Crown dendrimers

Figure 5.5. Reduction of the carbonyl moieties of the crowned arborols (Scheme 2.24) afforded these more lipophilic crowned dendrimers.

Scheme 9. Another convergent strategy for the preparation of crown dendrimers.

As shown in Section H.B., it was possible to use an aza crown as the core of a new type of dendrimer. So we tried to combine our new stategy in synthesizing TREN cascades with the experience we collected in synthesizing our crown dendrimer (44). We selected cyclam (59), as the... 

Experiments are in progress to generate information about the complexing behavior of these crown dendrimers. One of the interesting aspects will be the influence of the dendritic surrounding on the selectivity of the aza crown. 

Multi-crown dendrimers (Fig. 3.7-12) were found to exhibit good solubihty in solvents of low polarity, making them attractive as extractants for separation processes. The combination of protonated tertiary amine groups as anion binding sites together with cation-active crown ether moieties represents an approach for the simultaneous binding of cations and anions. Extraction studies performed with sodium pertechnetate and mercury(II) chloride have shown that the guest molecules are mainly bound in the interior of the protonated polyamine skeleton [18]. 

Figure 42 The formation of crowned dendrimers containing intenvoven structures at the surface.

Template effects have been used in rotaxane synthesis to direct threading of the axle through the wheel. Since macrocycHc compounds such as cyclodextrins, crown ethers, cyclophanes, and cucurbiturils form stable complexes with specific guest molecules, they have been widely used in the templated synthesis of rotax-anes as ring (wheel) components. Here, we briefly discuss macrocycles used in the synthesis of rotaxane dendrimers and their important features. 

Fig.1. Rotaxane dendrimers from crown ether type macrocycle ring, bipyridinimn derivatives rod component and Frechet-type dendron stoppers...

Fig. 14. Rotaxane dendrimers from crown ether-terminated polypropylimine dendrimer complexation with dibenzylammonium ion...

In a way related are the complexes formed by Hg salts and multicrown dendrimers of different generations (dendrimers with a polypropylene amine interior of different volume and benzo[15]-crown-5 ether periphery), studied by extraction methods using radioactive 203Hg2+.210 Up to 12 Hg2+ ions were found to be bound per dendrimer molecule, obviously in the amine-dominated interior, not in the crown-ether periphery. 

Fig. 5. Dendrimer 22-[G4] generation 4,48 terminal crown ether units...

In contrast to Bosman et al., who only found metal complexation in the periphery of polypropylene imine) dendrimers, Tomalia and co-workers reported on the incorporation of copper ions into the interior of PAMAM dendrimers judging from EPR and UV/Vis studies [220, 221]. Metal binding in the dendrimer interior has also been observed for dendrimers carrying multiple ligands for metal complexation within their framework such as crown-ethers [222, 223] (Cs(I)-complexes), piperazine [224] (Pd(II)- and Cu(II)-complexes) or triazocyclononane [225] (Cu(II)- and Ni(II)-complexes). In most cases addition of the metal-salt to the dendrimer led to the formation of 1 1 complexes. 

Supramolecnlar derivatives containing crown ether [228], calixarenes [264, 265] or dendrimers [267, 279] have been synthesized with the precursors shown in Table 4.10. Fullerene-flavonoid [268] or the fuUerene glycoconjugate [258] derived from 237 and 235, respectively, are expected to show biological activity. 

Porphyrin-based self-assembled molecular squares 389 can form mesoporous thin films in which the edge of a square, thus the size of the cavity, can be adjusted by appropriate choice of substituents [8]. Fibers that form coil-coiled aggregates with distinct, tunable helicity are built from crown ethers bearing porphyrins 390 [9]. In addition to the porphyrin applications discussed in Sections 6.3.2.2 and 6.4, dendrimer metalloporphyrins 391 to be applied in catalysis [10] and the water-soluble dendritic iron porphyrin 319 modelling globular heme proteins [11] can be mentioned. 

It also proved possible to attach aza-crown ethers to the periphery of a POPAM dendrimer via amidoferrocenyl units (Fig. 4.5) [9a] and also to decorate the outer shell of a second-generation POPAM dendrimer alternately with eight F-stilbene and eight 4-tert-bulylbenzenesulphonic acid units [9b]. Various other... 

Fig. 4.5 POPAM dendrimer with terminal aza-crown ether and ferrocene units...

Carbosilane dendrimers functionalised eight-fold with crown ethers (Fig. 4.45) can accommodate a total of eight potassium ions in their peripheral crown ether units [86]. 

On-chip chiral separation of enantiomers of adrenaline, noradrenaline, and dopamine was achieved using a mixture of carboxymethyl-p-cyclodextrin (CMCD) and a polyamidoamine dendrimer (Starburst) [120], On-chip chiral separation of enantiomers of homovanillic acid, DOPA, cDOPA, methoxy-tyramine (MT), metanephrine, and normetanephrine was achieved using a mixture of 18-crown-6-ether and carboxymethyl-P-cyclodextrin [120],... 

Fig. 119. (Top) Crown-cryptand for the analysis of barium and ethylenediamine ligand for anion sensing (tpen). (Bottom) Dendrimer-like ligand for anion sensing.

A special type of ABA block copolymer is formed when dendrimers are used as stoppers on rotaxanes. Rotaxanes are linear polymers onto which one or several large macrocycles are threaded. In order to prevent unthreading, sufficiently bulky groups must be attached to the two ends of the rotaxane. The [G-3] po-ly(benzyl ether) dendrimer proved to be highly suited for trapping [128] (see Scheme 16). The linear polymer is made of stiff bipyridinium groups that spontaneously assemble with diparaphenylene-34-crown-10 to form the rotaxane. The dendrimers are attached via quaternation of pyridine with benzyl bromide... 

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