Rotaxanes slipping

In their first step towards building dendritic rotaxanes Stoddart et al. [140] reported the construction of such materials via a so-called slipping method (Fig. 24). Thus, treatment of bisparaphenylene-34-crown-10 (BPP34C10) with tris(bipyridinium) compound 54 at 50 °C in acetonitrile for 10 days afforded mono-, di-, and tris-rotaxanes 55, 56, and 57, respectively. ES-MS was used to determine the molar masses of these macromolecules while upfield shifts in the... 

Kinetically stable superarchitectures can be assembled by relying upon both noncovalent bonds and mechanical coercion. Thus, at elevated temperatures, rotaxane-like complexes (a) are generated when macrocycles slip over the stoppers of chemical dumbbells, while hemicarceplexes (b) are created when guests squeeze through the portals of hemicarcerands. 

Slipping. The slipping method exploits the kinetic stability of the rotaxane. Provided the end groups of the dumbbell are of appropriate size, they are able to reversibly thread through the macrocycle at higher temperatures, but by lowering the temperature the dynamic complex becomes kinetically trapped as a rotaxane. 

Asakawa M, Ashton PR, Ballardini R, Balzani V, Belohradsky M, Gandolfi MT, Kocian O, Prodi L, Raymo EM, Stoddart JF, Venturi M (1997) The slipping approach to self-assembling [n]rotaxanes. J Am Chem Soc 119 302-310... 

Figure 1. Threading (a and b) and slipping (c) approaches to [2]iotaxanes. Clipping (not shown) is another synthetic approach to rotaxanes. It involves the macrocyclization of the ring component around the preformed dumbbell-shaped component.

Figure 12. The slipping approach to the [2]rotaxane 14-PF6 and its solid-state structure.

A very simple novel slipping method meets both requirements. When the wheels 65 or 89 were melted together with axle 90 by heating in a tube to 350°C for 1 min and quenched thereafter in cold water the rotaxanes 91 and 92 were isolated in 8 and 3% yields, respectively (Figure 34) [51]. 

These promising results led us to believe that rotaxanes, which do not have attractive interactions between wheel and axle and cannot therefore benefit from a template effect, should also be accessible by a slipping process performed in the melt. Indeed the first tetralactam rotaxane 94 containing a pure hydrocarbon dumbbell 93 was synthesized in 6% yield (Figure 36) [53]. 

The synthesis of 94 is noteworthy, because no templating interactions seem to be required to form the rotaxane in preparative yields The slipping approach performed in the melt seems to offer universal access to otherwise not obtainable rotaxanes. The absence of solvent not only assists this slipping by guaranteeing high concentrations, but also eases the reaction because no desolvation processes need to take place. 

Figure 34. A new slipping process of tailor-made wheels and axles in the melt enables easy access to rotaxanes and pseudoiotaxanes.

Figure 35. Schematic representation depicting the self organization of rotaxane-like entities using the slipping approach in the melt (high concentrations of wheel and axle). R Rotax-ane where the hydrogen bonds between wheel and axle have not yet been formed. R Rot-axane with hydrogen bonds between wheel and axle. Entropy loss upon formation of the mechanical bond is not considered in the graph.

Figure 36. The synthesis of a [2]rotaxane with a pure hydrocarbon axle 93 reveals the strong point of the slipping process performed in the melt the nontemplate synthesis is statistical yet leads to preparative yields of 94.

The amide hydrogen-bonding motif has been shown to support the clipping, threading, and slipping approaches to the formation of rotaxanes. The wide scope of the neutral template synthesis is enriched by the promising possibilities of the non-template synthesis performed in the melt. Numerous rotaxanes were obtained on a preparative scale and are available for further chemistry. 

As depicted in structures 1 and 2, rotaxanes are molecular composites consisting of cyclic and linear molecules in which the two components are connected together without any covalent bond, i.e., mechanically. Structure 1 is a pseudorotax-ane because the cyclic, represented by an open ring, can be disassociated from the linear species by external forces, e.g., dilution and heating in the solution state. Structure 2 is a true rotaxane, because the cyclic is permanently confined between two bulky blocking groups (BG), represented by the two balls, and can not slip off the linear molecule. 

The procedure of the anion-templated synthesis was later used for the preparation of various rotaxanes by the groups of Smith [41] and Schalley [34-36], The anion-templation technique was carefully tested by Schalley et al. Altering the length of the axle-center pieces and the size of stopper groups they synthesized a variety of rotaxanes and analysed their yields and de-slipping rates [34, 35],... 

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