Rotaxanes anion template effect

Scheme 15 A new anion template effect for the syntheses of ether rotaxanes.

Because earlier reviews deal explicitly with these template effects [4, 7, 12], the following discussion is restricted to a recently discovered anion template effect (Scheme 6.5.3). fn non-competitive solvents, the tetralactam macrocyde strongly binds anions such as chloride, bromide, or phenolate. Phenolate stoppers bound to the macrocycle can act as wheeled nudeophiles and react through the wheel with a semi-axle generated in situ to yield ether rotaxanes in yields of 57% to 95% [13]. Consequently, the macrocyde not only represents an anion receptor, but as a concave template simultaneously provides the correct orientation of the guest for threading the axle into the wheel. 

Scheme 6.5.3. Anion template effect for efficient rotaxane synthesis. 

The most recently discovered template effect is the one that makes use of anions. Vogtle and coworkers [12] have found that a phenolate equipped with one stopper can bind in the cavity of the tetralactam macrocycle by two strong hydrogen bonds. Then this nucleophile complex is reacted with an electrophilic semi-axle to obtain rotaxane in very high yields up to 95%. 

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. 

Fluorescence Sensing of Anions, p. 566 Guanidium-Based Anion Receptors, p. (575 Halogen Bonding, p. 628 Macrocyclic Synthesis, p. 830 Molecular Squares, Boxes, and Cubes, p. 909 Naked Anion Effect, p. 939 Organometallic Anion Receptors, p. 1006 Rotaxanes and Pseudorotaxanes, p. 1194 Self-Assembly Definition and Kinetic and Thermodynamic Considerations, p. 1248 The Template Effect, p. 1493... 

Fig. 2 Template syntheses of rotaxanes the Cu(I) ion binds a phenanthroline ligand inside a macrocycle (top left). A l f5-paraquat macrocycle is clipped around an axle bearing a hydroquinone center piece (top right). Hydrogen bonding permits the use of nonionic template effects for the preparation of amide-type rotaxanes (bottom left). Phenolate anions bound to the macrocycle react as a supramolecular nucleophile (bottom right).

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). 

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