Threading pseudorotaxanes

The first approach is to use copper(I) to thread a linear multinitrogen-donor molecule, such as the 2,9-diphenyl-1,10-phenanthroline [177] derivative L1059 [178] 

Alternatively, NMR or UV-Vis spectroscopy can be used to determine association and dissociation rate constants, and thence estimate association constants, as in the study of the threading of L1074 through L1058 [194]. Kinetic and equilibrium characteristics of association have also been monitored for model systems in which 1,4-disubstituted benzenes interact with the ubiquitous cyclophane LI058 (1951. 

Optically active (pseudo)rotaxanes can be built up by the use of a rigid chiral unit L1087 in place of one or both phenylene units in the cyclophane L1058. L1087 may be prepared from L1088 and L1089 on a l,4-bis 2-(2-hydroxyethoxy)ethoxy benzene template (Eq. 4.60) [197]. 

4 Template synthesis of three-dimensional macrocyclic systems OH 

A further type of threading is involved in amide (pseudo)rotaxanes. In this class of compounds both components are amides (e.g. L1097 with L1098 Eq. 4.64) [204]. There are many sites for hydrogen-bonding and n-n (stacking, donor-acceptor) non-covalent interactions between the components [205]. 

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Jiang, W, Schafer, A., Mohr, PC., Schalley, C.A. (2010) Monitoring Self-sorting by Electrospray Ionization Mass Spectrometry Formation Intermediates and Error-correction during the Self-assembly of Multiply Threaded Pseudorotaxanes. J. Am. Chem. Soc. 132 2309-2320. 

Copper(I)-threaded pseudorotaxanes can be capped by such large moieties as porphyrins [228] or fullerenes [229] - indeed there is a tris-Cu pseudorotaxane which can be deemed a rotaxane if the two terminal copper-bonded centres are considered as caps for the central unit [179]. Porphyrin-capping is of particular interest as provi ng a means to have an electron donor and electron acceptor... 

Fig. 16. Coupling triply-branched amine and pseudorotaxane furnishes pseudorotaxane containing CB[6] beads threaded on the dendritic framework...

Kim et al. synthesized similar dendritic pseudorotaxanes as shown in Fig. 16 [58]. Reaction of triply-branched amine 42 with pseudorotaxane 43 incorporating CB[6] threaded on a string with a carboxylic acid terminus in DMF in the presence of EDC produces branched [4]pseudorotaxane 44, which is a G-1 dendritic pseudorotaxane. Coupling between 42 and pseudorotaxane dendritic wedge 45 under similar conditions yields G-2 dendritic pseudorotaxane 46, which contains 9 beads threaded on a dendritic framework ([lOjpseudorotax-ane). These dendritic pseudorotaxanes have been characterized by NMR spectroscopy, but further characterization remains to be done. 

More recently, Kim et al. synthesized dendritic [n] pseudorotaxane based on the stable charge-transfer complex formation inside cucurbit[8]uril (CB[8j) (Fig. 17) [59]. Reaction of triply branched molecule 47 containing an electron deficient bipyridinium unit on each branch, and three equiv of CB[8] forms branched [4] pseudorotaxane 48 which has been characterized by NMR and ESI mass spectrometry. Addition of three equivalents of electron-rich dihydrox-ynaphthalene 49 produces branched [4]rotaxane 50, which is stabilized by charge-transfer interactions between the bipyridinium unit and dihydroxy-naphthalene inside CB[8]. No dethreading of CB[8] is observed in solution. Reaction of [4] pseudorotaxane 48 with three equiv of triply branched molecule 51 having an electron donor unit on one arm and CB[6] threaded on a diaminobutane unit on each of two remaining arms produced dendritic [ 10] pseudorotaxane 52 which may be considered to be a second generation dendritic pseudorotaxane. 

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). 

Upon threading the oligosilane molecules into the asymmetric y-cyclodextrin cavity to form a pseudorotaxane-type aggregate,335 a bisignate Cotton effect became apparent at about 290 nm, indicating induced PSS helicity. The reason for the bisignate nature of the signal was not clear. 

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... 

Photochemically-driven dethreading and rethreading of an azobenzene-based pseudorotaxane in acetonitrile occurs when (5) threads with (6) (Figure 12.17). 

A pseudorotaxane is the supramolecular product of the self-assembly of two components (i.e., a [2]pseudorotaxane), a molecular ring (macrocycle) and a molecular wire (thread) threaded into it. The occurrence of specific interactions between the two components lowers the free energy of the assembled system with respect to the separate components and drives the self-assembly of the pseudorotaxane (threading process). Threading occurs generally in solution and, upon changes of solvent... 

The ability of cyclodextrins to bind acyclic guests with pseudorotaxane geometries has been employed (Figure 30) by Harada et al. [46, 47] to self-assemble [2]rotaxanes after the threading procedure depicted schematically in Figure la. The diamine 92 is bound by the methylated a-cyclodextrins 93 and 94 with 1 1 stoichiometries in H20. Reaction of the terminal amine groups of the guest with... 

Nakashima et al. [49] have self-assembled (Figure 32) a Tight-driven molecular shuttle by relying upon the threading of one a-cyclodextrin (99) on to an acyclic guest, followed by the covalent attachment of two stoppers at both its ends. The azobenzene-based guest 104 is bound by a-cyclodextrin 99 with a pseudorotaxane... 

Steinbrunn and Wenz recently reported poly(amide-CD rotaxane)s 53 via a very imaginative approach [90,91]. First, the CD was threaded on to an a,co-amino acid in water to give a pseudorotaxane monomer. An NMR study showed that two CD molecules were threaded per linear molecule for 11-aminoundecanoic acid. The X-ray powder pattern indicated that these rotaxanes stacked like channels in the solid state this provided the basis for solid state polymerization at 200°C to afford polyamide 53 with m/n=2 ... 

Combination of 10.41 with crown ether derivative 10.48 and Ag0) gives a trimetallic sheathed rack in which a rack of Ag(I) ions is threaded through the cavities of three heterocrowns (Figure 10.42). This complex is an example of a pseudorotaxane (stricdy a [4] pseudorotaxane because there are four components - three loops and an axel) and we will return to these kinds of assemblies, which are precursors for the synthesis of complex interlocked molecules by postmodification techniques, in Section 10.7. 

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