Polyether template

The template-directed preparation of cycloi is(paraquat-4,4 -biphenylene (a molecular square ) has been achieved the use of a macrocyclic hydroquinone-based polyether template incorporating an ester moiety in one polyether chain afforded a 1 1 mixture of two topologically stereoisomeric [3]catenanes <96CEJ877>. 

The corresponding n=4 polyether template 40 was synthesized according to Scheme 8-14. Coupling of the methyl ester of protocatechuic acid (39) with two equivalents of the diastereomerically pure tosylate 38 gave 40 in 83% yield. 

We have not attempted to cover all or even most aspects of crown chemistry and some may say that the inclusions are eclectic. We felt that anyone approaching the field would need an appreciation for the jargon currently abounding and for the so-called template effect since the latter has a considerable bearing on the synthetic methodology. We have, therefore, included brief discussions of these topics in the first two chapters. In chapters 3—8, we have tried to present an overview of the macrocyclic polyethers which have been prepared. We have taken a decidedly organic tack in this attempting to be comprehensive in our inclusion of alkali and alkaline earth cation binders rather than the compounds of use in transition metal chemistry. Nevertheless, many of the latter are included in concert with their overall importance. 

As discussed in the previous section, perfluorinated polyether pony-tails can be covalently grafted onto PPI dendrimers, and it has previously been shown that such materials are soluble in liquid CO2 [17]. HRTEM confirms that particles prepared within such dendrimer templates are nearly mono-disperse and are of a size that is consistent with the dendrimer template (Sect. 2.5.4). Such dendrimer-encapsulated metal particles are quite soluble in liquid and SC CO2. 

The reaction sequence outlined in Scheme 12 illustrates how macrocyclic polyether-thiono diesters such as RR)-lfi6 can be prepared (184) from 0,0-dimethyl 2,6-pyridinedicaibothiolate and (RR)-S4, Potassium thiocyanate forms a 1 1 crystalline complex with (RR)-1S6 and presumably the potassium ion serves as a template for the (1 -I-1) cyclization. Raney nickel desulfurization of (/ R)-186 yields the chiral pyridino-18-crown-6 derivative RR)-191. 

Higher yields are obtained when the reaction rate is increased by using dipolar aprotic solvents, with the possible operation of a template effect (77) of a precomplexed cation (64). Again, this shows that high dilution techniques are not necessary in many cases, but they would certainly increase the yields. Synthesis of polyether-sulfides are generally facilitated by the greater reactivity of — S compared to — O- in nucleophilic displacement reactions. However, the yield is low when path b) is followed (74). 

Porphyrin-containing [2]catenanes have been synthesized by Gunter et al.,1311 following the template-directed synthetic strategy outlined in Figure 15. The bis(hexa-fluorophosphate) salt 15-2PF6 was treated with the dibromide 16 in the presence of the previously formed macrocyclic polyether 22 or 23. After counterion exchange,... 

In this system, however, intermolecular dimerization may take place competitively with intramolecular dimerization. To rule out this possibility, compound 5, in which two anthracenes are linked by two polyether chains, was synthesized.171 It was found that intramolecular photodimerization proceeds rapidly in the presence of Na+ as the template metal cation. Compound 6 was also synthesized.181 Although this compound has not been applied in a photoswitch system, it displays a remarkable fluorescence change upon binding with RbC104 or H3N+(CH2)7NHj.[81 Yama-shita et al.[9] also synthesized 7, in which intermolecular photodimerization of anthracene is completely suppressed. The photochemically produced cyclic form 8 displayed excellent Na+ selectivity. 

Reinhoudt and co-workers (101-105) have reported a series of Schiff base macrocyclic polyether ligand complexes prepared via barium cation-templated Schiff base condensation of the appropriate polyether dialdehyde with a diamine, in the presence of a transition metal or uranyl acetate, followed by removal of the Ba2+ template cation on subsequent addition of guanidinium sulfate (Scheme 19). The copperdl) and nickeldl) complexes (62) and (63) exhibit reversible redox couples... 

It is important to appreciate that these ditopic macrocycles contain a polyether moiety on one side and present a diamido motif on the other. Addition of a charged pyridinium cation to the substrate-free form of these macrocycles then generates a highly efficient receptor for chloride, presumably as the result of both template-induced organization and electrostatic effects. The further incorporation of hydro-quinone groups and polyether functionalities into the cyclic framework presumably contributes to the stabilization of the cationic pyridinium component within the final interpenetrated structure. Interestingly, while titration of the pyridinium cation building block alone revealed a preference for oxoanions, the final receptor-anion... 

The macrocyclization reaction described above has been used to generate a great number of catenane (12) and rotaxane (13) architectures (Scheme 10.4) using both crown ethers [preformed macrocyclic components 14 (strategy A)] and hydroqui-none-based dumbbell-shaped polyethers [preformed acyclic components 15 ( clipping )] as templates [14b, 15]. These templates are also relatively robust with regard to the substitution of different groups into both the tetracationic cyclophane and the neutral frameworks. 

A great variety of polyether macrocycles, containing from 3 to 20 oxygen atoms and with ring sizes from 9 to 60, have been prepared by Pedersen (105-108). The synthetic routes used in cyclic polyether formation all require NaOH, but a template role for the Na+ ion has not been established ... 

Four donor acceptor [2]catenanes possessing cyclobis(paraquat-p-phenylene), as the 7i-electron acceptor, and 1,5-dioxynaphthalene-containing macrocyclic polyethers, as the 7i-electron donor moieties, have been constructed employing Cu+-catalyzed Huisgen 1,3-dipolar cycloaddition and Cu+2-mediated Eglinton coupling condition in the final step <07JA8236>. Desymmetrized [2]catenanes were synthesized by means of a template synthesis of pseudorotaxanes between 7i-rich crown ethers and a 7i-deficient pyromellitic subunit,... 

The cholates 8.117-8.119 were designed for the preparation of dynamic hbraries with different binding affinities for alkah metal ions. The presence of a polyether chain in position 7 of 8.117 provided a recognition element for metal binding that was absent from the disubstituted p-methoxybenzyl substitution pattern of 8.118, while the 7-deoxy derivative 8.119 was even less prone to metal coordination. The three monomers were submitted to transesterification/cyclization protocols, either without metal templates or using different alkali metal salts as templates. The relative abundances of cyclic dimers, trimers, tetramers, and pentamers for each experiment are reported in Table 8.7. 

Thus, cationic polymerization of oxiranes is of little synthetic value, if the preparation of linear polymers is attempted. The high tendency for cyclization may be employed, however, for preparation of macrocyclic polyethers (crown ethers). Polymerization of ethylene oxide in the presence of suitable cations (e.g., Na+, K+, Rb +, Cs + ) leads to crown ethers of a given ring size in relatively high yields, due to the template effect [105], Thus, with Rb+ or Cs+ cations, cyclic fraction contained exclusively 18-crown-6. 

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