Crown ether units

Frensch and Vdgtle have recently appended three crown ether units to the cyclo-triveratrylene unit . Note that Hyatt had previously prepared the open-chained relatives of this structure (see Sect. 7.3 and Eq. 7.6). Whereas Hyatt prepared the cyclo-triveratrylene skeleton and then appended polyethyleneoxy arms to it, Frensch and Vogtle conducted the condensation reaction (formaldehyde/HCl) on the preformed benzocrown. Thus benzo-15-crown-5 was converted into the corresponding tris-crown (IS) (mp 203.5—205.5°) in 4% yield. The yield was somewhat higher for the condensation of benzo-18-crown-6, but in both cases, yield ranges were observed. These species formed 1 3 (ligand/salt) complexes with sodium and potassium ions. 

Most of the compounds in this class have been prepared from preexisting crown ether units. By far, the most common approach is to use a benzo-substituted crown and an electrophilic condensation polymerization. A patent issued to Takekoshi, Scotia and Webb (General Electric) in 1974 which covered the formation of glyoxal and chloral type copolymers with dibenzo-18-crown-6. The latter were prepared by stirring the crown with an equivalent of chloral in chloroform solution. Boron trifluoride was catalyst in this reaction. The polymer which resulted was obtained in about 95% yield. The reaction is illustrated in Eq. (6.22). 

Macro-azo-initiators containing crown ether units were successfully synthesized by Yagci et al. [37,38] condensing ACPC with the c s or trans forms of 4,4 -diaminodibenzo-18-crown-6 (Scheme 8). The polymeric... 

Polyphosphazenes bearing crown ethers (12-crown-4,15-crown-5 and 18-crown-6) as single or as mixed substituents with trifluoroethoxy or methoxy-ethoxyethoxy groups were synthesized by Cowie [601,602] and Allcock [484] and their conductivity studied because it was shown that the incorporation of crown ether molecules into a polymer electrolyte could increase their ionic conductivity. In these macromolecules, the crown ether units were linked to the backbone through oxymethylene spacer groups. 

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

In compound (4) (Figure 12.15) the fluorescence of the anthracene group is quenched by electron transfer from both the crown ether unit and the amino group. The electron transfer processes from the crown ether and the amino group can be prevented by adding Na+ and H+, respectively. 

In 1990 we reported the synthesis of new redox-responsive crown ether molecules that contain a conjugated link between the crown ether unit and a ferrocene redox-active centre (Beer et al., 1990a). Examples of some of the species synthesized are shown in Fig. 5. The electrochemical behaviour of these species was investigated and also the electrochemical behaviour of their analogues with a saturated link between the ferrocene unit and the crown ether. The changes in the CVs of [2a] upon addition of magnesium cations are shown in Fig. 6. The metal cation-induced anodic shifts of [2a], [2b] and also their saturated analogue [3] and vinyl derivatives [4a], [4b] are shown in Table 1. 

FABMS has been used as a semiquantitative indication of the selectivity of receptors for particular guest metal cations (Johnstone and Rose, 1983). The FABMS competition experiment on [7] with equimolar amounts of the nitrates of sodium, potassium, rubidium and caesium gave gas-phase complex ions of ([7] + K)+ ion (m/z 809) and a minor peak ([7] + Rb)+ ion (m/z 855) exclusively. The relative peak intensities therefore suggested a selectivity order of K+ Rb+ Na+, Cs+, indicative of the bis-crown effect, the ability of bis-crown ether ligands to complex a metal cation of size larger than the cavity of a single crown ether unit, forming a sandwich structure. 

Carbohydrates remain an attractive source of chirality in preparation of ligands for asymmetric catalysis. Functionalized phospholanes, 192 [167], and chiral bisphosphinites 193 [168] with an attached crown ether unit were obtained recently from D-mannitol and from phenyl 2,3-di-0-allyl-4,6-0-benzylidene-p-D-glucopyranoside, respectively (Figure 18). Compounds 194 and 195 were obtained in the photochemical addition of H2P(CH2)3PPH2 onto the crresponding alkenes - Pd-complexes of these new bisphosphines were successfully applied as catalysts in the copolymerization of CO and... 

Several cylindrical cryptands based on other basic crown ether units containing different numbers of oxygen or heteroatoms have been prepared (80T46l,80PAC244l). The synthetic strategies are generally the same. 

Wozniak and coworkers described recently the first heterodinuclear bismacrocyclic transition metal complex 34 + (Fig. 14.5) that exhibits potential-driven intramolecular motion of the interlocked crown ether unit.25 26 Although the system contains transition metals, the main interaction between the various subunits, which also allowed to construct catenane 34+, is an acceptor-donor interaction of the charge transfer type. 

The reported heterodinuclear catenane should allow a controlled translocation of the crown ether unit back and forth between two different metal centers in response to an external stimulus, specifically a potential applied to the electrode (Fig. 14.6). 

