Fluorous dendrimer

Biphasic catalysis in an organic/fluorous solvent system by Pd/dendrimer nanocomposites. 

The team of Crooks is involved in the synthesis and the use of dendrimers and, more particularly, poly(amidoamine) dendrimers (PAMAM), for the preparation of dendrimer-encapsulated mono- or bimetallic nanoparticles of various metals (Pt, Pd, Cu, Au, Ag, Ni, etc.) [55, 56]. The dendrimers were used as nanocatalysts for the hydrogenation of allyl alcohol and N-isopropylacrylamide or other alkenes under different reaction conditions (water, organic solvents, biphasic fluorous/or-ganic solvents or supercritical COz). The hydrogenation reaction rate is dependent on dendrimer generation, as higher-generation dendrimers are more sterically... 

Homogeneous Catalysis in Fluorous Solvents Using Dendrimer-... 

In this section we describe two approaches for using dendrimer-encapsulated metal particles to perform biphasic catalysis. The first is hydrogenation catalysis using dendrimers rendered soluble in the fluorous phase by electrostatic attachment of perfluoroether groups [103]. The second demonstrates the use of perfluoroether groups covalently hnked to the dendrimer exterior to carry out a Heck reaction [100]. 

Fig. 22. Schematic illustration of the approach used to carry out fluorous biphasic catalysis using dendrimer-encapsulated metal nanoparticles modified on their exterior with perfluoroether ponytails. Note that the ponytails can be attached by either electrostatic or covalent means. Reprinted with permission from Ref. 103 Copyright 2000 American Chemical Society...

The key result from this study is that the Pd/dendrimer nanocomposites are catalytically active under fluorous biphasic conditions. Indeed, one catalyst was... 

Table 2. Substrate, structures, and turn-over frequencies obtained for hydrogenation reactions in fluorous biphasic systems employing dendrimer-encapsulated Pd nanoparticles ...

To demonstrate further the powerful utiHty of the fluorous/organic biphasic approach to catalyst recycling, a dendrimer-encapsulated catalyst (DEC) with covalently attached perfluorinated polyether chains was synthesized [17] and metallic nanoparticles were introduced into the interior. 

These composites were prepared by mixing a fluorocarbon solution of the perfluorinated PPl dendrimers with an aqueous ethanolic solution of Pd, thereby forming an emulsion. After phase separation, the fluorous phase, which contains the dendrimer-encapsulated Pd +, was treated with a reducing agent yielding zero-valent Pd nanoclusters trapped within the dendrimer (Fig. 24). 

Various approaches have been used for the immobilisation of hydroformylation catalysts. These include (i) anchoring the catalyst to a dendrimer,16-101 polymer111"131 or inorganic solid 114 151 (ii) fluorous biphasic... 

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. 

Another possibility, taking advantage of the biphasic environment, is to use fluorous organic solvents as the catalyst phase instead of water [155]. Crooks and coworkers prepared dendrimer-stabilized colloid catalysts soluble in the fluorous phase and used the catalysts in hydrogenation [156] and in a Heck reaction [157]. In both cases the colloidal catalyst in the fluorous phase was recyclable and showed some interesting selectivities and products unique to the nanoenvironment in the dendrimer interior. 

Polypropylene imine dendrimers with covalently attached perfluorinated poly-(propylene oxide) end-groups have been employed for the stabilization of palladium colloids in Heck reactions in fluorous solvents by Crooks et al. [36] (Table 2). Relatively low activities were obtained, which were further reduced upon re-use of the fluorous phase in a second cycle. From the results of repeated Heck reactions without an added base, it can be asstuned that the reduction in activity upon recycling is due to protonation of the dendrimer scaffold, serving as a base. No leaching of palladium from the fluorous phase was detected (< 0.01 ppm) however, this value was not related to the overall palladium loading (cf. also Section 4.2). 

Recently, a combination of fluorous substituents and a sugar-derived structure allowed the preparation of the scC02-soluble copolymer 53 as a novel phase-transfer catalyst (Figure 4.8). ° Dendrimers with fluorous substituents were also prepared for the same use. ° They are soluble in dense carbon dioxide and can solubilize otherwise C02-insoluble compounds such as Pd-nanoparticles (Scheme 92). The resulting dendrimer-encapsulated Pd catalyzes the hydrogenation of styrene and the Heck reaction of phenyl iodide. 

Chechik, V. and Crooks, R.M. (2000) Dendrimer-encapsulated Pd nanopartides as fluorous phase-soluble catalysts. Journal of the American Chemical Society, 122,1243. 

It is noteworthy that the fluorine content differences can be generated by attaching either similar reaction domains to different fluorous tags [13] or the same fluorous tag to reaction domains of different sizes (Figure 7.1). For example, a class of fluorinated dendrimers with the same fluorinated domain (fluorous tag), G0-G3, can be separated from each other by fluorous HPLC because of the over 10% differences in their fluorine content (Figure 7.3). Comparing the HPLC profile of dendrimers on the fluorous phase with that on normal phase and reverse phase, it is clear that fluorous HPLC gives the best separation. 

Figure 7.3 Fluorous HPLC separation of fluorinated dendrimers. (a) Structure of fluori-nated dendrimers C0-C3 and (b) HPLC profile of fluorinated dendrimers on fluorous, normal phase, and reverse phase.

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