Core-functionalized dendrimers

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. 

In core- (and focal point-) functionalized dendrimers, the catalyst may benefit from the site isolation created by the environment of the dendritic structure. Site-isolation effects in dendrimers can also be beneficial for other functionalities (a review of this topic has appeared in Reference (10)). When reactions are deactivated by excess ligand and when a bimetallic deactivation mechanism is operative, core-functionalized dendrimers can minimize the deactivation. 

Another noteworthy difference between core- and periphery-functionalized dendrimers is that much higher costs are involved in the application of core-functionalized dendrimers due to their higher molecular weight per catalytic site. Furthermore, applications may be limited by the solubility of the dendrimer. (To dissolve 1 mmol of catalyst/L, 20 g/L of core-functionalized dendrimer is required (MW 20 000 Da, 1 active site) compared to 1 g/L of periphery-functionalized dendrimer (MW 20 000 Da, 20 active sites). On the other hand, for core-functionalized systems, the solubility of the dendritic catalyst can be optimized by changing the peripheral groups. 

The physical properties of dendrimers such as solubility, arising from their hyperbranched globular shapes and the peripheral groups, can be modified by end-group modification. In core-functionalized dendrimers, the immiscibility of the wedges with a solvent enables precipitation and subsequent separation by filtration. 

BINAP core-functionalized dendrimers were synthesized by Fan et al. (36), via condensation of Frechet s polybenzyl ether dendritic wedges to 5,5 -diamino-BINAP (26—28). The various generations of BINAP core-functionalized dendrimers were tested in the ruthenium-catalyzed asymmetric hydrogenation of 2-[p-(2-methyl-propyl)phenyl]acrylic acid in the presence of 80 bar H2 pressure and in a 1 1 (v/v) methanol/toluene mixture. As later generations of the in situ prepared cymeneruthe-nium chloride dendritic catalysts were used, higher activities were observed (TOF values were 6.5, 8.3, and 214 h respectively). Relative to those of the BINAP... 

Since the pioneering studies of asymmetric catalysis with core-functionalized dendrimers reported by Brunner (88) and Bolm (89), several noteworthy investigations have been described in this field. Some examples of the dendritic effects observed in enantioselective catalysis with dendrimers having active sites in the core were discussed in Section II, such as the catalytic experiments with TADDOL-cored dendrimers described by Seebach et al. (59) the asymmetric addition of Et2Zn to aldehydes catalyzed by core-functionalized phenylacetylene-containing dendrimers reported by Hu et al (42)-, the asymmetric hydrogenation investigations with (R)-BINAP core-functionalized dendrimers synthesized by Fan et al. (36) or the results... 

Copper-zinc reagents, in asymmetric addition reactions, 9, 110 Core-functionalized dendrimers as catalyst hosts, 12, 803 as organometallic hosts, 12, 800 Core-valence ionization correlations, and photoelectron spectroscopy, 1, 394... 

Core-functionalized metallodendrimers have the advantage of creating isolated sites due to the environment of the dendritic framework. In the case of core-functionalized dendrimers, the molecular weight per catalytic site (ligand/catalyst) is higher than for periphery-functionalized dendrimers, which therefore involves higher costs from a commercial point of view. The... 

Fig. 6 The application of a noncovalently core-functionalized dendrimer (2) in a CFMR in the allylic amination of crotyl carbonate in dichloromethane (Koch MPF-60 NF membrane, molecular weight cut-off = 400 Da)...

Fig. 14 BINAP core-functionalized dendrimers containing long alkyl chains in the periphery...

Fig. 1 Schematic renderings of a peripheral, b core, and c open core functionalized dendrimers...

In the case of the core-functionalized dendrimers, it is expected that a steric shielding or blocking effect of the specific microenvironment created by the dendritic structure might modulate the catalytic behavior of the core [11, 26]. This site-isolahon effects in dendrimer catalysts may be beneficial for some reactions, whereby the catalysts often suffer from deactivahon caused by coordination saturation of the metal centers, or by the irreversible formation of an inactive metallic dimer under conventional homogenous reaction conditions. The encapsulation of such an organometallic catalyst into a dendrimer framework can specifically prevent the deachvahon pathways and consequently enhance the stability and... 

The benefits of an organometallic inclusion process have been widely demonstrated for core-functionalized dendrimers.Actually, they may benefit from the local catalyst environment created by the dendrimer which may be viewed as a molecular enzyme-like structure. Dendrimers that contain bis(diarylphosphine) ligands have been especially targeted because they can increase the stability of the catalyst situated at the core. 

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