The introduction at the C- or N-terminal position of a crown ether unit has been used as a strategy to control the aggregation of poly(benzyl glutamate) derivatives 19 The incorporation of the crown unit at the C-terminal position is performed using (benzo-15-crown-5)-4-amine as initiator of the polymerization of l-G1u(OBz1)-NCA. Physical properties of such crown derivatives can be modulated by the formation of sandwich 2 1 complexes driven by the addition of specific alkali metal ions. In the reported case, the formation of K+ sandwich complex between two C-terminal benzo-15-crown-5 modified helical polypeptides induced aggregation. In a similar approach,f20 addition of Cs+ to 18-crown-6 terminated helical peptides results in the formation of supramolecular assemblies having membrane ion conductivity activities. 

The general design concept for liquid crystalline crown ethers discussed in this section consists of a rigid core with terminal chains and a laterally attached crown ether unit (Fig. 3). 

In the crown ether derivatives 33, only monotropic phases were observed. All compounds exhibited a very small nematic phase with a relatively broad smectic phase. Decreasing the size of the crown ether unit resulted in higher clearing points... 

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]. 

Ushakov, E.N., Gromov, S.P., Fedorova, O.A., Pershina, Y.V., Alfimov, M.V., Barigelletti, F., Flamigni, L., Balzani, V. (1999) Sandwich-Type Complexes of Alkaline-Earth Metal Cations with a Bisstyryl Dye Containing Two Crown Ether Units, J. Phys. Chem.A, 103, 11188-11193. 

Probably the best known early crown ether example is the chundle reported by Jullien and Lehn (Jullien and Lehn, 1988). Their strategy used a central crown ether unit with sidearms radiating from it. The stereochemistry of the sidearms was fixed by incorporation of tartaric acid units within the macrocycle. The name was given because the compound was a channel formed from a bundle of fibers. In this first report, no information about insertion or transport appeared, and the assertion that the compound was a channel apparently rested on the intent of the design. Later work from this group showed that related compounds, called bouquet molecules, did conduct cations, albeit rather slowly (Canceill etal., 1992). 

Receptor 14b was designed for ion pair recognition as it also contains a cavity suitable for cation complexation (ester groups on the lower rim). Similar recognition ability [17] was found for derivative 15 bearing additional crown ether units. In both cases the relationship between the cation present and the binding of anions was studied. 

The incorporation of crown ether units into a cobaltocenium receptor has been shown to allow the switchable sensing of anions. Proton NMR titrations of receptor 8 in CD3CN solution gave log K values of 3.1 for chloride and 3.0 for... 

By introducing a crown ether unit at the C-terminal region of hydrophobic helical peptides, Otoda et al.19 were able to demonstrate increased stabilization of the peptide aggregate in the membrane by the formation of sandwich-type complexes with large cations. Ion channel activity was also increased due to the ability of the crown peptide to bind ions to the terminal portion of the hydrophobic helix bundle at the water-lipid interface. Ueda et al.20 considered the problem of insolubility of hydrophobic peptides which restricts the distribution of peptides in water to a phospholipid bilayer membrane. In consequence they constructed a hydrophobic helix bundle shielded by hydrophilic peptides that acts rather like an umbrella. 

Langmuir films of rod-shaped amphiphilic ionophores 9 with laterally grafted crown ether units of different ring size (n = 1—3) have been successfully exploited to investigate alkali metal ion molecular recognition processes at the air-water interface <1998L5245>. 

Dialkyl-substituted anthracene-bridged bis-crown ethers 92 formed 1 1 and 1 2 (crown to metal) complexes with alkali metal ions <1999J(P2)1193>. The stability constants of the latter were suggestive of a negative cooperation effect between the two crown ether units < 0.25). The derivative with R = Et showed a decrease in... 

The photochromism of spirobenzopyrans is a well-documented phenomenon that arises from the photoinduced reversible isomerization between spiropyran and merocyanine forms . In spirobenzopyrans carrying a crown ether moiety (e.g., Ill), this interconversion process is affected by metal ion complexation. A strong interaction of the crown ether unit with a metal ion caused the thermal isomerization of the spirobenzopyran residue to the corresponding merocyanine form with simultaneous suppression of the UV-induced isomerization process (negative photochromism) (Scheme 3). Conversely, a weak metal ion interaction induced a positive photochromism <2001JOC1533, 2002EJ0655>. 

Di- and polytopic host-guest systems have provided a convenient starting point for the construction of larger assemblies and many systems of this type are now known. For example, in an early study Kimura et synthesised the catecholamine complex 17. The crown ether unit of this ditopic host was known to be an effective receptor for primary alkyl ammonium salts, whereas the partially protonated form of the hexamine ring had been documented to bind anionic substrates (such as car-boxy lates) or electron-donor substrates (such as catechols). Accordingly, this host forms stable 1 1 complexes with zwitterionic guests such as amino acids, pep-... 

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