, 112]. These dendrimers were prepared either by the homocoupling of 3,5-dibromophenyl boronic acid under modified Suzuki conditions, or by aryl-aryl coupling reactions involving 3,5-dihalo-phenyl Grignard reagents in the presence of Ni(II) catalysts as shown in Scheme 7. 

The periphery of convergently synthesized den-drimers has also been modified to allow the assembly of monolayers,494 to support dendritic catalysts,495 to control the intermolecular assembly of porphyrin dendrimers,246 to probe the effect of photo isomerization,319 and to enable cross-linking of the periphery followed by removal of the core.496 These studies in peripheral modification highlight the versatility of the convergent synthesis. In particular, the ability to selectively modify the periphery and focal functionalities of a dendron enables the design of complex macromolecules that involve the interaction between multiple functional components. 

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. 

The first example of a catalytically active metallodendrimer, having catalytic groups at the periphery, was reported by van Koten, van Leeuwen and coworkers [20]. These authors prepared the nickel(II) complexes containing carbosilane dendrimers, which were successfully employed in the homogeneous regioselective Kharasch addition of polyhalogenoalkanes to the terminal C=C double bonds. Since these early studies there has been a steadily increasing number of dendrimer catalysts which have been synthesized and studied [15]. In this section, the details of peripherally modified chiral dendrimer catalysts for different asymmetric catalytic reactions will be summarized. 

There have been multiple efforts toward supported catalysts for asymmetric transfer hydrogenation, and the 4 position on the aryl sulfonate group of 26 has proven a convenient site for functionalization. Thus far, this ligand has been supported on dendrimers [181,182], polystyrenes [183], silica gel [184], mesoporous siliceous foam [185], and mesoporous siliceous foam modified with magnetic particles [186]. The resulting modified ligands have been used in combination with ruthenium, rhodium, and iridium to catalyze the asymmetric transfer of imines and, more commonly, ketones. 

In the first approach, prolinamides have been supported on micelleforming species, dendrimers (32a-c), polystyrene (26, 31a-d), poly-vinylidene chloride, phenolic polymers, ionic liquids, silica (28, 29), other inorganic supports (30), ° and polymer-modified small peptides. Supported prolinamide catalysts have also been prepared by acrylic and styrene (27) copolymerisation. 

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