O

rotation shuttling rotation

circitm-

rotation circitm-

rotation

Scheme 6.5.2. Molecular motion feasible in rotaxanes (top) and catenanes (bottom).

ping by two bulky stopper groups that mechanically trap the axle. Consequently, the axle is free to move inside the wheel, because there is no covalent bond between the two components. Several different types of motion are feasible. Besides rotation of the axle inside the wheel, the wheel can move back and forth along the axle, if it is long enough. This process can be regarded as one-dimensional diffusion along the thread. Pivotal rocking of the axle is also possible, as indicated by the arrows in Scheme 6.5.2 (top).

For efficient rotary motion it is necessary to guide the axle properly [6]. Pivotal rocking and the shuttling along the axle should be reduced to a minimum, whereas rotation should be freely possible. It should, however, be mentioned here that the shuttling and the rotation in rotaxanes become identical in catenanes (Scheme 6.5.2, bottom), another kind of mechanically bound molecule in which two wheels are interlocked just like two members of a chain. The process of one wheel rotating through the other is called circumrotation. Because of this analogy of catenanes and rotaxanes, we will include both species in the following discussion.

Rotaxane Synthesis via Template Effects

The use of mechanically bound molecules as the basis for functional devices requires efficient strategies for their preparation. The statistical synthesis of rotax-anes, that is their accidental formation, is usually a low-yield process and thus does not permit their production on a larger scale. Template effects [7] must necessarily be used to thread the axle through the wheel of the rotaxane. Quite an arsenal of effects exist which enable synthesis of a large number of structurally different ro-taxanes in high yield. Rotaxane synthesis is increasingly becoming routine. Among the different template effects are those involving metal coordination to a convex

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