Dendrimer supports

Van Koten et al. reported on a negative dendritic effect in the Kharasch addition reaction. [3 9,40] A fast deactivation for the carbosilane dendrimer supported NCN pincer catalyst (Figures 4.28 and 4.29) was observed by comparison with a mononuclear analogue. This deactivation is expected to be caused by irreversible formation of inactive Ni(III) sites on the periphery of these dendrimers. 

Attachment of dendritic wedges of either the carbosilane or benzylphenyl ether type to the para-hydroxy aryl site in [2,6-(ArN=CMe)2C5H3N (1 R = Me, Ar = 2-Me-4-OHC6H3), has been shown to proceed in good yield [162], Complexation with iron(II) chloride allows access to dendrimer-supported precatalyst 42 (Scheme 13). Using MAO as a co-catalyst, it was shown that 42 are active in the oligomerisation of ethylene the activity of these new catalysts is not, however, related to the type of dendritic wedge employed. 

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]. 

The different carbosilane dendrimer supports (generation 0, 1 R=H, Me) were then used for the synthesis of the / -lactam (13). As shown in Scheme 7.2, the first step was again an immobilization of a carboxylic acid via ester bond formation. Treatment with LDA and ZnCl2 yielded in situ the corresponding zinc ester enolate (11) which reacts with N-(trimethylsilyl)phenylimine (12) to form the final four membered lactam ring (13). The last reaction step includes several intermediates. The last one is a supported /9-amino ester which undergoes spontaneous... 

Homogeneous catalysis with a dendrimer supported catalyst was introduced by van Koten et al. in 1994 [68] and since then has rapidly expanded [5, 64—67]. The advantage of dendrimers over linear or irregular polymeric supports is their well-... 

A remarkable dendritic effect in the Heck arylation of olefins by a dendrimer supported Pd-cat-alyst has recently reported by Portnoy et ah A. Dahan, M. Portnoy, Org. Lett. 2003, 5, 1197-1200. 

The dendrimer framework also plays an important role. The catalytic performance measured by activity, selectivity, stability, and recyclability depends on the dendritic architecture, and it is important to distinguish periphery-functionalized, core-functionalized, and focal point-functionalized dendrimers (Fig. 1). Periphery-functionalized dendrimers have catalytic groups located at the surface where they are directly available to the substrate. In contrast, when a dendrimer is functionalized at its core, the substrate has to penetrate the dendrimer support before it reaches the active center, and this transport process can limit the rate of a catalytic reaction if large and congested dendrimers are involved. 

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

In a batch process, all dendritic catalysts showed very high activity. When a substrate-to-Pd molar ratio of 2000 was used, the conversions after 5 min were 49, 55, 45, and 47% when dendrimers with 4, 36, 8, and 24 phosphine ligands were used, respectively. These results show that all the active sites located at the periphery of the dendrimer support acted independently as catalysts. 

Fig. 7. Schematic representation of the non-covalent immobilization of ligands to a dendrimer support and the actual supramolecular dendritic complex containing 32 phosphine ligands 21).

The same authors recently described the synthesis of similar rhodium-complexed dendrimers supported on a resin having both interior and exterior functional groups. These were tested as catalysts for the hydroformylation of aryl alkenes and vinyl esters (52). The results show that the reactions proceeded with high selectivity for the branched aldehydes, with excellent yields, even up to the tenth cycle. The hydroformylation experiments were carried out with first- and a second-generation rhodium-complexed dendrimers as catalysts, with a mixture of 34.5 bar of CO and 34.5 bar of H2 in dichloromethane at room temperature. Each catalyst was easily recovered by simple filtration and was reusable for at least six more cycles without... 

Rhodium-complexed dendrimers, supported on a resin, have been reported to show high activity for the carbonylative ring expansion of aziridines with carbon monoxide to give p-lactams (Scheme 59), [150]. 

The chemistry of carbonylation has long been known and widely applied in organic synthesis as a convenient, versatile, and powerful method [153-205]. Recently, Lu and Alper [206] investigated the catalytic efficiency of dendrimer-supported... 

Most studies performed partly on molecular models [33] but also on real POPAM and PAMAM dendrimers support the latter model concept [34]. Careful studies on the three-dimensional structure of flexible dendrimers in solution were performed by Ballauff et al. by means of SANS (Small Angle Neutron Scattering) [35] (see Section 7.6). 

Peripheral ferrocenyl-functionalized dendrimers are in fact one of the most common type of metallodendrimers synthesized today. Due to the chemical and thermal stabilities of this kind of metallocene, formation of the intended dendrimer-supported metallocene complex can be performed in a single reaction step the metallocene fragment is introduced onto the surface of a (commercially available) dendrimer as the final construction step [56,57]. 

The groups of Reek and Meijer have also applied the noncovalent approach for catalyst anchoring to a dendrimer support [62], Phosphine functionalized ligands were attached to the periphery of polypropylene imine) dendrimers via combined ionic interactions and H-bonding using a specific binding motif that is complementary to that of the support (Fig. 6). 

Nonetheless, the transposition of homogeneous catalytic reactions from unsupported to dendrimer-supported catalysts is still not straightforward. Various dendritic effects , positive and negative ones, on the activity, selectivity, stability and solubility of metallodendrimer catalysts have been observed in this respect. In our own research we have found that a high concentration of metal centers at periphery-functionalized metallodendrimers may translate into a decrease in the catalytic performance due to undesirable side-reactions between the catalytic sites at the dendrimer surface (Fig. 4 and Scheme 4). In contrast, when the exact same catalyst is located at the focal point of a dendron, this matter is avoided by isolating the active site, thereby providing a more stable albeit less active catalyst (Scheme 13). 

Whether dendritic catalysis can compete successfully in commercial applications with other strategies that allow recycling of the catalyst remains to be seen. Dendrimer supports are still relatively expensive, but for applications that do not require the well-defined structure of the dendrimer support, hy-perbranched polymers can offer a cheaper alternative. A recent novel strategy in this research area that might provide the added value required to make the step towards commercial applications involves the noncovalent functionalization of dendritic support with catalysts (Fig. 1). This offers several advantages above the traditional covalent approaches. In this chapter we will review the progress in the area of supramolecular dendritic catalysis. 

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