Chiral dendritic structure

Mention of chirality in dendritic architectures can be traced back to patents of Denkewalter et al., which describe the construction of peptide-like dendritic structures from L-lysine units [1]. In spite of the demanding nature of some of these syntheses, numerous chiral dendritic structures have meanwhile been prepared and characterised [2]. This cannot be explained solely by the somewhat academic interest in the effect of chiral monomeric building blocks on the chirality of the overall molecule. The prospect of using chiral dendrimers as model... 

Multiple sequential AD gave access to interesting dendritic structures as shown by Sharpless [19], Thus, dihydroxylations of 3- and 4-vinylbenzyl chlorides followed by coupling of the products with other chiral diols in a double-exponential manner afforded novel dendrimers with chiral polyether subunits. 

The contribution of individual chiral building blocks to the overall rotation of the dendrimer remains almost constant for flexible dendritic structures regardless of the generation number. 

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. 

Seebach and coworkers[341 successfully utilized a convergent approach to the preparation of chiral dendrimers. They wished to address three basic questions (1) Will the incorporation of chiral cores impact asymmetry to dendritic structures (2) Can enantio-selective host-guest interactions occur near the core (3) Can chiral recognition be translated to the dendritic surface ... 

Jansen, de Brabander-van Den Berg, and Meijer describe the entrapment of molecules in a dendritic box,f ° based on poly(propyleneimine) dendrimers with a chiral shell of protected amino acids (Fig. 7). The resulting dendritic structure, 5 nm in size, possesses a dense shell (as a result of bulky surface... 

Dendrimers including a chiral part, carried by either the central core or the branching points distributed in the dendritic structure, have attracted recent interest. Three research groups ventured into this area, but the main contribution was due to Seebach s team [32]. These researchers relied on Frechet s synthetic approach to first make aryl ether dendrons and then attach them to various chiral cores having either a true chiral center (Scheme 9A) or unsym-... 

Chiral dendrimers are a class of compounds which offer the possibility to investigate the impact of chirality in macromolecular systems. Their specific properties are based on their well defined highly ordered structures with nano-scopic dimension (in this report we refer to dendrimers if the molecule has a core with at least three branches attached and a defined structure otherwise we will use the term dendritic compound). 

In 1994 we published the first chiral dendrimers built from chiral cores and achiral branches [ 1,89], see for instance dendrimer 57 with a core from hydroxy-butanoic acid and diphenyl-acetaldehyde and with twelve nitro-groups at the periphery (Fig. 21). As had already been observed with starburst dendrimers, compound 57 formed stable clathrates with many polar solvent molecules, and it could actually only be isolated and characterized as a complex [2 (57- EtO-Ac (8 H20))]. Because no enantioselective guest-host complex formation could be found, and since compounds of type 57 were poorly soluble, and could thus not be easily handled, we have moved on and developed other systems to investigate how the chirality of the core might be influencing the structure of achiral dendritic elongation units. 

Other efforts based on the macromonomer approach to homopolymers having dendritic side chains, include the work of Draheim and Ritter on acrylate and methacrylate derived structures having dendritic chiral side chains based on L-aspartic esters [17a], and of Xi and coworkers with poly(methacrylate) structures containing very small benzyl ether dendritic side-chains [17b]. Unfortunately, both of these approaches met with limited success due to a significant drop in degree of polymerization (DP) when the size of the dendron used as pendant group in the macromonomers increased from G-l to G-2. 

A number of groups have reported the preparation and in situ application of several types of dendrimers with chiral auxiliaries at their periphery in asymmetric catalysis. These chiral dendrimer ligands can be subdivided into three different classes based on the specific position of the chiral auxiliary in the dendrimer structure. The chiral positions may be located at, (1) the periphery, (2) the dendritic core (in the case of a dendron), or (3) throughout the structure. An example of the first class was reported by Meijer et al. [22] who prepared different generations of polypropylene imine) dendrimers which were substituted at the periphery of the dendrimer with chiral aminoalcohols. These surface functionalities act as chiral ligand sites from which chiral alkylzinc aminoalcoholate catalysts can be generated in situ at the dendrimer periphery. These dendrimer systems were tested as catalyst precursors in the catalytic 1,2-addition of diethylzinc to benzaldehyde (see e.g. 13, Scheme 14). 

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. 

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