Rotaxanes recognition sites

Reaction (Scheme 2) of salt 5-2PF6 with an excess of 6, in the presence of the macrocydic polyether BPP34C10, provided1181 [BPP34C10-7] [PF6]4 via a threading-followed-by-stoppering approach.1231 This rotaxane incorporates a free 4,4-bipyridinium recognition site within its dumbbell-shaped compo-... 

Rotaxanes are made of dumbbell-shaped and ring components which exhibit some kind of interaction originating from complementary chemical properties. In rotaxanes containing two different recognition sites in the dumbbell-shaped component it is possible to switch the position of the ring between the two stations by an external stimu-... 

Rotaxanes Containing Identical Recognition Sites for the Ring in their Dumbbell Component... 

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... 

When a rotaxane contains two different recognition sites in its dumbbell component, it can exist as two different equilibrating coconformations, the populations of which reflect their relative free energies as determined primarily by the strengths of the two different sets of noncovalent bonding interactions. In the schematic representation shown in Fig. 13.8, it has been assumed that the molecular ring... 

Figure 13.8 Schematic operation of a two-station rotaxane as a controllable molecular shuttle, and idealized representation of the potential energy of the system as a function of the position of the ring relative to the axle upon switching off and on station A. The number of dots in each position reflects the relative population of the corresponding coconformation in a statistically significant ensemble. Structures (a) and (c) correspond to equilibrium states, whereas (b) and (d) are metastable states. An alternative approach would be to modify station through an external stimulus in order to make it a stronger recognition site compared to station A.

As discussed in Section 13.2.2, when a rotaxane contains two different recognition sites in its dumbbell component, it can behave as a controllable molecular shuttle, and, if appropriately designed by incorporating suitable redox units, it can perform its machine-like operation by exploiting electrochemical energy inputs. Of course, in such cases, electrons/holes, besides supplying the energy needed to make the machine work, can also be useful to read the state of the systems by means of the various electrochemical techniques. 

The first example of electrochemically driven molecular shuttles is rotaxane 284+ (Fig. 13.25) constituted by the electron-deficient cyclophane 124+ and a dumbbellshaped component containing two different electron donors, namely, a benzidine and a biphenol moieties, that represent two possible stations for the cyclophane.10 Because benzidine is a better recognition site for 124+ than biphenol, the prevalent isomer is that having the former unit inside the cyclophane. The rotaxane... 

After this first report, a remarkable number of electrochemically controllable molecular shuttles have been designed, constructed, and studied. Rotaxane 294+ (Fig. 13.26), for instance, incorporates the electron-deficient cyclophane 124+ and a dumbbell containing two kinds of electron-rich units, namely, one 2,6-dioxyanthra-cene and two 1,4-dioxybenzene moieties.34 In solution, the rotaxane is present as the isomer with the 2,6-dioxyanthracene unit inside the cyclophane, owing to the fact that this unit is a better station in comparison to the 1,4-dioxybenzene recognition sites. 

Figure 10. The self-assembly of the [2]rotaxane 12 3PF6 incorporating two different recognition sites within its dumbbell-shaped component.

Catenaries and Rotaxanes Incorporating Amide Recognition Sites... 

Over the last decade, noncovalent bonding interactions between appropriate Jt-elec-tron-deficient and Jt-electron-rich recognition sites have been exploited in our laboratories for the synthesis of catenanes and rotaxanes. 271 In the example illustrated in Figure 10, the bis(hexafluorophosphate) salt 11-2PF6 was treated 28 with trons-bis(pyridine)ethylene (12) in the presence of the previously formed macrocyclic polyether bis-p-phenylene-34-crown-10 (13). The resulting [2]catenane 14-4PF6 was isolated in 43 % yield after counterion exchange. 

Rotaxanes incorporating dialkylammonium recognition sites have been synthesized,[36] as illustrated in Figure 22. Treatment of the bis(hexafluorophosphate) salts [30-H]-2PF6 and [31-H]-2PF6 with the benzylic bromide 32 in the presence of... 

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