Templated rotaxane synthesis

Scheme 16 anion-templated rotaxane synthesis via a Michael addition reaction. 

Schalley, C.A, Silva, G., Nising, C.F., Linnartz, P. Analysis and improvement of an anion-templated rotaxane synthesis, Helv. Chim. Acta, 85 (2002), 1578-1596. 

Fig. 1.4. Anion-templated rotaxane synthesis. The axle center piece is threaded through the macrocycle s cavity by hydrogen bonding. Stopper attachment to both axle ends traps the wheel on the axle. Inset hydroquinone-based center piece which can also be used, but with lower efficiency.

Analysis and improvement of an anion-templated rotaxane synthesis. Helv. Chim. Acta, 2002. 85. 1578. 

Template effects have been used in rotaxane synthesis to direct threading of the axle through the wheel. Since macrocycHc compounds such as cyclodextrins, crown ethers, cyclophanes, and cucurbiturils form stable complexes with specific guest molecules, they have been widely used in the templated synthesis of rotax-anes as ring (wheel) components. Here, we briefly discuss macrocycles used in the synthesis of rotaxane dendrimers and their important features. 

If during the template-directed synthesis of a rotaxane, the location of two identical recognition sites ( stations ) within its dumbbell component can be arranged (Fig. 13.3a and b), a degenerate, coconformational equilibrium state is obtained in which the macrocyclic component spontaneously shuttles back and forth between... 

Figure 19. The template-directed synthesis of the [2]rotaxane 56-4PF6.

The reaction of fumaryl dichloride (59a) with 52 in the presence of macro-ring 32 resulted indeed in 26% [2]rotaxane 60a and 26% of the pure thread 61a [19]. The remarkably high yield of 60a indicates that arene units are not needed for this nonionic template-assisted reaction. When, however, the 2,4-hexadiene diacid dichloride (59b) was used for rotaxane synthesis the [2]rotaxane 60b, even though detected by mass spectrometry, could not be isolated, either at room tem-... 

This seems to be the first rotaxane synthesis based on an anionic template and latest results indicate that this new strategy can be extended to ester- and acetal-based axles [70]. Hence the anionic procedure represents a powerful alternative to the rotaxane synthesis assisted by a neutral template. 

VIII. The Next Surprise Rotaxane Synthesis Mediated by a Template... 

A very surprising and fruitful result was obtained when a control experiment related to an amide templated synthesis was made. Dibromo compound 19 utilized in the reaction looked similar to the axle centerpiece used in the amide template synthesis but lacked the amide in the middle which was crucial for this purpose. However, when the reaction was complete, it was found that rotaxane 24 was formed with 80-95% yield [12] (Figure 9). It seemed reasonable to assume that this time not the axle but the stopper coordinated to the macrocycle [27], This suggestion was supported by the high binding constant of the deprotonated stopper-wheel complex 21 18 (> 105 M"1) derived from H NMR titrations. In the rotaxane synthesis, this complex reacts with the semiaxle 23 producing a rotaxane. 

Raymo, F.M., Stoddart, J.F. Organic Template Directed Synthesis of Catenanes, Rotaxanes and Knots. In ref. 4b, p.143. 

Scheme 6.5.3. Anion template effect for efficient rotaxane synthesis. 

For accounts and reviews on the use of this recognition motif to template the synthesis of catenanes and rotaxanes, see (a) Amabilino, D. B. and Stoddart,... 

The -electron-deficient pyridinium ring has played an important role in template-directed synthesis in which the interaction of a -electron acceptor and a -electron donor facilitate self-assembly. This has led to the construction of a variety of supramolecular assemblies including catenanes (linked rings) and rotaxanes (wheel and axle) . 

A template-directed synthesis of [2]-rotaxanes, with a yield in one case of 72%, has been reported. The experiments employed dumb-bell-shaped components incorporating terminal triisopropylsilyl stoppers connected to a central 1,5-dioxynaph-thalene recognition site by [-CH2CH20-] spacers (n = 1-3). - These components were used as templates for the synthesis ( clipping reaction ) of the corresponding rotaxanes incorporating cyclobis(paraquat-p-phenylene) as the ring component. The... 

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