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How to Achieve Unidirectional Rotation in Artificial Molecular Motors?

Any artificial rotaxane-based mimic of the natural ATP synthase motor must bear a chiral element that helps to define the direction of rotation. Chirality can, of course, be implemented in such molecules by adding chiral groups as the stoppers or the wheel. Rotaxanes with cyclodextrins as the wheels have been described [11], and rotaxanes with glucose-containing stoppers are known [14]. Rotaxanes with elements of planar chirality have also been realized [15].

More challenging, however, is the preparation of cycloenantiomeric, topologically chiral rotaxanes [16] and the resolution of their two enantiomers [17]. In these compounds, neither the axle nor the wheel is chiral but both together give a cyclo-enantiomeric species (Scheme 6.5.4). Such molecules can be prepared if both axle and wheel bear groups with a certain directionality. For example, the sulfonamide group in the wheel in Scheme 6.5.4 defines such a directionality and any rotaxane built from an axle with two different stoppers and the sulfonamide wheel exists in two enantiomeric forms, as shown in the diagram. Based on the same basic idea, a cyclodiastereomeric [3]rotaxane - a rotaxane with two wheels on one axle - has been synthesized that exists as a pair of enantiomers plus an additional meso form [18]. Separation into the stereoisomers by HPLC on a chiral stationary phase is often possible, and the CD spectra have been recorded for many of them. Topo-logically chiral rotaxanes offer an interesting future perspective to realize molecular rotors which not only have an axle free to rotate within the wheel, but which couple this rotation to chirality in order to provide unidirectionality. The problem of how to couple topological chirality to the rotational sense has not yet been solved satisfactorily, however. Thus, as an illustration of the principle, let us discuss briefly an artificial minimal system, which is not based on rotaxanes but provides continuous unidirectional motion. This system, which was developed by Feringa et al. [19], contains an overcrowded double bond with chiral substituents (Scheme

Scheme 6.5.3. Anion template effect for efficient rotaxane synthesis.

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Scheme 6.5.4. A topologically chiral rotaxane. Arrows indicate the directionality imposed by the sulfonamide group in the wheel.

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