Neutral Catenanes

The first of the above [2]-catenane systems was prepared by a one-pot double-macrocyclisation reaction to give the linked product in 34% yield, with the other (non-catenated) products from this reaction being the corresponding single and 

Ottens-Hildebrandt, M. Nieger, K. Rissanen, J. Rouvinen, S. Meier, G. Harder and F. Vdgtle, J. Chem. Soc., Chem. Commun., 1995, 111. 

The [2]-catenane derived from the furano-containing macrocycle 41 was demonstrated to exist as a mixture of isomers of the types 42 and 43. No evidence for 

Vogtle, R. Jager, M. Handel and S. Ottens-Hildebrandt, Pure Appl. Chem., 1996, 68, 225. 

A number of related catenanes, including systems assembled directly from commercially available precursors and a system based on sulfonamide linkages, have also been described. 

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Hydrogen-bonding in neutral rotaxanes The independent discovery in 1992 by Hunter [25], and Vogtle and coworkers [26, 102] of catenanes made of interlocked amide cyclophanes led to the development of a novel family of neutral catenanes and rotaxanes. In an example of rotaxane synthesis [103] macrocycle (38) and isophthaloyl dichloride (39) were mixed in dichloromethane, and the trityl amine... 

Fujimoto and coworkers used a self-assembled Janus [2]-pseudorotaxane406 to obtain a Janus[2]rotaxane 407 (Figure 8.2.4) [33]. A quadruply stranded alkaline earth metal containing helical catenate, a charge-neutral heterotopic homodinuclear [2]catenane, was reported by Castro et al. [34]. 

Figure 25. The self-assembly of the neutral [2]catenanes 73 and 74 and the solid state structure of the [2]catenane 73.

Scheme 10.23 Strong ion-pairing in the pyridinium salt in non polar solvents allows the coordination of a neutral hydrogen bond donor and hence the creation of the precursor to a crossover in a catenane using the chloride ion as an auxiliary linkage.80...

However, produced a neutral [2]catenane (Scheme 11.5) [lib] bearing a mechanically interlocked bis-NDI macrocycle in 53% yield. This yield is far more attractive for a bis-NDI macrocycle than the 10% yield originally obtained in macrocycles [30a]. This approach involved an oxidative coupling of two acetylenic naphthalene diimides in the presence of crown ether. The efficiency of the catenation can be attributed to the crown ether component acting as a permanent template for the forming cyclophane. 

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. 

Sanders and coworkers have used RCM as the final ring-forming step in their neutral donor/acceptor -stacking templated catenanes (64 and 65) with both pyr-omellitimide (66) and naphthodiimide (67) acceptors (Scheme 10.12) [48]. The addition of Lil during synthesis of the pyromellitimide-containing [2]catenane... 

Complexation/decomplexation of metal ions or of neutral organic molecules, protonation/deprotonation reactions, and oxidation/reduction processes can all be exploited to alter reversibly the stereoelectronic properties of one of the two recognition sites, thus affecting its ability to sustain noncovalent bonds [30-34, 41]. These kinds of switchable [2 catenanes can be prepared following the template-directed synthetic strategy illustrated in Figure 5, wherein one of the two macrocyclic components is preformed and then the other one is clipped around it with the help of noncovalent bonding interactions. 

For the electrochemical charaterization of a neutral [2]catenane incorporating i-electron-rich and Tj-electron-deficient components, see D.G. Hamilton, M. Montalti, L. Prodi, M. Fontani, P. Zanello, J.K.M. Sanders, Chem. Eur. J., 2000, 6, 608. 

Catenanes and rotaxanes as components of supramolecular systems self-assembled by donor-acceptor interaction between neutral organic subunits 04MI27. 

Fig. 27. The rationale behind the design of an electrochemically switchable [2]catenane. The neutral form of TTF is bound very strongly within the cyclophane CBPQT4+. However, when it is oxidized to either of its cationic forms, TTF shows no affinity whatsoever for the cyclophane. 1,5-Dimethoxynaphthalene exhibits moderate binding toward the cyclophane. All binding constants Ka were measured in acetonitrile at 298 K...

Fig. 29. The UV-Vis spectrum of [2]catenane 21-4PF6 shows - a by virtue of a strong charge transfer band at 830 nm - exclusive occupancy of the cyclophane cavity by the TTF unit, b Addition of one equivalent of Fe(Cl04)3 oxidizes the TTF unit to its radical cation, which results in the circumrotation of the crown ether, such that the 1,5-dioxynaphthalene unit resides within the cyclophane s cavity, as indicated by the appearance of a charge-transfer band at 515 nm. c Addition of a further equivalent of iron perchlorate yields the TTF dication which remains outside the cavity. Chemical reduction of the TTF dication back to its neutral form yields the [2]catenane in which the TTF resides exclusively within the cyclophane s cavity...

Although there have been several approaches to porphyrinic catenanes and rotaxanes [46-48], these have been based on assembly through charged [49] or neutral [50,51] 7r-donor/acceptor concepts, or by using coordination chemistry [47]. Nevertheless, there is a series of non-porphyrinic amide-based catenanes and rotaxanes that have received considerable attention [52,53]. The primary recognition factor in these systems is H-bonding. Gunter [54] has extended these systems to include porphyrinic supramolecu-les that use similar H-bonded amide-based motifs for the self-assembly of multiporphyrin catenanes and rotaxanes. 

Since the work of Pedersen, Lehn and Cram, an enormous amount of research has been conducted using macrocycles for binding cation, anion and neutral species, sensing and catalysis, as well as the synthesis of self-assembled rotaxanes, catenanes and knots (see Chapter 3). Subsequent sections give a brief overview of some of the most important classes of compound. 

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