Dendritic rotaxanes

In their first step towards building dendritic rotaxanes Stoddart et al. [140] reported the construction of such materials via a so-called slipping method (Fig. 24). Thus, treatment of bisparaphenylene-34-crown-10 (BPP34C10) with tris(bipyridinium) compound 54 at 50 °C in acetonitrile for 10 days afforded mono-, di-, and tris-rotaxanes 55, 56, and 57, respectively. ES-MS was used to determine the molar masses of these macromolecules while upfield shifts in the... 

Rotaxane 80e can even be dimerized by reaction with l,3,5-tris(4-methyl-phenyl)benzene (105) [42]. Considering the steric crowding, it is remarkable that the resulting wheel-tris[2]rotaxane ([4]rotaxane) 106 is formed in 35% yield (Figure 42). Extension of the strategy of coupling rotaxanes to multifunctional units should enable easy access to amide-linked dendritic rotaxane structures. 

Stoddart et al.l<,lb have reported the self-assembly of branched [njrotaxanes in an investigation aimed at the preparation of larger, dendritic rotaxanes. Kato et al. 9lcl have reported the preparation of supramolecular liquid crystalline networks based on self-assembly of carboxylic acid-based, trigonally branched, //-bonding donors and bipyridine-type //-bonding acceptors. 

As described earlier, we classify dendritic poiyrotaxanes in which rotaxane building units grow like a dendrimer, as Type III rotaxane dendrimers. Depending on whether ring components are located on the branches or at the branching points. Type III rotaxane dendrimers are further classified as 111-A and 111-B, respectively. 

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. 

The properties of these rotaxane dendrimers are quite different from those of the individual rotaxanes or dendrimers and often a blend of both. In view of the versatile characteristics that a dendron or dendrimer can manifest, several new properties can be imparted to the rotaxanes. For example, the solubility of rotaxanes in organic solvents as well as in water can be significantly improved when large dendrimer units are appended enhancing the prospects of their use as molecular machines. The dendritic units can also influence the photo/electro-chemical properties of the rotaxanes. Employing photo-receptive dendron units, photo chemically driven molecular machines may be developed, where the dendrons act as antenna for photo-harvesting [62]. 

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. 

Fig. 25. Rotaxanes with dendritic stoppers synthesized by Stoddart et al.

Stoddart and co-workers have made use of Frechet-type dendrons as dendritic stoppers for self-assembled [n]rotaxanes [65]. The solubility enhancement that results from incorporating dendritic wedges at the termini facilitated the purification of these materials by column chromatography despite the polycationic nature of their bipyridinium backbone. Again, the dendritic wedges did not alter the electrochemical characteristics of the viologen subunits. Flowever, the enhanced solubility resulting from the presence of the dendritic components en-... 

The [2]rotaxanes, which contain tetrathiafulvalene (TTF) and 1,5-dioxynaphtha-lene (DNP) as the two recognition stations and cyclobis(paraquat-p-phenylene) (CBPQT4 + ) as the cyclic moiety, have been comprehensively investigated by Stoddart and coworkers.68 We have recently reported two new TTF-DNP-CBPQT4+ [2]rotaxanes 62 and 63. In these two [2]rotaxanes, the TTF moiety is the 4,4 (5 )-dialkylthiotetrathiafulvalene that is rather easily accessible based on the synthetic procedure developed by us previously,69 and two different spacers are used the cyclohexyl and alkyl chains. Two stopper units are the G2-dendritic moieties. 

A similar template-directed approach has been employed to self-assemble [n]rotaxanes incorporating up to four mechanically-interlocked components and large dendritic stoppers Amabilino, D.B., Ashton, P.R., Balzani, V., Brown, C.L., Credi, A., Frdchet, J.M.J., Leon, J.W., Raymo, F.M., Spencer, N., Stoddart, J.F., Venturi, M., J. Am. Chem. Soc. 1996, 118, 12012-12020. 

Fig. 6.24 Synthesis of a rotaxane (schematic) with dendritic stopper (cone) and tri-tylphenol stopper (sphere). SN = nucleophilic substitution. The grey arrows indicate dethreading of the wheel (red ellipse) - to the left or the right according to choice - from...

In addition, chiral dendrimers (see Section 4.2) can be resolved with the aid of HPLC into their enantiomers, if the silica gel material used as stationary phase has optically active substances bound to its surface [9]. Since the chiral stationary phase (CSP) [10] undergoes different intensities of interaction with the enantiomeric dendrimers, they are retained to different degrees, and in the ideal case two completely separated (baseline separated) peaks are obtained. This separation technique was successfully applied inter alia to racemic mixtures of planar-chiral dendro[2.2]paracyclophanes, cycloenantiomeric dendro[2] rotaxanes, topologically chiral dendro[2]catenanes [11] as well as topologically chiral, dendritically substituted molecular knots (knotanes) [12] (Section 4.2.3). 

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