Catenane formation

Each catenane consists of two identical, interlocked 26-membered rings with a relatively small internal cavity (with dimensions of 4x6 A). This interlocked species was the first amide-catenane to be structurally characterised (although Hunter s and Vogtle s catenanes were reported earlier). The structure supported the proposal that the driving force for catenane formation is hydrogen bonding between the newly formed 1,3-diamine units and carbonyl groups of the acid... 

For catenane formation from two separate rings, the most reasonable explanation involves dissociation of a ring, threading another ring on the thread, and reconnection of the ends of the thread (Scheme 2 a). However, careful mechanistic consideration of the rapid interconversion suggested a Mobius strip mechanism which involves molecular topology reminiscent of the well-known Mobius strip. 

Catenane formation from fluorinated ligand 19 is apparently more effective than that from 12. This difference is most probably because of attractive edge-to-face or CH-jt interaction working in catenane 13, as observed in the crystal structure of platinum-incorporated catenane 14 (Figure 2). In this structure, we can observe efficient edge-to-face aromatic contacts between two phenylene units or a phenylene and a pyridine ring. 

Table 1. The prediction of the catenane formation on the basis of force-field calculation3.

Figure 15. The template-directed synthesis of the [2]catenanes 32-4PF6-384PF6. The insets show (i) the intermediate [2]pseudorotaxane formed during the course of catenane formation and (ii) the solid-state structure of the [2]catenane 324+.

A) the in/out isomer 14 and the out/out isomer 15 were obtained, depending on which of the two possible niches of the first formed macrocycle is used for the intercalation of the monoamide. The third possible (in/in) isomer 16 could finally be formed when the substitution pattern was reversed (pathway B). The two pathways differ in the embedding compound, and interestingly for route B, where the nesting component bears the substituent, significant steric hindrance seems to accompany catenane formation - catenane 14 was obtained in only 2.7% yield, whereas route A leads to 17% of the same catenane. 

A second experiment should prove that macromonocycles are actually the intermediate supramolecular templates in the course of catenane formation. Therefore macromonocycle 17 was reacted with 5 and 3, and the first [2]catenane 18 of the amide type consisting of two different macromonocycles was isolated (Figure 8). Unsymmetric catenanes like 18 can be identified unambiguously by mass spectrometry, because the corresponding tetrameric macromonocycle can not be formed in this reaction sequence. This confirms the presumption that catenation here proceeds via a macrocycle rather than via intertwining open chain units. 

Figure 7. Mechanism of catenane formation (amide type) the guest is orthogonally embedded in an intermediate macrocycle, the concave template. Depending on the substitution pattern of the reactants (pathways A and B) isomeric catenanes are obtained. For the sake of clarity the diacid dichloride is drawn here to be the nesting guest even though there is clear indication that the effective interactions take place between the corresponding monoamide and the macrocycle.

A third example where the modification of the building blocks hindered catenation is shown in Figure 11 [16]. When diamine 5 is reacted with 2,5-fiiranedicar-boxylic acid dichloride (23) only macrocycle 24 is obtained, whereas catenane formation seems to be negligible. 

Catenane 38 was obtained even though, according to CPK models, the cavity of its partial aliphatic macrocycle should be too tight for catenane formation. The successful synthesis of catenanes containing aliphatic chains indicates that n-n interactions are not necessarily required for pre-organization. Instead hydrogenbonding seems to be the main driving force for catenane formation. 

This conformation resembles that determined by X-ray single-crystal structure analysis. Apart from a close interaction of the -system of the amide groups with the para-substituted aryl moieties, no further n-n interactions are observed. The template-assisted catenane formation is on this account mainly driven by hydrogen-bonding, and n-n interactions are of minor importance only. 

After the synthesis of the first amide-linked [2]rotaxanes, Vogtle et al. set out to study the limits of molecular recognition, which in terms of Emil Fischer means to discover if the lock (macrocycle) is specific to a certain key, or if several keys (monoamide threads) fit. It turned out that - in contrast to catenane formation - rotaxane synthesis is very tolerant towards the variation of the building... 

Hydrophobic forces are also important in the assemblies of metallo-supramolecular catenanes. One of the most interesting examples is formed when one of the unpolar bipyridine ligands of one macrocycle is included spontaneously in the other macrocycle s internal cavity [39]. Here, the benzene unit of the one macrocycle serves as a guest molecule for the other macrocycle, and the cyclization is favored by n-n interactions. In addition, the minimization of hydrophobic surfaces in polar medium constitutes the second driving force for the catenane formation. The quantitative formation of the [2]catenanes 31a and 31b based on this principle are depicted in Figure 13. Formation of catenane 31b was found to be reversible. Even at room temperature, two monomeric ring structures equilibrate quickly due to the labile nature of Pd-N bond and interlocked molecular ring system 31b is formed. 

Scheme 10.1 Cu+-templated catenane formation first reported by Sauvage and coworkers. 

Table 10.1 Selected examples of catenane formation via quaternization of aromatic amines.

Scheme 10.6 Self-assembly of the hydrogen bond-directed [2]catenane formation by Leigh and coworkers.

Scheme 10.16 Anion-templated [2]rotaxane and [2]catenane formation reported by Beer and coworkers.

Scheme 10.25 [2]Catenane formation by metallomacrocyclic clipping reported by Quintela and coworkers.

Scheme 10.26 Metallomacrocyclic/[2]catenane formation templated by second-sphere coordination of the PdCI2 subunit reported by Wisner and coworkers.

Instead of using a metal ion as a clasp , it is also possible to use weak interactions between ring components for catenane interlocking. In the example shown in Fig. 3.25, a C-shaped precursor containing viologen-type cyclophane was complexed to a cyclophane with benzene ring moieties. The cationic bipyridine moiety in the C-shaped component was then sandwiched by electron-rich benzene units. Cyclization of the C-shaped precursor using a dibromo compound resulted in interlocked catenanes. This electronic interaction between two kinds of species results in more efficient catenane formation. 

Supramolecular interactions are an important factor in catenane formation. Such interactions can be disrupted after the catenane has been built, making the catenane structure more flexible. This flexible nature can be an advantage because the catenane structure is then free to respond when external stimuli are applied. The catenane shown in Fig. 3.26 is one example where this structural flexibility is utilized. One of the rings of this catenane contains two kinds of ligands, and the nature of the coordination to the copper ion depends on the oxidation number of the copper. When the copper ion is in the Cu(I) state, fourway coordination is stabilized. However, five-way coordination becomes more favorable upon oxidation to Cu(II), and to accommodate this, the ring rotates... 

Figure 3.27. Catenane formation through a self-assembled process...

The macrocyclic polyether, tris(l,5-naphtho)-57-crown-15 (Figure 5.12) contains a sufficiently large ring size to permit the simultaneous accommodation of two threaded 4,4 -bipyridinium units, which are then appropriately orientated for further template formation of tetracationic cyclophanes. However, the efficacy of template formation for [3]-catenane formation proved to be less than anticipated. The... 

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