Dendrimer olefin metathesis

Frechet and Liang synthesised a fifth-generation benzyl ether dendrimer with 22 internal allyl groups. Subsequent olefin metathesis [1] led on the one hand to intramolecular linking, and on the other hand to creation of a covalent linkage site for olefin-functionalised guests (Fig. 6.1) [2]. In this way, intermolecular cross-linking - between G5 dendrimer molecules - could also be attained. 

Fig. 6.1 Olefin metathesis for cross-linking (shown in grey) within the dendrimer molecules (some of the bonds are lengthened for the sake of clarity according to Frechet, Liang et al.)...

Catalytic crossed olefin metathesis can also be used to stereoselectively mono-or bifunctionalise polyolefin dendrimers [6]. Grubbs et al. were already able to... 

Olefin metathesis has become a very important reaction in polymer chemistry and natural product synthesis [47-49]. Garber et al. have used the physical properties of dendrimers in order to improve the separation between the dendritic metathesis catalyst and products on silica gel column chromatography [50]. The Van Koten group has reported on the synthesis of different generations of carbosilane dendrimers functionalized with ruthenium metathesis catalysts [51]. 

The olefin metathesis between alkenyl groups has been frequently used in recent years to stabilize molecules (e.g., dendrimers [46]) or molecular assemblies [47] by additional covalent connectivities [48] and to synthesize macrocydes or more demanding structures like multiple catenanes [24]. Therefore, this reaction was chosen for the connection of alkenyl residues attached to a calix[4]arene via the urea residues. To avoid complications by ds/trans isomerism around the newly formed double bond, the crude reaction mixture was always hydrogenated before working up (Scheme 5.10). 

Ring-Opening Olefin Metathesis Polymerization (ROMP) Dendrimers Polymers That Look Like Trees Silicones Summing Up References Cited Other Reading... 

C. Ornelas, D. Mery, J.-C. Blais, E. Cloutet, J. R. Aranzaes, and D. Astruc, Efficient Mono- and Bifunctionalization of Polyolefin Dendrimers by Olefin Metathesis, Angew. Chem. Int. Ed. 44, 7399-7404 (2005). 

A few examples of polymer supported olefin metathesis have been reported recently. Hoveyda formed styrenyl ether complexes of several mthenium-based olefin metathesis catalysts that were then incorporated into a dendrimer stmcture (273) (Scheme 23), reporting good conversions over 6 cycles albeit with diminishing mthenium content. Yao used a similar chelating ligand to incorporate olefin... 

Figure 22 Dendrimer 36 used in the preparation of cored-dendrimer unimolecular imprints, by olefin metathesis to the covalently linked template cavity within a cross-linked shell 37 and removal of the template core to yield the binding cavity 38 (adapted from Ref 32).

THE OLEFIN METATHESIS REACTIONS COMBINED WITH ORGANO-mON ARENE ACTIVATION TOWARDS DENDRIMERS AND POLYMERS... 

Other exotic structures have been obtained by olefin metathesis reactions around metal coordination spheres 1-11 and dendrimers. Organic and... 

Following these successful reports, olefin cross-metathesis of suitably substituted ferrocene derivatives was used in the synthesis of complex molecules, including dinuclear Zr/Fe polymerization catalysts (see Scheme 12.26) [29], rotaxanes [30], dendrimers [31]. 

Keywords metathesis, polymerization, dendrimer, arene, iron, olefin... 

Figure 35).Another alternative to dendritic monomers involves the preparation of a dendritic core terminated in olefins which are available for metathesis with a catalyst s alkylidene such that the catalyst becomes incorporated at the ends of the dendrimer. From there, ROMP of monomers such as norbomene elongates the dendritic ends into linear arms to yield a star polymer with a dendritic core (Figure 36). Interestingly, prior to ROMP, the catalysts attached to the dendrimer reversibly dimerized at low temperatures and high concentrations. 

This concept was refined by the research group of Li, who employed the thiol-yne reaction instead of the olefin cross-metathesis reaction as key step [60]. In this case, the carboxylic acid component served as anchor, whereby terminal alkynes were introduced by the remaining components (5-hexyn-l-al and propargyl isocyanoacetamide). Interestingly, thiol-yne addition of 3-mercaptopropionic acid to the pendant alkynes enabled not only the incorporation of further carboxylic acids, but also resulted in additional branching. Therefore, the second generation dendrimer, synthesized in three steps, exhibited 16 peripheral triple bonds. Moreover, this concept offers the opportunity to introduce structural diversity into the dendrimer architecture because the use of only one alkyne-functionalized compound in the Passerini-3CR still results in branching due to the thiol-yne reaction. Here, a structural sequence of employed phenylacetaldehyde and 2-nitrobenzaldehyde was demonstrated. 

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