Dendrimer optically active

The conformational flexibility and the lack of difference of the electronic properties of the polyether branches in 32 have been forwarded to explain this zero rotation. Therefore a similar dendrimer 34 has been prepared which carries a more sterically demanding branch, leading to a more rigid structure [66] interestingly, this dendrimer indeed exhibited a very small but measurable optical activity, which underlines the thesis that nanoscopic chirality depends on the rigidity of the investigated structure. 

When f-butoxy methoxy benzyl acetate groups are attached to the same dendrimers, which are similar in shape to but less dependent on the solvent than the phenylalanine moieties, a roughly constant optical rotation per end group is obtained for all generations [2]. In this case the contributions to optical activity of the end groups seem to be additive and insensitive to differences in packing. 

In another experiment, alkyl chains have been introduced as spacers between the surface NH2 groups of the dendrimer and the N-BOC-(S)-phenylalanine groups. In this case, too, the optical activity per end group remained constant for both, the dendrimer of the 1st (with four end groups) and of the 5th generation... 

Fig. 15. Optical activities of poly(propylene imine) dendrimers, functionalized at the periphery with protected phenylalanine or f-butoxy methoxy benzyl acetate groups, depend on the number of end groups [2]...

Vogtle et al. have prepared chiral poly(imine) dendrimers of various generations by condensation of non-racemic 5-formyl-4-hydroxy[2.2]paracyclophane moieties with poly(amine) dendrimers [71]. They have found that the optical activity of these dendrimers was nearly constant with increasing generation number. 

McGrath et al. have also thoroughly studied the chiroptical properties of dendrimers such as 40. They compared the optical activities of the series of 1st-, 2nd- and 3rd-generation compounds of type 40, considering the molar rotation per chiral unit ([ ]D/n) [75]. A big difference of the values was found between the generations which could possibly indicate chiral conformations inside the dendrimers, that enhance the optical rotation values per unit when... 

Following the convergent procedure, dendrimers of type 58,59 and 60 have been prepared from the chiral core triol 54 and achiral Frechet-type [62] ben-zylic branch bromides. In the series of dendrimers with aromatic spacers (60) and without spacers (58), the optical activity [a]D decreased on going from the 1st (not shown in Fig. 21) to the 2nd generation, whereas with aliphatic spacers... 

Comparison of the optical activity showed that the dendrimer 63 with branches of (S)-configuration has a specific rotation and a molecular ellipticity which clearly deviate from the expected values [88,90]. All other 2nd-generation dendrimers (even those with additional spacers between the branches and the core) have specific rotations that are comparable to those expected by simple addition of appropriate values for their building blocks. The deviation may therefore signal the presence of chiral conformational substructures in the 2nd-generation dendrimer 63. 

Pugh VJ, Hu QS, Zuo X et al (2001) Optically active BINOL core-based phenyleneethyny-lene dendrimers for the enantioselective fluorescent recognition of amino alcohols. J Org... 

Metallocenes have frequently been used as terminal moieties in dendrimer chemistry - as already demonstrated in previous sections. They are of interest primarily because of their potential application in catalysis [123]. An unusual metallodendrimer with peripheral ferrocene entities and optically active ferro-cenyldiphosphine ligands (josiphos ligands) was prepared by Togni et al. (Fig. 4.58) [124]. Adamantanetetracarboxlic acid was one of the core units employed. 

The first dendrimers with chiral cores for studies on the influence of the stereo-genic centres of a core unit on the chiroptical properties of the overall molecule were presented by Seebach s group [18]. These workers first synthesised dendrimers based on a chiral tris(hydroxymethyl)methane core unit. To these were attached zeroth- to second-generation Frechet dendrons, either directly or separated from the core by an aliphatic (n-propyl) or an aromatic spacer (p-xylylene) (Fig. 4.62). Remarkably, the dendrimers with aliphatic spacer showed no significant optical activity. This loss of chiral information was attributed to a dilution effecf, resulting from linkage of the achiral dendron to the chiral core unit,... 

The local chirality of a core unit is not necessarily manifested in a macroscopic chirality of the dendrimer. This is clearly apparent from the example of the dendrimers constructed by Meijer et al. from a glycerol unit as chiral core unit and four Frechet-type dendrons of different generations (Fig. 4.66). No optical activity could be measured with available instrumentation for the two enantiomeric forms (S)-l and (i )-l [2 a]. 

Information about transmission of chirality in dendrimer molecules also came from various generations of dendrimers derived from optically active alkaloids. Starting from atropine or quinine, corresponding ammonium salts 1 or 2, respectively, of up to the third generation were synthesised by quatemisation at the aliphatic nitrogen with Frechet-type dendritic benzyl bromides (Fig. 4.67) [22]. 

