Dendritic polymer catalyst

Scheme 7.33 Asymmetric epoxidation using a dendritic polymer catalyst and a nanomembrane...

Ultrafiltration has been used for the separation of dendritic polymeric supports in multi-step syntheses as well as for the separation of dendritic polymer-sup-ported reagents [4, 21]. However, this technique has most frequently been employed for the separation of polymer-supported catalysts (see Section 7.5) [18]. In the latter case, continuous flow UF-systems, so-called membrane reactors, were used for homogeneous catalysis, with catalysts complexed to dendritic ligands [23-27]. A critical issue for dendritic catalysts is the retention of the catalyst by the membrane (Fig. 7.2b, see also Section 7.5). 

Dendritic Polymers as High-Loading Supports for Catalysts I 331... 

Recently, dendrimers, which are hyperbranched macromolecules, were found to be an appropriate support for polymer catalysts, because chiral sites can be designed at the peripheral region of the dendrimers (Scheme 5). Seebach synthesized chiral dendrimer 14, which has TADDOLs on its periphery and used an efficient chiral ligand in the Ti(IV)-promoted enantioselective alkylation [21]. We developed chiral hyperbranched hydrocarbon chain 15 which has six p-ami-no alcohols [22], It catalyzes the enantioselective addition of diethylzinc to aldehydes. We also reported dendritic chiral catalysts with flexible carbosilane backbones [23]. 

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]. 

Kleij AW, Klein Gebbink RJM, van Koten G (2002) Dendritic polymer application catalysts. In Frechet J, Tomalia D (eds) Dendrimers and other dendritic polymers. Wiley, New York... 

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... 

Another application of hyperbranched polymers as supports for catalysts is their use as backbones for the covalent attachment of organometallic fragments. NCN-pincer complexes (NCN-pincer = 2,6-bis[(dimethylamino)-methyl] phenyl anion) are attractive building blocks for catalytic reactions [20,21], Covalent introduction of the transition-metal complexes can also be of interest for visualization and imaging of dendritic polymers by transmission electron microscopy (TEM). 

Nonracemic Ti-BINOLate (BINOL = l,l -bi-2-naplilli()l) and Ti-TADDOLate (TADDOL = a,a,a, a -tetraaryl-2,2-dimethyl-l,3-dioxolan-4,5-dimethanol) complexes are also effechve chiral catalysts for the asymmetric alkylation of aldehydes [9-11]. Seebach developed polystyrene beads with dendritically embedded BINOL [9] or TADDOL derivatives 11 [10, 11]. As the chiral ligand is located in the core of the dendritic polymer, less steric congeshon around the catalyhc center was achieved after the treatment with Ti(OiPr)4. This polymer-supported TiTADDOLate 14 was then used for the ZnEt2 addition to benzaldehyde. Chiral 1-phenylpropanol was obtained in quantitahve yield with 96% ee (Scheme 3.3), while the polymeric catalyst could be recycled many times. 

---