Chiral dendritic catalysts

Kollner et al. (29) prepared a Josiphos derivative containing an amine functionality that was reacted with benzene-1,3,5-tricarboxylic acid trichloride (11) and adamantane-l,3,5,7-tetracarboxylic acid tetrachloride (12). The second generation of these two types of dendrimers (13 and 14) were synthesized convergently through esterification of benzene-1,3,5-tricarboxylic acid trichloride and adamantane-1,3,5,7-tetracarboxylic acid with a phenol bearing the Josiphos derivative in the 1,3 positions. The rhodium complexes of the dendrimers were used as chiral dendritic catalysts in the asymmetric hydrogenation of dimethyl itaconate in methanol (1 mol% catalyst, 1 bar H2 partial pressure). The enantioselectivities were only... 

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... 

Chiral dendritic catalysts 194 derived from BINAP was prepared and used for the asymmetric hydrogenation of quinolines (Scheme 3.63) [126]. The corresponding cyclic amine products were obtained with high enantioselectivities up to 93% ee. The dendritic catalyst showed excellent catalytic activities (TOP up to 3450h" ) and productivities (TON up to 43 000). The dendritic catalyst was recovered by precipitation and filtration and reused at least six times, with similar enantioselectivity. 

Direct catalytic Michael addition of aldehydes to nitrostyrenes proceeds in good yield, syn diastereoselectivity, and enantioselectivity (up to 82/90/99%, respectively) using a recyclable dendritic catalyst bearing chiral pyrrolidine moieties.200 High-yielding enantio- and diastereo-selective direct Michael addition of ketones to nitroalkenes to give aldol products employ modular acyclic primary amino acid derivatives as catalysts.201... 

A dendritic catalyst 9, which like 7 and 8 has a chiral metal complex as core, was synthesised by Seebach et al. [54, 55]. The core building block was a,a,a ,a -tetraaryl-l,3-dioxolan-4,5-dimethanof (TADDOL), to which both chiral and achiral dendrons and those with peripheral octyl groups can be linked (Fig. 6.36, cf. Section 4.2.3, Fig. 4.62). 

The first example of the integration of a molecular catalyst into the core position of a dendrimer was reported by Bruner et al., who studied the influence of a chiral dendritic periphery on the performance of cyclopropa-nation catalysts [63]. Ever since, a series of reports on the application of chiral core-functionalized metallodendrimers in asymmetric catalysis have... 

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. 

Recently, Majoral et al. described the synthesis of a third-generation phosphorus-containing dendrimer possessing 24 chiral iminophosphine end groups derived from (2S)-2-amino-l-(diphenylphosphinyl)-3-methylbutane (Fig. 11) [32]. The dendritic catalyst was tested in allylic substitution reactions, using rac-(.E)-diphenyl-2-propenyl acetate or pivalate as substrates. The observed enantioselectivities were good to excellent (max. 95% ee) in all reactions. After completion of the catalytic reaction, the catalyst could be reused at least twice after precipitation and filtration. A slight decrease... 

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. 

In this chapter, we attempt to summarize the recently developed chiral dendrimer catalysts with their chiral catalytically active species located either at the core or at the periphery of the dendritic macromolecular supports. The discussion will also be focused on dendrimer effects and the development of new methodologies for the recovery and reuse of chiral dendrimer catalysts, with special emphasis on their applications in enantioselective synthesis. The published data have been classified according to the type of reachon in each of the following three sections. 

Brunner s concept (dendrizyme) of attaching dendrihc chiral wedges to a catalytically achve achiral metal complex represents the first example of asymmetric catalysis using a core-funchonalized dendrimer catalyst [21]. In view of the extremely poor asymmetric induction effected by the chiral dendritic shucture, the bulk of the attenhon has been focused on the immobilizahon of the well-established chiral hgands and/or their metal complexes into an achiral dendrimer core. The important early examples included TADDOL-centered chiral dendrimers, which were reported by Seebach et al. in 1999 [28]. In this section, we ahempt to summarize the recently reported chiral core-funchonalized dendrimers with special emphasis on their applications in asymmetric synthesis. 

Figure 4.5 Chiral dendritic ruthenium catalysts containing BINAP- and DPEN-cored dendrimer ligands.

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