Chirality periphery

The molar ellipticity of these dendrimers was found to increase proportional to the number of chiral end groups. This is to be expected, in the absence of interactions between the terminal tryptophane moieties. No higher-generation dendrimers of this type have been reported. Other amino-acid-containing chiral dendrimers have been described by Meijer et al. who attached various amino acid derivatives to the periphery of poly(propylene imine) dendrimers (see Sect. 3) and more recently by Liskamp et al. (modification of polyamide dendra) [22] and Ritter et al. (synthesis of grafted polymerizable dendrimers containing L-aspartic acid components) [23]. 

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. 

To catalyze asymmetric transformations, catalytically active sites can be incorporated in different areas of a dendrimer a) chiral sites at the periphery, b) chiral sites in cavities or at the core, c) achiral sites which are surrounded by chiral branches in the interior of the dendrimer. 

The latter effect has been demonstrated by Meijer et al., who attached chiral aminoalcohols to the peripheral NH2-groups of polypropylene imine) dendrimers of different generations [100]. In the enantioselective addition of diethyl-zinc to benzaldehyde (mediated by these aminoalcohol appendages) both the yields and the enantioselectivities decreased with increasing size of the dendrimer (Fig. 28). The catalyst obtained from the 5th-generation dendrimer carrying 64 aminoalcohol groups at its periphery showed almost no preference for one enantiomer over the other. This behavior coincides with the absence of measurable optical rotation as mentioned in Sect. 3 above. The loss of activity and selectivity was ascribed to multiple interactions on the surface which were... 

In our group, dendrimers carrying the catalytically active part either on the periphery or in the core were investigated. In both cases a,a,a, a -fetraaryl-l,3-dioxolane-4,5-dimethanoZs (TADDOLs) have been employed as ligands in chiral... 

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). 

Brunner et al. [26] synthesized and applied so-called dendrizymes in enan-tioselective catalysis. These catalysts are based on dendrimers which have a functionalized periphery that carries chiral subunits, (e.g. dendrons functionalized with chiral menthol or borneol ligands). The core phosphine donor atoms can be complexed to (transition) metal salts. The resultant dendron-enlarged 1,2-diphosphino-ethane (e.g. 16, see Scheme 17) Rh1 complexes were used as catalysts in the hydrogenation of acetamidocinnamic acid to yield iV-acetyl-phenylalanine (Scheme 17) [26]. A small retardation of the hydrogenation of the substrate was encountered, pointing to an effect of the meta-positioned dendron substituents. No significantly enantiomerically enriched products were isolated. However, a somewhat improved enantioselectivity (up to 10-11% e.e.) was... 

Their results show that the chemical yields of 1-phenylpropanol and the enantioselectivities decreased with increasing generation of the dendrimer when G5 was used, almost no enantioselectivity was observed (57% yield, 7% ee), a result that was attributed to the dense packing of the chiral end groups at the periphery. 

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). 

The ring system of vitamin B12, like that of porphyrins (Fig. 16-5), is made up of four pyrrole rings whose biosynthetic relationship to the corresponding rings in porphyrins is obvious from the structures. In addition, a number of "extra" methyl groups are present. A less extensive conjugated system of double bonds is present in the corrin ring of vitamin B12 than in porphyrins, and as a result, many chiral centers are found around the periphery... 

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]. 

Alternatively, dendrimers with chirality in the molecular periphery can also be constructed by the convergent approach starting from chiral monomeric units. 

Fig. 4.75 POPAM dendrimer with chiral Boc-protected amino acids in the molecular periphery (according to Meijer et ai.)...

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. 

A further question which may be answered by X-ray is that of planarity. Recent papers206,207 disclose that of the thousands of possible PAHs with 10 rings, the vast majority should be nonplanar, even chiral, structures resulting from steric factors. Highly notable examples of nonplanar PAHs include helicenes, fullerenes, bowl-shaped components of fullerenes, and nanotubes.206,207 Several intriguing species have been prepared in which the rings are forced out of planarity due to intramolecular steric interaction between substituents. These interactions become important when the angles by which substituents protrude from the sp2 periphery are such that they would approach adjacent substituents closer than allowed by van der Waals radii. 

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