Stopper groups

The term rotamne derives (G. Schill, Histoiy , In Catenaries, Rotaxanes, and Knots, Academic, New York, 1971, pp. 1-4) from the Latin words rota, meaning wheel, and axis, meaning axle. In these molecular compounds, several wheel and axle components are constrained to be bound to one another mechanically, Le., without the aid of any valence forces, since the axle(s) is/are endowed with bulky stopper groups that prevent the extrusion of the wheel(s). 

Squaraines 17b and 17c have terminal acetylene residues, which allowed to convert the squaraine dyes and tetralactam macrocycles into permanently interlocked rotaxane structures using copper-catalyzed and copper-free cycloaddition reactions with bulky stopper groups [58]. 

The procedure of the anion-templated synthesis was later used for the preparation of various rotaxanes by the groups of Smith [41] and Schalley [34-36], The anion-templation technique was carefully tested by Schalley et al. Altering the length of the axle-center pieces and the size of stopper groups they synthesized a variety of rotaxanes and analysed their yields and de-slipping rates [34, 35],... 

Affeld, A., Hubner, G.M., Seel, C., Schalley, C.A. Rotaxane or pseudorotaxane Effects of small structural variations on the deslipping kinetics of rotaxanes with stopper groups of intermediate size, Eur. J. Org. Chem. (2001), 2877-2890. 

The synthesis of fourfold [2]rotaxanes by clipping requires the efficient formation of heterodimers between a tetra-urea substituted by bulky stopper groups and an octaalk-enyl urea 6. We initially hoped that the steric crowding in homodimers of a tetra-tritylphenylurea calix[4]arene would be sufficient to shift the equilibrium toward the heterodimers in a mixture with 6. However, the distribution of the dimers was close to the statistical ratio and the desired rotaxane could be obtained in only 5% yield [59]. 

On a simpler scale the Rybak-Akimova group found that when 4/-(amino-methylene)benzo[18]crown-6 was treated with an acid salt, mass spectrometric evidence indicated that dimers were the most stable species [13]. This was backed up by an X-ray crystal structure of the compound crystallized from methanol which revealed pairs of mutually interlocked crowns. The amine termini had become pro-tonated and the resultant ammonium group, illustrated in Fig. 8.4, formed a complex with a second crown. Although yet to be attempted, it would be intriguing to react the complex with a linear component containing a second amine and a bulky stopper group to generate a metal-free, interlocked pair of rotaxanes with two amine stations . The contraction and extension of the complex could then be controlled as a function of pH. 

When supramolecular polymers are treated with bulky stopper groups, they may form poly[2]rotaxane daisy chains [32,60-68]. Cyclic tri[2]rotaxanes (daisy chain necklace) containing cyclodextrins have been prepared from the mixture of 6-(4-aminocinnamoyl)-Q -CD and 2,4,6-trinitrobenzene sulfonic acid sodium salt [50,59] in an agueous solution (Fig. 21). If the molecule changes its conformation (or co-conformation), the ring may expand or shrink by external conditions (temperature, solvents, photochemically, elec-trochemically). These compounds are important because the cycle can be used as a chemical valve as seen in ion channels in biological membranes. 

In other studies, matched complementarity between the cavity size of the macro-cyclic component and the size of the bulky stopper groups has been exploited to allow preparation of rotaxanes by means of the slippage mechanism mentioned previously. 

Such rotaxanes incorporate a macrocyclic structure (representative of a bead) on an acyclic chain (thread), with bulky stopper groups at their ends to prevent dethreading. The position of the bead on the thread, and hence representation of information, is controlled by the incorporation of stations on the thread upon which the bead preferentially resides. Requirements of a successful system are that the bead can be controllably shuttled between stations by an external stimulus and that the position of the bead can be subsequently read out. 

Table 14 Synthesis of polyrotaxanes by terminal coupling of stopper groups at polymer ICs with X coverage, number of CD tings per two polymer repeat units...

Fig. 5.9. Example for a rotaxane, which consists of an axle mechanically trapped inside the cavity of the macrocycle by large stopper groups attached to its two ends.

Rotaxanes are molecules consisting of one or more macrocyclic rings (beads) threaded by a linear component with bulky stopper groups at both ends [5]. The sliding motion of the macrocyclic rings is limited by the stopper groups,... 

---