Rotaxanes occurring

Pseudorotaxanes are precursors of both rotaxanes and catenanes they consist of a guest molecule threaded through a macrocyclic host. Stoppering both ends of the threaded molecule gives a rotaxane, cycliza-tion of the thread gives a catenane. Pseudorotaxane formation may occur by spontaneous self-assembly, or may be template-controlled. Anion size can be of paramount importance for such templates - Cl- is effective, Br, I- less good, and PFe ineffective when the recognition motif demands a small template (454). 

Scheme 15 Templated synthesis of rotaxane 24 employing the threading-followed-by-capping methodology. The shift of the proton from the terminal amine to the secondary amine allows for the threading to occur yielding the pseudorotaxane 26... 

The dynamic nature of the system offers the crown ether access to the ammonium centre allowing self-assembly of the corresponding dynamic [2]ro-taxanes 31 to occur. Fixing of the interlocked assembly can be achieved by reducing the imine groups in 31 to the corresponding amine so that a kinetically inert [2]rotaxane forms. 

On the basis of these observations, the four processes of rotaxane 14+ can be easily and unequivocally assigned as follows (Fig. 13.5). The first reduction, which occurs at the same potential as the first process of dumbbell 34+, is related to the bipyridinium unit not surrounded by the ring the second one, occurring at a potential that coincides with the first process observed for rotaxane 44+, concerns the first reduction of the bipyridinium unit surrounded by the ring and therefore involved in CT interactions. Finally, the third and fourth processes can be assigned to the second reduction of the free and engaged with the ring bipyridinium unit, respectively. 

The electrochemical behavior of rotaxane 76+ can be straightforwardly explained on the basis of the above discussion. On reduction, a first monoelectronic process, assigned to the first reduction of the free bipyridinium unit, is followed by a bielectronic process, assigned to the first reduction of the two bipyridinium units encircled by the ring. Even the second reduction of the three bipyridinium units, which occurs at more negative potentials, occurs with the same 1 2 pattern. On oxidation, the behavior of rotaxane 76+ is again similar to that of rotaxane 66+, with a more intense process in correspondence of oxidation of the DMB units. 

In rotaxane 86+, the three bipyridinium units are expected to be electrochemically equivalent because each one is encircled by the ring. In agreement with this expectation, it has been found two trielectronic processes corresponding to the first and the second reduction of the bipyridinium units, a situation similar to that observed for compound 56+. In the case of the rotaxane, however, the processes occur at more negative potentials because of the CT interaction. As far as oxidation is concerned, the behavior of 86+ is in line with that of 66+ and 76+. 

The electrochemical behavior of 4(4)+ in a CH2CI2/CH3CN solution has been studied by CV and is represented in Fig. 14.8a. A reversible signal appears at 0.54 V. In the rotaxane 4(4)+, where the metal is tetracoordinated, the signal occurring at... 

The cyclic voltammetry behavior of the Cu(II) rotaxane, 4(5)2+ (Fig. 14.8b), is very different from that of 4, t l +. The potential sweep for the measurement was started at - 0.9 V, a potential at which no electron transfer should occur, regardless of the nature of the surrounding of the central Cu(II) center (penta- or tetracoordinate). Curve i shows two cathodic peaks a very small one, located at + 0.53 V, followed by an intense one at —0.13V. Only one anodic peak at 0.59 V appears during the reverse sweep. If a second scan ii follows immediately the first one i, the intensity of the cathodic peak at 0.53 V increases noticeably. The main cathodic peak at —0.15 V is characteristic of pentacoordinate Cu(II). Thus, in 4(5)2+ prepared from the free rotaxane by metalation with Cu(II) ions, the central metal is coordinated to the terdentate terpyridine of the wheel and to the bidentate dpp of the axle. On the other hand, the irreversibility of this peak means that the pentacoordinate Cu(I) species formed in the diffusion layer when sweeping cathodically is transformed very rapidly and in any case before the electrode potential becomes again more anodic than the potential of the pentacoordinate Cu2 + /Cu+ redox system. The irreversible character of the wave at —0.15 V and the appearance of an anodic peak at the value of + 0.53 V indicate that the transient species, formed by reduction of 4(5)2 +, has undergone a complete reorganization, which leads to a tetracoordinate copper rotaxane. The second scan ii, which follows immediately the first one i, confirms this assertion. 

Assembling [2] rotaxanes (wheel and axle) can involve three basic processes (Scheme 13). One of these (93CC1269) involves slippage, in which the axle (115) is linked by a 4,4 -bipyridine, and the wheel (116) is a bisparaphenylene-34-crown-10 ether. Heating the two components in acetonitrile at 60°C yields the rotaxane, which can be characterized by FABMS and H and l3C NMR, but extrusion of the wheel occurs at 100°C. In other developments (94NAT(369)133), the pyridine component may be incorporated in the wheel, as in structure (117), where two bipyridinium units are connected by p-xylyl groups, and here the rotaxane acts as molecular switch. At room temperature in acetonitrile the wheel... 

So far the rotaxanes were bridged via the macrocycles, but coupling can also occur at sulfonamide-bearing axles. Reaction of 57m with 95 results in 48% of the axle-bis[2]rotaxane 107, which can be considered as a [3]rotaxane (Figure 43) [42]. The translational freedom of the wheel along the thread is strongly impeded not only by the second wheel, but also by the attached podand-like chain. 

Numerous examples of catenanes 1, rotaxanes 2, and trefoil knots 3 (Scheme 1) have been previously reported in the literature and are still attracting considerable attention (see Chapters 4 and 6-8) [1-5]. These aesthetically appealing molecules have in common that the topological bonds occurring in catenanes 1 and trefoil knots 2 and the mechanical bonds connecting the component parts of rotaxanes 3 are defined at a molecular scale without ambiguity [1, 2, 4]. 

To study the effect of the BG on threading process, triphenylmethyl-based azo compounds were designed as BG initiators for the preparation of polystyrene rotaxane 32 [67-69]. The BG fragments were shown to end up at both chain ends of the polystyrene since termination occurs nearly exclusively by combination. However, the threading efficiencies did not increase with 29 as cyclic and increased very little in other instances relative to those without BG. 

Similar to that in copoly(ester rotaxane)s 64, min for these poly(urethane rotax-ane)s increased with increasing BG, i.e. higher x values [116,117]. However, compared with the copoly(ester rotaxane), the dethreading occurred to lesser extent in these polyrotaxanes this is attributed to the fact that the NH groups retard dethreading by hydrogen bonding with the threaded crown ether as in structure 67. A linear relationship between min values and x was revealed. 

Paraquat, an ionic molecule, is more rigid than polymeric crown ethers. In addition, die crown ether has to adopt a restricted conformation for complexation to occur. The total increase of the rigidity for derived polyrotaxanes 84 depended on the min value [118, 119]. The higher the value of min, the higher Tg of the poly-rotaxane. Paraquat is a crystalline material but the polyrotaxanes are amorphous. 

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