Dendrimer-Supported Chiral Catalysts

Dendrimers are a class of macromolecules with highly branched and well-defined structures, and have recently attracted much attention as soluble supports for (chiral) catalyst immobilization [55-65]. As stated above, the catalysts anchored onto or into insoluble supports often possess an uneven catalytic site distribution and partly unknown structures, and generally suffer from diminished activity due to the mass transfer hmitations. Dendrimers, on the other hand, allow for the precise construction of catalyst structures with uniformly distributed catalytic 

It is clear tliat the attachment of chiral catalysts to dendrimer supports offers a potential combination of the advantages of homogeneous and heterogeneous asymmetric catalysis, and provides a very promising solution to the catalyst-product separation problem. However, one major problem which limits the practical application of these complicated macromolecules is their tedious synthesis. Thus, the development of more efficient ways to access enantioselective dendritic catalysts with high activity and reusability remains a major challenge in the near future. 

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Table 13.7 Screening of polyether dendrimer supported chiral phosphine catalysts.

The use of soluble polymers or dendrimers as chiral catalyst supports is another interesting way for catalyst separation [13]. Behaving like a homogeneous catalyst during the reaction, the catalyst can easily be separated by precipitation at the end of the reaction. High catalytic activities were reported using this approach. In addition, even use in membrane reactors may be possible using the ball-shaped dendrimers. 

Peng M, et al. A highly efficient and recyclable catalyst dendrimer supported chiral salen Mn(III) complex for asymmetric epoxidation. RSC Adv 2013 3 20684-92. 

In a subsequent paper, the authors developed another type of silica-supported dendritic chiral catalyst that was anticipated to suppress the background racemic reaction caused by the surface silanol groups, and to diminish the multiple interactions between chiral groups at the periphery of the dendrimer 91). The silica-supported chiral dendrimers were synthesized in four steps (1) grafting of an epoxide linker on a silica support, (2) immobilization of the nth generation PAMAM dendrimer, (3) introduction of a long alkyl spacer, and (4) introduction of chiral auxiliaries at the periphery of the dendrimer with (IR, 2R)-( + )-l-phenyl-propene oxide. Two families of dendritic chiral catalysts with different spacer lengths were prepared (nG-104 and nG-105). 

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]. 

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. 

The majority of studies into the catalytic behaviour of dendrimers with chiral catalytic centres at the periphery of the dendritic support have concerned non-phosphine-based catalysts. As has become apparent in these studies, the effect of the dendrimer fixation on the catalytic performance generally depends on the individual system. Factors such as the high local density of catalytic sites, the interaction of functional groups in the dendrimer backbone with the catalysts and the structural rigidity or flexibility of the dendrimers seem to play a role in many cases. 

In another example of the dendronization of solid supports, Rhee et al. described the design of silica-supported chiral dendritic catalysts for the en-antioselective addition of diethylzinc to benzaldehyde (Fig. 28) [60-62], The immobilized dendritic systems were formed in two different ways one by stepwise propagation of dendrimers and the other by direct immobilization... 

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. 

Recently, Soai et al. reported the synthesis of series of chiral dendrimer amino alcohol ligands based on PAMAM, hydrocarbon and carbosilane dendritic backbones (Figure 4.31) [99-102]. These chiral dendrimers were used as catalysts for the enantioselective addition of dialkylzincs to aldehydes and N-diphenylphosphi-nylimines (Scheme 4.25). The molecular structures of the dendrimer supports were shown to have a significant influence on the catalytic properties. The negative dendrimer effect for the PAMAM-bound catalysts was considered due to the fact that the nitrogen and oxygen atoms on the dendrimer skeleton could coordinate to zinc. 

In 2002, Sasai et al. reported the synthesis of dendrimer heterobimetallic multi-funchonal chiral catalysts, containing up to 12 chiral BINOL units at the periphery (Figure 4.36) [107]. The insoluble dendrihc heterobimetallic mulhfunchonal chiral AlLibis(binaphthoxide) (ALB) complexes were obtained by treahng these dendrimer ligands with AlMcj and n-BuIi. The resulhng dendrimer-supported ALB... 

Solid-Supported Chiral Dendrimer Catalysts for Asymmetric Catalysis... 

In the dendritic [Co(salen)] complexes prepared by Breinbauer and Jacobsen the dendrimer again serves as - covalent - support material for the catalytic entities attached to the periphery [62]. These dendritic Jacobsen catalysts were obtained by reaction of the corresponding PAMAM dendrimers with active ester derivates of chiral ]Co(II)-(salen)] units according to standard peptide coupling methods. In hydrolytic kinetic resolution of vinylcyclohexane oxide the dendrimer 14 (Fig. 6.40) showed a dramatically increased reactivity compared to the commercially available monomeric Jacobsen catalyst [63-67]. Whereas the latter merely gave a conversion of less than 1% with an indeterminable ee, 14 afforded a conversion of 50% with an ee of 98 2. 

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