Shuttling processes

In a recent report [141] Stoddart et al. reported a new class of rotaxanes with dendritic stoppers by using a so-called threading approach (Fig. 25). Alkylation of bipyridinium based units with Frechet s third tier branched aryl ethereal dendron, in the presence of BPP34C10 afforded 58 as one of the products. Variable temperature H-NMR spectroscopy in different NMR solvents helped determine the novel shuttling process of BPP34C10 from one bipyridinium unit to the other in 58. The dendritic framework of 58 assists in its solubility in a wide range of solvents. 

The chain shuttling process offers a high level of control over the resulting OBC microstructure. The average number of blocks per chain can be adjusted by modifying the ratio of CSA to monomer higher values of [Zn]/[C2HJ result in shorter... 

Multiblock OBCs from chain shuttling polymerization have very different architectures. The overall chains and blocks within chains have distributions of molecular weights, with MJMn approaching 2.0. The statistical shuttling process produces chains with a distribution in the number of blocks per chain. The block junctions are precise since each block is grown on a different catalyst, and the compositions are homogeneous since the OBCs are produced at steady-state in a continuous reactor. 

Since, during an actual measurement, the shuttling process is repeated many times in a cyclic manner, the technique has been named field-cycling (FC) NMR relaxometry, a term which underlines the fact that it is the magnetic field variation that matters and not the manner in which it is achieved. 

Figure 20. The shuttling process associated with the [2]rotaxane 56-4PFg in solution.

With the larger Ms(calix) crown (3.127),65 two dynamic processes are observed. The intermolecular association/disassociation equilibrium in which the complex equilibrates with the uncomplexed ligand and free metal cation is slow on the NMR time scale. The complex also exhibits a faster intramolecular vibration of the metal cation from one binding site to another (Scheme 3.26). Compound 3.127 is also interesting because the mechanism of this fast, intramolecular cation shuttling process may have important implications on the analogous movement of metal cations through transmembrane ion channels (Section 2.2). 

Sauvage has demonstrated both electrochemical and photochemical control over ring motions in a catenate, 18 [57,58]. The observed behavior of the catenate is essentially similar to the analogous rotaxane, the only difference being that the 4-coordinate to 5-coordinate (dpp -> terpy) shuttling process is slower in the catenate and the reverse step is faster. Again, the issue of directionality is not addressed in this system. 

The shuttling process was confirmed by H NMR and NOE experiments, and significantly also resulted in changes in the circular dichroism spectrum, suggesting that the change in position also affected the influence of the chirality of the cyclodextrin upon the aromatic regions of the thread. 

Fig. 13. The shuttling process and energy barrier for the passage of the tetracationic cyclophane between the two degenerate recognition sites,...

No example has so far been reported of a shuttling process controlled by electron transfer chemical reactions. There are, however, very interesting examples of shuttling processes controlled by acid/base reactions. One case is that of the previously discussed compound 13 " (see Figure 14), in which the shuttling of the macrocycle component can be controlled not only electrochemically, but also by protonation/ deprotonation of the benzidine unit [43]. 

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