Rotaxane mechanism

As already pointed out in the case of rotaxanes, mechanical movements can also be induced in catenanes by chemical, electrochemical, and photochemical stimulation. Catenanes 164+ and 174+ (Fig. 19) are examples of systems in which the conformational motion can be controlled electrochemically [82, 83], They are made of a symmetric electron acceptor, tetracationic cyclophane, and a desymmetrized ring comprising two different electron donor units, namely a tetrathiafulvalene (TTF) and a dimethoxybenzene (DOB) (I64 1) or a dimethoxynaphthalene (DON) (174+) unit. Because the TTF moiety is a better electron donor than the dioxyarene units, as witnessed by the potentials values for their oxidation, the thermodynamically stable conformation of these catenanes is that in which the symmetric cyclophane encircles the TTF unit of the desymmetrized macrocycle (Fig. 19a, state 0). 

Also, from the dendrimer point of view, the introduction of mechanical bonds to dendrimers has an enormous potential to alter the properties of dendrimers in a controlled way. For example, in the synthesis of a Type II rotaxane dendrimers, the wheel components are introduced to the terminal groups of the dendrimers. This can improve the solubility of dendrimer in organic and/or aqueous media due to the formation of complexes soluble in such solvents. 

In particular, rotaxane dendrimers capable of reversible binding of ring and rod components, such as Type II, pseudorotaxane-terminated dendrimers, can be reversibly controlled by external stimuli, such as the solvent composition, temperature, and pH, to change their structure and properties. This has profound implications in diverse applications, for instance in the controlled drug release. A trapped guest molecule within a closed dendrimeric host system can be unleashed in a controlled manner by manipulating these external factors. In the type III-B rotaxane dendrimers, external stimuli can result in perturbations of the interlocked mechanical bonds. This behavior can be gainfully exploited to construct controlled molecular machines. 

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). 

R. Jager, F. Vogtle, A New Synthetic Strategy towards Molecules with Mechanical Bonds Nonionic Template Synthesis of Amide-Linked Catenanes and Rotaxanes , Angew. Chem Int. Ed. Engl. 1997,36,930-944. 

In this kind of pseudorotaxanes, rotaxanes, and catenanes, the stability of a specific (supra)molecu-lar structure is a result, at least in part, of the CT interaction. In order to cause mechanical move ments, such a CT interaction has to be destroyed. 

Kinetically stable superarchitectures can be assembled by relying upon both noncovalent bonds and mechanical coercion. Thus, at elevated temperatures, rotaxane-like complexes (a) are generated when macrocycles slip over the stoppers of chemical dumbbells, while hemicarceplexes (b) are created when guests squeeze through the portals of hemicarcerands. 

Scheme 20 Synthesis of [2] rotaxane 45 by a three-component threading-followed-by-capping mechanism...

Scheme 24 Templated synthesis of [2]rotaxane 50 by the clipping mechanism...

Rotaxanes are mechanically-linked molecules consisting of a molecular strand with a cyclic molecule linked around it. The molecular strand is terminated with bulky end groups at both ends. Rotaxanes find use in the fact that there are a number of positions (stations) along the molecular strand to which the cyclic molecule can temporarily attach. 

Leigh DA, Venturini A, Wilson AJ, Wong JKY, Zerbetto F (2004) The mechanism of formation of amide-based interlocked compounds prediction of a new rotaxane-forming motif. Chem Eur J 10 4960 -969... 

Aucagne V, Bema J, Crowley JD, Goldup SM, Hanni KD, Leigh DA, Lusby PJ, Ronaldson VE, Slawin AMZ, Viterisi A, Walker DB (2007) Catalytic active-metal template synthesis of [2]rotaxanes, [3]rotaxanes, and molecular shuttles, and some observations on the mechanism of the Cu(I)-catalyzed azide-alkyne 1, 3-cycloaddition. J Am Chem Soc 129 11950-11963... 

Besides their topology, rotaxanes and catenanes are also appealing systems for the construction of molecular machines because (i) the mechanical bond allows a large variety of mutual arrangements of the molecular components, while conferring stability to the system, (ii) the interlocked architecture limits the amplitude of the intercomponent motion in the three directions, (iii) the stability of a specific... 

An interesting example of a system whose components become mechanically bound upon surface immobilization is rotaxane trans-21 (Fig. 13.24), consisting of a ferrocene-functionalized /3-cyclodextrin (fi-CD) macrocycle threaded on a molecule containing a photoisomerizable azobenzene unity and a long alkyl chain.33 A monolayer of trans-21 was self-assembled on a gold electrode. Therefore, the ring... 

Figure 13.27 Dual-pathway square scheme mechanism that accounts for the rearrangements induced by the monoelectronic reduction of deprotonated rotaxane 92+. The species A and C represent the stable structure of the deprotonated rotaxane and its monoreduced form, respectively, whereas and D are metastable intermediates. Note that the exact position of the macrocycle along the axle in the reduced forms and C is not known. From a simple digital simulation of the cyclic voltammetric patterns, the following values have been obtained = - 0.59V, E°dc = - 0.34V, /cAD 0.15S- da<2.5s kBC > 100 s and kCB 1 s V...

The redox-controlled mechanical switching in SAMs of disulfide-functionalized bistable TTF-DMN rotaxanes consisting of cyclophane 124+ and a dumbbell-shaped component containing TTF and DMN stations was also extensively investigated.49... 

---