Metallodendritic catalysts

Recently, two new approaches for the preparation of metallodendritic catalysts were described. The first one involves the use of PAMAM dendrimers [11] as both template and stabilizer of metal ions. The cavities of these dendrimers can serve as host type nano-reactors for metal ion guests. This strategy is referred to as reactive encapsulation . The concept was first elegantly demonstrated by... 

METALLODENDRITIC CATALYSTS AND MEMBRANE CATALYSIS CATALYST RECOVERY... 

Recovery of the metallodendritic catalysts with Cy substituents was achieved by precipitation with pentane, and the dendrimers were reused five times without significant loss of activity. However, the tBu-substituted catalysts could not be recovered by this method as a consequence of their high solubility in pentane. 

In this opening chapter, we will focus our discussion on metallodendritic catalysts that localize their catalytic functions either at the periphery or at the core of a dendritic macromolecule. These types of dendritic catalyst have been by far the most widely applied of the metallodendrimers in combination with membrane separation technologies. 

One of the best-studied examples of recoverable metallodendritic catalyst was reported in 1997 by Reetz for the Heck reaction using Pd dendrimers that were derived from the dendritic phosphines DAB-dendr-[N(CII2PPh2)2]x ob-... 

Moreover, the metallodendritic catalysts can be recovered after the reaction, which was indeed achieved by precipitation of 23b and 23c using pentane. However, the catalysts 23d-f are too soluble in pentane and other common solvents for recovery because of the presence of the f-Bu substituents. 

A few metal-earbene metallodendritic catalysts with four branches were known before our study and metathesis activity had been recorded, but good recyclability was still a challenge. The diffieulty resides in the need to sustain both metathesis aetivity and stability of the metallodendrimer. Thus, we selected the ruthenium (Ru) family of catalysts, and designed metal-lodendrimers eontaining Ru-benzylidene fragments loeated at the dendrimer... 

An interesting novel approach to the synthesis of (metallo)dendrimer catalysts could be the use of random hyperbranched polymers [38]. Obviously, these hyperbranched polymers have comparable but less defined structures, but to arrive at dendrimers with similar sizes, a larger number of preparative steps are required, which may be an economic disadvantage. Furthermore, materials involving heterogeneous supports with well-defined metallodendritic subunits [15] can be a promising future direction giving rise to new types of supramolecu-lar catalysts that can easily be recovered from production streams. 

Fig. 2 Schematic representations of metallodendritic architectures according to the metal (catalyst) location A at the periphery of a dendrimer or of a dendron B at the core of a dendrimer or at the focal point of a dendron C at branching points of a dendrimer or of a dendron D dendrimer-encapsulated metal nanoparticles (DEMNs)...

The applicability of this type of dendritic catalyst in a CFMR was tested for G2-I7. It was observed that the catalytic activity remained almost constant for up to eight hours of reaction time, which is in contrast with the peripheral-functionalized catalysts (Gi-7, Scheme 7) applied under similar conditions. Although resulting in an overall lower activity per catalytic center, the location of the catalytic site within the dendritic sphere seems to protect the active species against deactivation via interaction with the membrane or with other metallodendritic species. 

Generally, the Sonogashira coupling reaction is achieved by a palladium-copper catalyzed reaction of aryl or vinyl halide and terminal alkyne [70-72], The presence of the copper co-catalyst is an obstacle, however, towards the metallodendritic approach of the system. In this context, only a few examples of copper-free procedures have been reported [73-77], involving for instance, in situ Pd(0) complex formation with bulky phosphines [78]. 

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