Catenanes anion-templated

The recently developed anion template methodology developed by Beer and coworkers (Scheme 10.16) has been applied in conjunction with RCM to synthesize both [2]rotaxanes (81) and catenanes (82) from their respective starting materials (83-85) [55,56], In this case, the two organic components are organized orthogonally around... 

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

Molecular interlocked systems such as catenanes and rotaxanes can also be prepared using anion-directed approaches. Although anionic templates did not make their way into this area until relatively recently, nowadays there are several examples that demonstrate the utility of this approach for the synthesis of this topologically interesting species. 

Using the same general strategy described above. Beer reported the first anion-templated synthesis of catenanes [43,44]. Mixing macrocycle 21 with... 

Figure 3.36 An anion-templated [2]-catenane (3.19) formed by post-synthetic modification to a pseudorotaxane precursor.

Anionic templation in general, and of interlocked structures in particular, is an immature field, certainly in comparison to cationic templation." An anion-binding [2]catenane 43 was reported by Sessler et al. in 1998 (Figure 21). It was found to bind anions such as fluoride, chloride, and dihydrogen phosphate strongly in <(2-tetrachloroethane, but addition of these anions during the synthesis of catenane 43 failed to increase the very low yields (<5%) of catenane formation." ... 

With the aim of exploiting the unique interlocked cavities of catenanes for anion recognition and sensory applications. Beer et al. synthesized the first example of an anion-templated Plcatenane." Following the observation by Crabtree that a neutral isophthalamide unit will form... 

Figure 23 Syntheses of anion-templated [2]catenanes by double clipping (a) chloride-templated [2]catenane and (b) sulfate-...

SCHEME 17.3 Anion templated synthesis of [2]catenane and its closure by ring closing metathesis (RCM). ... 

Pseudorotaxanes are precursors of both rotaxanes and catenanes they consist of a guest molecule threaded through a macrocyclic host. Stoppering both ends of the threaded molecule gives a rotaxane, cycliza-tion of the thread gives a catenane. Pseudorotaxane formation may occur by spontaneous self-assembly, or may be template-controlled. Anion size can be of paramount importance for such templates - Cl- is effective, Br, I- less good, and PFe ineffective when the recognition motif demands a small template (454). 

Anions have also been used as templates in the synthesis of catenanes and rotaxanes, as recently described by Beer and coworkers [15]. They used the self-assembly of diamides, known as anion receptor molecules, around a chloride anion to produce, for example, [2]- and [3]catenanes by olefin metathesis [16] (Scheme 5.4). In this special example, the template effect was increased by hydrogen bonding between the N-methyl group to the crown ether chain and by n-n stacking interactions. 

However, preorganization by reversible interactions via hydrogen bonds does not necessarily need a further template such as an anion. Direct intermolecular hydrogen bonds between derivatives of isophthalic acid and 2,6-pyridine diacid were used to synthesize a series of new catenanes [17,18], rotaxanes [18c,19], and trefoil knots [20]. The interaction of dibenzylammonium cations and dibenzo[24]crown[8] [21] led to the preparation of rotaxanes by the attachment of stoppers to the thread ends (Scheme 5.5) [22], or similarly by clipping of the crown ether chain wrapped around the ammonium dumbbell [23]. Recently, several novel catenanes were synthesized via the macrocydic connection of three ammonium cations threaded through three crown ether rings attached to a triptycene core [24]. 

Scheme 5.4 Synthesis of a [2]catenane by self-organization of amides around a chloride anion as the template. The pseu-dorotaxane formed between the linear and the cyclic amides is held together additionally by various hydrogen bonds and n-n interactions.

The interest in rotaxanes, pseudorotaxanes, and catenanes (i.e., molecules that contain non-covalently interlocked components) stems from their potential use as building blocks in molecular devices. Their syntheses usually rely on some sort of template assistance, such as the preorganization of the assembly s components around a metal center. While cationic templates have been widely used in this context, only a few examples of anion-directed synthesis of interlocked molecules have been reported. In fact, although rotaxanes and pseudorotaxanes have been prepared in this way (as discussed in this section), to date there is no reported example of anion-directed synthesis of catenanes. 

Anion-Directed Assembly, p. 51 Enzymes Characteristics and Mechanisms, p. 554 Hydrogen Bonding, p. 658 Molecular Squares, Boxes, and Cubes, p. 909 Molecular-Level Machines, p. 931 Protein Supramolecular Chemistry, p. 1161 Racks, Ladders, and Grids, p. 1186 Self-Assembling Catenanes, p. 1240 Self-Assembly Definition and Kinetic and TTjermot/ynam-ic Considerations, p. 1248 Self-Assembly in Biochemistiy, p. 1257 The Template Effect, p. 1493... 

Halogen bonding between bromide anion and bromoimidazolium moieties was used to template the formation of catenane Br PFe through ring-closing metathesis (Scheme 1) [56]. The combination of precursors 16 PFs and 16 Br in the presence of the Grubbs second-generation ruthenium benzylidene complex led to the formation of the catenated structure in 24% yield. In the absence of the... 

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