Fig. 4.66 While no optical activity could be measured for the enantiomers of the cryp-tochiral dendrimer 1 (with 3,5-substituted...

They functionalised up to second-generation polyether amide dendrimers (ar-borols see Chapter 1) with enantiomerically pure tryptophan units (Fig. 4.74). The measured optical activities per chiral terminal group were roughly constant for all molecules of this dendrimer series. 

In addition, chiral dendrimers (see Section 4.2) can be resolved with the aid of HPLC into their enantiomers, if the silica gel material used as stationary phase has optically active substances bound to its surface [9]. Since the chiral stationary phase (CSP) [10] undergoes different intensities of interaction with the enantiomeric dendrimers, they are retained to different degrees, and in the ideal case two completely separated (baseline separated) peaks are obtained. This separation technique was successfully applied inter alia to racemic mixtures of planar-chiral dendro[2.2]paracyclophanes, cycloenantiomeric dendro[2] rotaxanes, topologically chiral dendro[2]catenanes [11] as well as topologically chiral, dendritically substituted molecular knots (knotanes) [12] (Section 4.2.3). 

Chiroptical methods used in dendrimer research exploit the optical activity as a characteristic property of chiral dendrimers for characterisation of their structures. 

Brunner et al. reported core-functionalised dendrimer metal catalysts ("dendri-zyme ) for use in enantioselective catalysis [lb, 10, 11]. Optically active dendrimer ligands were used, for example for the reaction of styrene with ethyl diazoacetate in a Cu(I)-catalysed enantioselective cyclopropanation. The starting compound for the preparation of these ligands was L-aspartic acid. However, (IS,2 S)-2-amino-1-phenyl-1,3-propanol can also serve as reactant for the synthesis of optically active dendrimer ligands. 

In addition, sulfonamides bear more acidic, compared to carboxamides, protons that can be selectively A-alkylatcd or V-acylatcd. Thus, alkylation of sulfonamide catenane and rotaxane with Frechet-dendrons of 2nd generation led to the first representatives of the dendrocatenane 15 and dendrorotaxane 16 [46], respectively. These compounds were of especial interest, since they allowed for the first time to study chiral induction of topologically chiral cores on appended dendrons and to compare to the analogous centrochiral dendrimers for which phenomenon of the crypto-optical activity was postulated [47],... 

The octaester 54 was then treated with additional activated ester 50 to afford an A-pro-tected dendrimer 55 possessing 16 terminal ester moieties and 15 chiral centers, all with the identical L-configuration. No optical activity data were reported. 

Brunner and Fiirst[431 reported the synthesis of a series of optically active, extended chelate phosphines, one of which was derived from the reaction of 5-bromo-l,3-di(bor-neoxymethyl)benzene (69) and bis(dichlorophosphino)methane (70). Technically, the product (-)-l,l-bis 3, 5 -di(borneoxymethyl)phenyl]phosphino methane (71) is both a first tier P- and 1,3-aryl-branched dendrimer. Excluding distortions, the maximum distance between the P atom and the most distant H atom(s) is 11.9 A for the two-layer ligand 71. The Rh-catalyzed [Rh(cod)Cl]2) hydrogenation of (a)-/V-acetamidocinnamic acid in the presence of 71 afforded a disappointingly low [5.2 % ee (/ )] optical induction. 

Scheme 7.15. First generation phosphine-core dendrimers possessing optically active (—)-borneol terminated groups.

Reactions 1-3 produce intermediates which can be hydrolyzed to give dendrimers with anionic surfaces. Intermediates derived from reaction 11 yield cationic surfaces after quaternization. Dendrimers possessing chiral surfaces were readily prepared from optically active epoxides, as in reaction 4. Hydrophobic surfaces result from reactions 4-10, 11, and 20-22. The resulting dendrimers are soluble in organic solvents, although their interiors might be quite hydrophilic. Conversely, water-soluble dendrimers are obtained from reactions 1-3 (after hydrolysis), 4 (R = H), 14, and 16-19. 

Terunuma and coworkers reported the properties of a different type of carbosilane dendrimer (Nq = 3, Wb = 3) bearing either cyanobiphenyl species or an optically active unit derived from 2-phenylpyrimidine at the periphery (Fig. 15). Only three branches emanated from the central silicon atom, the fourth valency being blocked by a phenyl group leading to 3 (GO), 9 (Gl), and 27 (G2) terminal functional units. 

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