Catalysis dendritic effect

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. 

Dendrimers are not only unreactive support molecules for homogeneous catalysts, as discussed in the previous paragraph, but they can also have an important influence on the performance of a catalyst. The dendrons of a dendrimer can form a microenvironment in which catalysis shows different results compared to classical homogeneous catalysis while peripheral functionalized dendrimers can enforce cooperative interactions between catalytic sites because of their relative proximity. These effects are called dendritic effects . Dendritic effects can alter the stability, activity and (enantio)selectivity of the catalyst. In this paragraph, different dendritic effects will be discussed. 

Here we review recent progress and breakthroughs in research with promising, novel transition metal-functionalized dendrimer catalysts and discuss aspects of catalyst recycling and unique dendritic effects in catalysis. 

Already at an early stage in the research with dendritic catalysis, these novel systems were proposed to form a promising class of recyclable catalysts. Furthermore, new, interesting properties were proposed to arise by catalyst attachment to these large, structurally well-defined polymers. In the previous section we summarized the results obtained so far in the recycling of dendritic catalysts, and here we describe some of the dendritic effects observed in catalysis. Both negative and positive effects are discussed, in an attempt to provide a balanced view of the current state of affairs. 

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... 

There are reports of numerous examples of dendritic transition metal catalysts incorporating various dendritic backbones functionalized at various locations. Dendritic effects in catalysis include increased or decreased activity, selectivity, and stability. It is clear from the contributions of many research groups that dendrimers are suitable supports for recyclable transition metal catalysts. Separation and/or recycle of the catalysts are possible with these functionalized dendrimers for example, separation results from precipitation of the dendrimer from the product liquid two-phase catalysis allows separation and recycle of the catalyst when the products and catalyst are concentrated in two immiscible liquid phases and immobilization of the dendrimer in an insoluble support (such as crosslinked polystyrene or silica) allows use of a fixed-bed reactor holding the catalyst and excluding it from the product stream. Furthermore, the large size and the globular structure of the dendrimers enable efficient separation by nanofiltration techniques. Nanofiltration can be performed either batch wise or in a continuous-flow membrane reactor (CFMR). 

Measurements of molar rotation showed that this parameter is almost proportional to the number of chiral binaphthyl units and the molar rotation per binaphthyl unit varies only slightly. On catalysis of the Diels-Alder reaction of cyclopentadiene with 3-[(E)-but-2-enoyl]oxazolidin-2-one the branched catalysts 7 and 8 showed an approximately 25% higher reactivity than the monofunctional catalyst 6 however, the former led to just a slight improvement of ee and endo-selectivity compared to 6. It is thus inappropriate to speak of a dendritic effect on catalysis, although one does indeed exist in relation to the chiroptical properties. 

Kleij, A.W., Gossage, R.A., Gebbink, R.J.M.K., Brinkmann, N., Reijerse, E.J., Kragl, U., Lutz, M., Spek, A.L. and van Koten, G. (2000) A dendritic effect in homogeneous catalysis with carbosilane-supported arylnickel(II) catalysts observation of active-site proximity effects in atom-transfer radical addition. J. Am. Chem. Soc., 122, 12, 112. 

This result, caused by the proximity effect between peripheral catalytic sites, can translate into higher or lower catalytic activity of the metallodendrimer in homogeneous catalysis, and is commonly termed the dendritic effect. In the above case, a negative dendritic effect is observed. An interesting example of a positive dendritic effect on catalyst activity was reported by Jacobsen et al. in the hydrolytic kinetic resolution of terminal epoxides by peripherally Co(salen)-substituted PAMAM dendrimers [39]. 

In this system, the catalyst G3-I9 showed a similar reaction rate and turnover number as observed with the parent unsupported NCN-pincer nickel complex under the same conditions. This result is in contrast to the earlier observations for periphery-functionalized Ni-containing carbosilane dendrimers (Fig. 4), which suffer from a negative dendritic effect during catalysis due to the proximity of the peripheral catalytic sites. In G3-I9, the catalytic active center is ensconced in the core of the dendrimer, thus preventing catalyst deactivation by the previous described radical homocoupling formation (Scheme 4). 

Dendritic polymers can be covalently functionalized with organometallic complexes to obtain a dendritic catalyst with molecularly defined catalytic sites [5-7]. Moreover, a considerable number of reports on the applicability of functionalized dendrimers in catalysis have led to the idea of a dendritic effect on the catalyst activity/selectivity, which can either be positive or... 

Gade et al.120 reported a strong positive dendritic effect in the asymmetric catalysis in the allylic amination of 1,3-diphenyl-l-acetoxypropene with morpholine when using pyrphos-palladium-functionalized PPI and PAMAM dendrimers. A remarkable and unprecedented increase in catalyst selectivity was observed as a function of the dendrimer s generation. This steady increase of ee values for these allylic animations was less pronounced for the PPI-derived Pd-catalysts than for the corresponding PAMAM-catalysts, for which an increase in selectivity from 9% ee for a mononuclear reference system to 69% ee for the Pd64-dendrimcr was realized. 

The efficiency of catalysis decreased upon increasing the dendrimer generation. This second dendritic effect is thus a negative one, and it is probably related to the more difficult access to the metal center due to the increasing steric effect at the dendrimer periphery when the generation increases. 

Servin P, Laurent R, Gonsalvi L, Tristany M, Peruzzini M, Majoral JP, Caminade AM (2009) Grafting of water-soluble phosphines to dendrimers and their use in catalysis positive dendritic effects in aqueous media. Dalton Trans 4432-4434... 

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