Cyclophane electron-deficient

The versatility and reversibility of exo-active surfaces have been further explored by using pseudo-rotaxane architectures. Pseudo-rotaxanes generated from the electron-deficient cyclophane cyclobi s(paraquat-p-phenylene) (CBPQT4 +) (3) and... 

The first example of electrochemically driven molecular shuttles is rotaxane 284+ (Fig. 13.25) constituted by the electron-deficient cyclophane 124+ and a dumbbellshaped component containing two different electron donors, namely, a benzidine and a biphenol moieties, that represent two possible stations for the cyclophane.10 Because benzidine is a better recognition site for 124+ than biphenol, the prevalent isomer is that having the former unit inside the cyclophane. The rotaxane... 

After this first report, a remarkable number of electrochemically controllable molecular shuttles have been designed, constructed, and studied. Rotaxane 294+ (Fig. 13.26), for instance, incorporates the electron-deficient cyclophane 124+ and a dumbbell containing two kinds of electron-rich units, namely, one 2,6-dioxyanthra-cene and two 1,4-dioxybenzene moieties.34 In solution, the rotaxane is present as the isomer with the 2,6-dioxyanthracene unit inside the cyclophane, owing to the fact that this unit is a better station in comparison to the 1,4-dioxybenzene recognition sites. 

Stepwise synthesis of macrocyclic parts accompanied with catenation can be applied to the synthesis of oligo-catenane. Stoddart et al. prepared [7]catenane on the basis of the charge-transfer interaction between the electron-deficient cyclophane bearing 4,4 -bis(pyridinium salt) and the crown ether bearing electron-rich aromatics such as... 

Figure 5 A molecular shuttle (5) [9]. An electron-deficient cyclophane is threaded onto a linear component incorporating two sites differing by their electron-donor properties. In the resting state, ca. 84% of the macrocycle is bound to the more powerful clcctron-donor, the benzidine moiety. Electrochemical oxidation of the latter triggers the translation of the macrocycle to the biphenol moiety. The process is reversible.

Pyridinophanes (72C420) as well as mixed cyclophanes with electron-deficient and -excessive subunits (74JOC2570) have been prepared similarly. Recently, [8+6] and [6 + 6] cycloadducts (33) and (34), respectively, have been synthesized by means of a crossdimerization of vinyl-p-xylylene (35) with heteroxylylene-type intermediates (36) (78TL415). 

This [2]catenane is composed of a jt-electron-deficient tetracationic cyclophane interlocked with a Jt-electron-rich macrocyclic polyether. In addition to a mechanical bond, [jt Jt] stacking interactions between the complementary aromatic units, [C-H---0] hydrogen bonds between the a-bipyridinium hydrogen atoms and the poly-ether oxygen atoms, and [C-H---Jt] interactions between the 1,4-dioxybenzene hydrogen atoms and the p-phenylene spacers in the tetracationic cyclophane hold the two macrocyclic components together and control their relative movements in solution. As a result of the asymmetry of the tetracationic cyclophane, two transla-... 

Photoinitiated SET has been used to drive a molecular machine and absorption and fluorescence spectroscopy have been used to monitor it. A 1 1 pseudoro-taxane forms spontaneously in solution as a consequence of the donor-acceptor interactions between the electron-rich naphthalene moiety of the thread (380) and the electron-deficient bipyridinium units of the cyclophane (381). The threading process is monitored by the appearance of a charge transfer absorption band and disappearance of the naphthalene fluorescence. Excited state SET from 9-anthracenecarboxylic acid (9-ACA) reduces a bipyridinium moiety of the cyclophane, lessening the extent of interaction between the thread and the cyclophane and dethreading occurs. On addition of oxygen the reduced cyclophane is reoxidised and threading reoccurs. ... 

The controlled motion of an arm covalently linked to a ring, a further scorpionand-like system, has been designed also with purely organic systems, in the absence of metal ions. A recent example refers to system 4, in which a side-chain containing the 7i-electron rich 1, 5-dioxynaphthalene moiety is covalently linked to a tetracationic cyclophane, containing two re-electron deficient 4, 4 -bipyridinium subunits, as indicated in Fig. 7 [14]. 

One of the appealing aspects of catenanes that are based upon the interactions between 7t-electron-rich crown ethers and 7t-electron-deficient tetracationic cyclophanes is the modularity that can be employed (Fig. 25) in their template-directed syntheses. Virtually any 7t-electron-rich crown ether with identical or different recognition sites can be introduced into this class of catenanes. Also, by exploiting the different ways in which cyclophanes can be clipped around the preformed ring of a macrocyclic polyether, it is possible to incorporate [46] different 7r-electron-deficient units into the cyclophanes. 

Fig. 25. The modular synthetic scheme outlined in a shows how virtually any crown ether containing 7i-electron-rich units can be catenated with any cyclophane that contains two 71-electron-deficient units, b When both the crown ether and the cyclophane contain two different recognition sites, then four translational isomers can, in principle, be observed...

The rationale behind this design was justified upon electrochemical investigation of the [2]catenane 184+. This catenane - synthesized in 43% yield (Fig. 26) from crown ether BPP34C10, the bipyridinium dibromide derivative 192+ and ( )-l,2-bis(4,4 -bipyridyl)ethylene - was demonstrated to consist, in solution, of mainly co-conformer A, with the more powerful n-electron-accepting bipyridinium unit located inside the cavity of the crown ether. Upon electrochemical reduction of this bipyridinium unit, the cyclophane undergoes a circumrotational movement with respect to the crown ether such that the profoundly more electron-deficient 7t-extended bipyridinium unit resides inside the cavity of the crown ether, affording co-conformer B. When the bipyridinium radical cation is oxidized back down to its dicationic state, the opposite circumrotational process occurs and the system reverts back to co-conformer A, its ground state [49]. 

So far, we have addressed the construction and properties of polyrotaxanes, incorporating one or more tetracationic cyclophanes on 7C-electron rich dumbbells Figure 4a), Formalistically, we could also extrapolate in our minds from the solid state structures of the [2]catenane to polyrotaxanes which incorporate one or more bis-paraphenylene-34-crown-lO macrocycles onto 7C-electron deficient dumbbells Figure 4b). We shall now discuss the initial approaches that have been developed for constructing such polyrotaxanes. 

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