Interlocked species

In the following sections, examples of hydrogen-bonding templates for the synthesis of macrocycles, cages, interlocked species, helicates and for the photochemical reaction of olefins will be discussed. The use of hydrogen-bonding templates in dynamic combinatorial libraries will also be presented. 

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... 

The strong hydrogen bonding interactions observed between the oxygen atoms of crown ethers and the N-H groups of ammonium groups can be successfully employed to prepare pseudorotaxanes and rotaxanes by templated processes. This approach has been extensively utilised by Stoddart, Busch and others to obtain a wide range of interlocked species. 

This elegant synthesis has demonstrated the possibility of using a hydrogen bond-acceptor template to prepare template-free interlocked species (in a similar fashion to Sauvage s catenanes syntheses which make use of transition metals as templates). 

Hydrogen Bonding vs ti-tc Stacking in Templated Synthesis of Interlocked Species... 

Loeb has reported a series of pseudorotaxanes [84,85] and rotaxanes [86,87] where C-H- 0 hydrogen bonding interactions (together with N+- -O attractive forces) play an important contribution in templating the formation of the interlocked species. In particular, the formation of a pseudorotaxane was observed when equimolar amounts of [pyCH2CH2py]2+ and the crown ether 20 were mixed. The structural characterization of the resulting host-guest complex... 

Further studies by the same authors have led to the formation of [2]rotaxanes, [3]rotaxanes and pseudo-polyrotaxanes [85-87]. In all these interlocked species, in spite of the presence of aromatic rings in the axle and wheel, tt-ti interactions do not seem to play a role in the templating process. This highlights once again the importance of C-H---0 hydrogen bonding in the assembly of interlocked species. 

Attempts have also been made to carry out multiple sterically directed cyclisations on closely related systems with a view to generating multiply interlocked materials. Compound 10.97 was prepared and subjected to a triple ring closure reaction. Unfortunately, the inherent low yields of the crucial reaction steps resulted in immense difficulty in characterising the reaction products. Compound 10.97 is prepared in nine steps, and following multiple cyclisation, a total of only 1.7 % of product was obtained, which proved to be an isomeric mixture of three different interlocked species. 

Figure 10.89 Conceptual production of increasingly interlocked species through the use of multime-tallic helicates. (a) Doubly interlocked [2]catenane from three metal centres, (b) Pentafoil knot from four metal ions, (c) Triply interlocked [2]catenane. (Reproduced with permission from [98]).

Self-assembly is a particular powerful tool in synthesising up large scale nanostructures are topologically complex molecules such as molecular machines and topologically interlocked species. 

Fig. 17 (a) Seif-assembly of the triply threaded supramolecular system 14 j13H36+ and the subsequent synthesis of the triply interlocked species 15H39+. (b, c) Operation of I5H39+ as an acid-base controlled molecular elevator [78-80]... 

The synthesis of rotaxanes (and catenanes) carried out under kinetically controlled conditions has as a drawback the employment of an irreversible bond-forming final step, which may yield competitive or unwanted non-interlocked by-products. Methods allowing interlocking to occur in a thermodynamically controlled manner have therefore been developed, so that by-products can be recycled to afford the energetically, most favored, interlocked species, via reversible breakage/formation of covalent bonds ( dynamic covalent chemistry ) <2002AGE898>. 

Rotaxanes [a name derived from the Latin words rota (wheel) and axis (axle)] [1] and catenanes [from the Latin word catena (chain)] [1] are supramolecular (multi-component) species [1-10] strictly related (Figure 1) to pseudorotaxanes, which were described in Volume III, Part 2, Chapter 6. Whereas pseudorotaxanes can undergo dissociation into their wire-like and macrocyclic components, rotaxanes and catenanes are interlocked species, whose dissociation requires breaking of a covalent bond. It should be pointed out, however, that, as discussed in detail in the previous chapter 6, the boundary between rotaxanes and pseudorotaxanes is somewhat fuzzy because, for example, when the stoppers are not extremely bulky compared to the hole of the macrocyclic component, a rotaxane at low temperature might well be a pseudorotaxane at elevated temperature [11]. 

Since reliable and efficient strategies for the synthesis of rotaxanes and catenanes have been established, current research is focused on functionalization of the component parts of these interlocked species and on the construction of increasingly complex compounds in the search for new and valuable properties. 

A more accurate treatment requires to reduce the size of the system and to use model compounds. E.g. Schalley and coworkers [125, 126] carried out DFT calculations in order to gain insight into the details of the hydrogen bond patterns involved in the formation of mechanically interlocked species such as amide rotax-anes, catenanes, and knots. 

The s)mthetic route described above corresponds to strategy I of Figure 26. Actually, strategy II was tested at first, since it involves well-controlled steps. It requires the preliminary preparation of one single chelate macrocyclic component. However, once intermediate (F) (called precatenate) is formed, only a single cyclization reaction is needed to afford the interlocked species. Accordingly, yields as high as 42% were observed for this last step. 

Abstract This review presents an overview of the area of anion-templated synthesis of molecules and supramolecular assemblies. The review is divided into two main sections the first part deals with anion-templated systems where the final products are linked by bonds that are not reversible under the conditions of the experiment Several recent examples of macrocycles, cages and interlocked species are presented in this section. The second part of the chapter, presents a discussion of anion-templation in systems containing reversible bonds that give rise to dynamic combinatorial libraries (either by formation of coordination metal-ligand bonds or by reversible covalent bonds). 

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