Core-functionalized catalysts

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. 

When the catalyst is located in the core of a dendrimer, its stability can also be increased by site-isolation effects. Core-functionalized dendritic catalysts supported on a carbosilane backbone were reported by Oosterom et al. 19). A novel route was developed to synthesize dendritic wedges with arylbromide as the focal point. These wedges were divergently coupled to a ferrocenyl diphosphine core to form dppf-like ligands (5). Other core-functionalized phosphine dendritic ligands have also been prepared by the same strategy 20). 

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... 

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... 

The first example of the integration of a molecular catalyst into the core position of a dendrimer was reported by Bruner et al., who studied the influence of a chiral dendritic periphery on the performance of cyclopropa-nation catalysts [63]. Ever since, a series of reports on the application of chiral core-functionalized metallodendrimers in asymmetric catalysis have... 

Two interesting reports by Chan and co-workers of dendritic core-functionalized Ru-BINAP (BINAP = 2,2/-bis(diphenylphosphino)-l,l/-binaphthyl) catalysts, which were employed in asymmetric hydrogenations and which were fully recoverable, have appeared recently [40,41]. In particular, such dendrimers containing long alkyl chains in the periphery were synthesized and... 

As for the exodendrally functionalized dendrimer catalysts (Sect. 3.2), chiral diamine ligands have also been the objects of study in the investigation into the catalytic behaviour of core-functionalized dendrons. 

The combination of an efficient control over the environment of the active sites in a multi-functional catalyst and its immobilization within an insoluble macromolecular support was pioneered by Seebach et al. In their approach, the chiral ligand to be immobilized was placed in the core of a polymerizable dendrimer, followed by copolymerization of the latter with styrene as shown in Scheme 9 [58]. In this way, no further cross-finking agent was necessary, since the dendrimer itself acted as cross-linker. The dendritic branches are thought to act as spacer units, keeping the obstructing polystyrene backbone... 

To inspect and compare the activation overvoltage of the three catalysts in more detail, so-called Tafel plots are used, which plot the cell voltage as a function of the logarithm of the current density. Figure 3.3.16B shows the Tafel plots derived from Figure 3.3.15A. At a cell voltage of 0.9 V, where the overall reaction rate is limited by the chemical surface catalysis, the dealloyed core-shell catalysts perform three... 

Monger et al. (100) reported the synthesis and screening of a 1344-member discrete polymer library L15 as a source of catalysts for the dehydration of the p-hydroxy ketone 11.33 to the enone 11.34 (equation 1, Fig. 11.21). The main feamres of L15, obtained from poly(acrylic anhydride) 11.32 (101) as a scaffold and the amine monomer set Mi (11 representatives) are reported in Fig. 11.21. The protocols for library preparation followed the same principles seen for L14 in the previous section. The presence of both acidic (the COOH backbone, to protonate the OH in 11.33 and promote its departure) and basic groups (side chains in some Mi representatives, to promote proton abstraction) should fulfill the core functional requirements to exert the overall catalytic activity. 

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. 

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. 

One of the first applications of dendrimers as organometallic hosts was their use as enantioselective catalysts. Indeed, dendrimers that are functionalized with transition metals in the core potentially can mimic the properties of enzymes. Brunner introduced the term dendrizymes for core-functionalized transition metal catalysts which might be used in enantioselective catalysis. The dendrimeric organometallic complex shown in Figure 34 is an example of such a dendrizyme inside which the chiral dendritic branches create a chiral pocket around the transition metal. 

The first indication of selective substitution reactions came from experiments with 1 and 2 as catalysts for alkene epoxidation. NMR experiments have shown that both compounds catalyze the epoxidation of cyclohexene with t-butyl hydroperoxide (TBHP). The catalytic activity is comparable to that of the model compound Hex7Si70i2Ti(0 Pr). " The measured turn over numbers indicate that all four Ti centers are involved in the catalytic process. The catalysts could be recovered quantitatively, a proof of core-functionalization and for the core stability during many catalytic cycles. A more detailed catalytic study has recently been performed with the cubic titanasiloxane [(2,6- Pr2C6H3) (Me3Si)NSi]40i2[Ti0 Bu]4 (12). This compound was prepared by the reaction of 9 with t-butanol and catalyzes the epoxidation of cyclohexene with TBHP. The titanium butylperoxo intermediate could be isolated after a stoichiometric reaction with TBHP. This intermediate then reacted with cyclohexene to produce cyclohexene oxide. A schematic representation of the catalytic process is given in Figure 28.4. 

These electron-deficient porphyrins and their metal complexes have proven to be valuable tools to study many aspects of porphyrin structure and function. An in-depth discussion of this area is beyond the scope of this review but examples will be cited. Thus, large substituent-dependent variations in electron-transfer properties of cobalt complexes were ascribed to widely varying inner sphere reorganization energies related to core expansion and contraction. In another example, a ruthenium complex functioned as a potential methane functionalization catalyst by virtue of the ability of... 

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