Rotaxanes process

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. 

In recent years, with increasing recognition of the roles played by specific noncovalent interactions in biological systems and chemical processes, the science of noncovalent assemblies- often called supramolecular science- has aroused considerable interest [76], The remaining part of this article reviews some important studies made on rotaxane and catenane, two classic types of supramolecular structure. 

NMR has been used for measurements of dynamics in few examples of CD complexes, and most applications are related to slow processes, such as those observed for rotaxanes. An example of such a slow process is the threading of a-CD onto a monomeric model of ionene, which could be followed directly by measuring the intensity changes of the signals due to complexed and uncomplexed material to give an association rate constant of 0.036 M-1 s 1.204... 

Rotational suspension separation encapsulation process, 16 450 Rotational viscometers, 21 731-737 computers in, 21 732 operation of, 21 736 Rotations, of molecules cause of color, 7 326t, 328 Rotavirus vaccine, 25 496 Rotaxanes, 17 61, 24 31, 33, 51 as molecular computers, 24 60 Rotenone (Chem-Fish Synergized,... 

The strong hydrogen bonding interactions observed between the oxygen atoms of crown ethers and the N-H groups of ammonium groups can be successfully employed to prepare pseudorotaxanes and rotaxanes by templated processes. This approach has been extensively utilised by Stoddart, Busch and others to obtain a wide range of interlocked species. 

Further studies by the same authors have led to the formation of [2]rotaxanes, [3]rotaxanes and pseudo-polyrotaxanes [85-87]. In all these interlocked species, in spite of the presence of aromatic rings in the axle and wheel, tt-ti interactions do not seem to play a role in the templating process. This highlights once again the importance of C-H---0 hydrogen bonding in the assembly of interlocked species. 

The squaraine rotaxane tetracarboxylic acid 15a is soluble in aqueous solution at physiological pH and acts as an excellent fluorescent marker with extremely high photostability, which allows trafficking processes in cells to be monitored in realtime, with constant sample illumination, over many minutes. This type of real-time monitoring cannot be done with other available NIR fluorescent probes, such as the amphiphilic styryl dye KM4-64 and water-soluble dextran-Alexa 647 conjugate, because they are rapidly photobleached. 

The clipping reaction used in [52, 53, 55] to synthesize tetralactam-based squaraine rotaxanes such as 14 and 15 afforded only moderate yields (ca. 28-35%) of the rotaxanes, possibly because of the unavoidable presence of nucleophiles that react with the chemically unstable squaraines during the reaction. The slippage approach [62] minimizes the squaraine dye s contact with nucleophiles during the rotaxane formation process and therefore can be used to efficiently encapsulate a squaraine dye such as 23 in a macrocycle such as 25 [63],... 

Buston JEH, Marken F, Anderson HL (2001) Enhanced chemical reversibility of redox processes in cyanine dye rotaxanes. Chem Commun 11 1046—1047... 

The pioneering papers by Stoddart and Sauvage have stimulated the design of a variety of movable rotaxanes and catenanes, whose controlled motion is promoted by a redox change. In all cases, the process of the redox-driven intramolecular motion can be described by a square scheme, as illustrated in Fig. 2.1. 

Rotaxane 14+ shows four distinct reduction processes, monoelectronic and reversible their assignment can be made by a comparison with the behavior of... 

Dumbbell 34+ exhibits two bielectronic and reversible processes that can be attributed to the simultaneous first and second reduction of the two bipyridinium units contained in its axle-like section. The bielectronic nature of the processes indicates, as expected, that the bipyridinium units are equivalent and behave independently. Also, model rotaxane 44+ shows two bielectronic and reversible processes that are straightforwardly assigned to the bipyridinium units contained in its dumbbell component they are, however, shifted to more negative potentials compared to dumbbell 34+. These shifts can be attributed to the CT interactions with the electron donor ring that make the electron acceptor bipyridinium units more difficult to reduce, whereas the bielectronic nature of the processes indicates the such units are noninteracting and equivalent—both of them are surrounded by a ring—in full agreement with the supramolecular structure of 44+. 

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 branched rotaxanes 66+, 76+, and 86+, besides the bipyridinium units of the triply branched backbone 56+, contain macrocycle 2 whose two DMB units are oxidized at distinct potentials the first oxidation process practically coincides with that of the model compound /i-dimethoxybenzene, whereas the second one is displaced to a slightly more positive potential. 

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+. 

Figure 13.25 Stmcture formula of rotaxane 284+ and its electrochemically controlled switching process.

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