Rotaxane threading

Fig. 94. Macrocycle (top) used to host the lanthanide-containing pseudo-rotaxane threads (middle and bottom)...

The ribosome structure is perhaps the most impressive natural rotaxane. Messenger RNA, as the rotaxane thread, is clamped by the ribosomal protein subunits which read each codon and transcribe it into a sequence of amino acids that are introduced by transfer RNAs to build up the desired protein. The importance of the ribosomes has been reflected in the number of Nobel Prize recipients associated with their discovery (Palade in 1974) and elucidation of their structures and functions (Ramakrishnan, Steitz and Yonath in 2009). 

Rotaxanes-Threading Molecular Rings Rotaxanes are obtained by threading linear polymers through molecular rings such as cyclodextrins, crown ethers and cyclophanes. Molecular shuttles based on the rotaxane structure have been proposed. 

Template effects have been used in rotaxane synthesis to direct threading of the axle through the wheel. Since macrocycHc compounds such as cyclodextrins, crown ethers, cyclophanes, and cucurbiturils form stable complexes with specific guest molecules, they have been widely used in the templated synthesis of rotax-anes as ring (wheel) components. Here, we briefly discuss macrocycles used in the synthesis of rotaxane dendrimers and their important features. 

Fig. 3. Rotaxane dendrimers incorporating cucurbituril threaded on l,6-di(bipyridmium)hex-ane with Fr chet dendrons as stoppers...

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. 

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 the late 1980s, the groups of Richey and Bickelhaupt reported rotaxanes in which linear diorganozinc compounds appear to have been threaded though the holes of crown ethers.87,88 Single crystal X-ray analyses proved that these... 

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... 

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). 

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-... 

Scheme 2. Rotaxane synthesis combining different meth- threading-followed-by-stopperingthen (2) slippage...

Scheme 12a-d There are several general methods to prepare rotaxanes a threading followed by capping b clipping c slippage d snapping... 

Scheme 13 Templated synthesis of rotaxane 22 employing the threading-followed-by-capping methodology...

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