Dendrimers, catalysis with

One of the main applications of dendrimers is in catalysis allowing easy recycling of the homogeneous catalyst by means of nanofiltration. Carbosilane dendrimers functionalized with diphenylphosphine groups at the periphery have been synthesized and characterized. Palladium complexes of these dendrimers have been used as catalysts in the allylic alkylation reaction. These dendrimeric catalysts can be used in a continuous process using a membrane reactor.509... 

Figure 4.19. Continuous catalysis with non-covalently functionalized dendrimers a) acid-, b) ester-functionalized guest. (Reprinted with permission from ref. 33. Copyright 2001 American Chemical Society)...

CATALYSIS WITH (METALLO)DENDRIMERS CONTAINING CHIRAL LIGANDS... 

Some conclusions can be drawn from comparison of the TOF values obtained using biphasic catalysis with literature data for the same reactions catalyzed by a polymer-supported Pd(0) catalyst (Table 2) [161]. First, although the TOFs for the dendrimer-encapsulated catalysts are consistently lower than for the polymer-supported catalysts, the values are in some cases comparable. Second, the selectivity pattern exhibited by the two types of catalysts is somewhat different. Specifically, the range of TOF numbers for biphasic catalysis is far greater than for conventional polymer catalysis, which suggests the possibility of the type of... 

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 general trend observed in many of the reports concerning catalysis with periphery-functionalized dendrimers is that the activity of the catalysts decreases with the dendrimer generation, which is usually attributed to the increasing steric bulk around the metal centers as the dendrimer generation increases. Some of these negative effects have already been discussed in Section II. 

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... 

Brunner s concept of attaching dendritic wedges to a catalytically active metal complex represented the first example of asymmetric catalysis with metal complex fragments located at the core of a dendritic structure [5,6]. Important early examples of catalysts in core positions were Seebach s TAD-DOL systems (TADDOL = 2,2-dimethyl-a,a,a/,a/-tetraphenyl-l,3-dioxolane-4,5-dimethanol) [38,39]. In general, the catalytic performance of such systems was either unchanged with respect to the simple mononuclear reference system or significantly lower. In no case has the potential analogy of this core fixation and the existence of efficient reactive pockets in enzymes been vindicated. This may be due to the absence of defined secondary structures in the dendrimers that have been employed to date. 

A decade after the initial pioneering studies, dendrimer catalysis has developed into a highly varied and complex field of research. While much of the fascination is derived from the aesthetic appeal of dendrimers (as with other applications of this class of macromolecule), dendrimer catalysts have vindicated many of the utilitarian aspects of this research. This volume provides a comprehensive overview of the current state of the art. 

The initial focus in dendrimer research was largely on their synthesis, but recently more importance has been given towards their functional aspects [23]. The successful blending of dendrimer chemistry with several contemporary themes such as host-guest chemistry [24], metallo-organic chemistry [25], luminescent materials [26], catalysis [20a], medicinal chemistry [18d] and polymers [27] has contributed enormously over the years to a rich chemistry with potential applications. As a detailed survey of this area is beyond the scope of this chapter, we will restrict ourselves to two topics involving these molecules (a) dendritic self-assembly [28] and (b) metallodendrimers [25,26]. 

This chapter will only deal with catalytic systems covalently attached to the support. Dendrimer [96-101], hyperbranched polymer [102, 103], or other polymer [100] encapsulated catalysts, micellar catalysis [104] and non-cova-lently bound catalysts (via ionic [105,106], fluorous, etc. intercations) are not being treated. Also catalysis with colloidal polymers [ 107,108] and biocatalysts, such as enzymes and RNA, will not be reviewed here. 

Several comprehensive reviews dealing with dendrimers in homogeneous catalysis have been pubhshed recently [58,83,91,112,122]. Catalytically active sites can be introduced into the core (monovalent), the branches, or the shell (multivalent) of dendritic systems. Core-fimctionahsed dendrimers have a very poor loading of catalyst and therefore catalysis with these supports will be much more expensive than with shell-fimctionalised dendritic structures. In addition, dendrimers themselves can act as catalysts. 

Heterogeneous catalysis also directly probes the surface properties of supported nanoparticles, and has been employed for dendrimer templated PtAu [37], PdAu [79], and PtCu [36] nanoparticles. Figure 4.17 shows CO oxidation for Pti Aui / Si02 catalysts compared with monometallic catalysts. Similar to the homogeneous catalysis studies, all three metal systems show synergism in catalytic activity CO oxidation catalysis, with the bimetalHc catalysts being more active than any of the corresponding monometallic catalysts. 

Catalyst recycling has been achieved using membrane reactors, biphasic solvent systems, as well as catalyst precipitation and subsequent filtration, although frequently with deteriorating catalyst performance over time. This utilitarian aspect of dendrimer catalysis has provided the motivation for much of the work on chiral dendrimer catalysts. 

Zhao MQ, Crooks RM (1999) Homogeneous hydrogenation catalysis with monodisperse, dendrimer-encapsulated Pd and Pt nanoparticles. Angew Chem Int Ed 38 364... 

Abstract The divergent synthesis, properties and functions of dendrimers terminated by metallocenyl redox groups are briefly illustrated in this micro-review, with emphasis on molecular electronics, sensing with regenerable derivatized Pt electrodes and efficient catalysis with dendrimer-stabilized Pd nanoparticles. 

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