Polymer-supported chiral dendritic catalysts

Soluble chiral catalysts bearing linear polymer supports or dendritic ligands. 

Portnoy and coworkers immobilized chiral hydroxyproline derivatives on polystyrene support functionalized with polyether dendrons (Scheme 15.45).These catalysts promoted the aldol addition of acetone to aromatic aldehydes with excellent enantioselec-tivities, significantly superior to those achieved in the same reaction with analogous catalyst lacking the dendritic interface. The same group prepared polymer-supported chiral bifunctional aminocarbamate and aminourea catalysts for nitro-Michael reaction (Scheme 15.46). However, in this case the dendritic catalysts were inferior to their simpler dendron-lacking analogues. 

Abstract Enantioselection in a stoichiometric or catalytic reaction is governed by small increments of free enthalpy of activation, and such transformations are thus in principle suited to assessing dendrimer effects which result from the immobilization of molecular catalysts. Chiral dendrimer catalysts, which possess a high level of structural regularity, molecular monodispersity and well-defined catalytic sites, have been generated either by attachment of achiral complexes to chiral dendrimer structures or by immobilization of chiral catalysts to non-chiral dendrimers. As monodispersed macromolecular supports they provide ideal model systems for less regularly structured but commercially more viable supports such as hyperbranched polymers, and have been successfully employed in continuous-flow membrane reactors. The combination of an efficient control over the environment of the active sites of multi-functional catalysts and their immobilization on an insoluble macromolecular support has resulted in the synthesis of catalytic dendronized polymers. In these, the catalysts are attached in a well-defined way to the dendritic sections, thus ensuring a well-defined microenvironment which is similar to that of the soluble molecular species or at least closely related to the dendrimer catalysts themselves. 

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. 

Figu re 4.1 Commonly encountered chiral catalyst immobilization on dendritic polymer supports (a) core-functionalized chiral dendrimers (b) peripherally modified chiral dendrimers (c) solid-supported dendritic chiral catalysts. 

Pan et al. give an extensive review of immobilized asymmetric catalysts according to reaction classes and the land of support [9]. Saluzzo and Lemaire reviewed the use of polymer-supported BINAP for hydrogenation and hydrogen-transfer reduction with diamines or amino alcohols, respectively [10]. The immobilization and recycling of chiral catalysts was the topic of a recent book [11]. Dendritic catalysts... 

The kinetically controlled stereoselection depends on very small increments of free activation enthalpy. It is therefore an excellent sensitive probe for dendrimer effects. As monodispersed macromolecules, chiral dendrimer catalysts provide ideal model systems for less regularly structured but commercially more viable supports such as hyperbranched polymers. However, the results obtained by a considerable number of research groups in the field have also established that the structural characteristics of the established dendrimer systems, such as the absence of a well-defined secondary structure, have limited the development of efficient abiotic enzyme mimics based on dendrimers. To achieve this ambitious goal, more effort in dendrimer synthesis will be necessary. The use of dendritic... 

Given the utility of chiral Cu(II)/bisoxazoline complexes in enantioselective Mukaiyama aldol reactions, a number of reports detailing the development of polymer-bound or dendritic bisoxazoline copper (I I) complexes have been developed. Development of such catalyst systems provides the potential for easy recovery and reuse of the relatively expensive catalyst. To this end, Salvadori and CO workers reported Mukaiyama aldol addition of ketene thioacetal (57) to methyl pyruvate catalyzed by a Cu(OTf)2 complex of polystyrene-supported bisoxazoline (89) (Scheme 17.18) [23]. The enantioselectivity of the addition remained high over eight cycles of the catalyst, however, reactivity was gradually reduced over time. 

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]. 

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