Directional Control by Steric Effects

Since the CD molecule has a conical shape in which the primary rim is narrow and the secondary side is wide, steric hindrance should depend on the direction of threading. If the bulky end group approaches the CD cavity from the primary rim, inclusion should be more hindered than for the approach from the secondary rim. Consequently, big pseudostoppers should force CD rings to thread with a preferential orientation. Preferential orientation can indeed be detected for unsymmetrical bola-amphiphiles composed from a pseudostopper and a real stopper, as shown in Fig. 11 and Table 9. 

Orientational selectivity depends on the size of the pseudostopper the smaller trimethyl ammonium group gives rise to a preference of 2 1 for the end group approaching the wide side a-CD [163], The preference ratio reaches 7 1 for the bigger 

Stopper Pseudo stopper Orientational selectivity AAGorient kJ mol-1 Ref. 

Finally, y-CD, with an internal diameter of 7.4 A, requires very large stoppers for rotaxane synthesis. Therefore, only a few y-CD rotaxanes are presently known. The Anderson group showed that m-terphenyl-4,4 dicarboxylic acid is sufficiently large. They were able to synthesize a [2]-rotaxane from the IC of y-CD and a stil-bene derivative using this stopper. The [2]-rotaxane obtained had sufficient space remaining to accommodate another axis molecule that could be stoppered, as well to furnish the first [3]-rotaxane with two axes through one CD ring. Both homo- and hetero- [3]-rotaxanes with two equal and two different axes, respectively, could be synthesized this way, as shown in Fig. 13 [183], 

In principle, CD rotaxanes can be used for molecular information processing if they are reversibly switchable between two states, such as CD [2]-rotaxanes comprising two different binding sites within their axes, as shown in Fig. 14. In the off state, the threaded CD recognizes the better binding site and settles there. Once the 

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