Cyclic catenane

Unlike the cyclic catenanes, rotaxanes are simpler species originally proposed by Wasserman and first demonstrated by Harrison [77], In these systems a cyclic molecule is threaded on a rigid or flexible molecular axle, attracted by complementary binding sites, to form a pseudorotaxane. Under normal entropically driven supramolecular chemistry the cyclic component would eventually slip off one or other end of the central axle, however, it can be kept in place if both ends of the axle react with bulky groups while the macrocycle is still threaded. Alternatively a macrocycle can be formed around an axle molecule that already possesses bulky termini, as shown in Fig. 1.22. Either method leads to an entanglement in which the cycle species can move along the thread without ever coming off. 

Because of space limitations, only meso- and macrocycles possessing heteroatoms and/or subheterocyclic rings are reviewed in general, lactones, lactams, and cyclic imides have been excluded. In view of the delayed availability of some articles appearing in previous years, several have been incorporated. The introduction of a systematic nomenclature of catenanes, rotaxanes, and derived assemblies has recently appeared <00JPC437> this should have appeared two decades ago, but is still timely in futuristic point of view. 

The examples summarized in Table 5 and in Sect. 5 of this review illustrate the applicability of RCM to the preparation of various macrocyclic perfume ingredients [30], pheromones [30], antibiotics [31-35], crown ethers [36], cyclic peptides [37],catenanes [38] and capped calixarenes [39]. 

Most of the reactions that can be used to prepare distannanes can be extended to the preparation of oligo- and polystannanes, which have attracted interest for potential use in electronic and optical devices.455 The structures of the products are not necessarily those of completely linear catenanes, (R2Sn) , and different degrees of branching confer different properties. The formation of polymers is also frequently accompanied by the formation of cyclic pentamers or hexamers, and samples prepared by different methods may show substantially different properties. 

It should also be recalled that a full electrochemical, as well as spectroscopic and photophysical, characterization of complex systems such as rotaxanes and catenanes requires the comparison with the behavior of the separated molecular components (ring and thread for rotaxanes and constituting rings in the case of catenanes), or suitable model compounds. As it will appear clearly from the examples reported in the following, this comparison is of fundamental importance to evidence how and to which extent the molecular and supramolecular architecture influences the electronic properties of the component units. An appropriate experimental and theoretical approach comprises the use of several techniques that, as far as electrochemistry is concerned, include cyclic voltammetry, steady-state voltammetry, chronoampero-metry, coulometry, impedance spectroscopy, and spectra- and photoelectrochemistry. 

Figure 13.18 Cyclic voltammograms for reduction of catenanes 18s+and 204+ (argon-purged MeCN/Et4NPF6 0.05 M, 298 K, glassy carbon electrode, scan rate 200 mV/s).

In the major isomer, the bipyridinium unit is located inside the cavity of the macrocyclic poly ether and the /7Y//7,v-bis(pyridinium)ethylene unit is positioned alongside, as confirmed by the electrochemical analysis. The cyclic voltammo-gram of the catenane shows four monoelectronic processes that, by a comparison with the data obtained for the free cyclophane, can be attributed as follows the first and third processes to the first and second reductions of the bipyridinium unit, and the second and fourth ones to the first and second reductions of the trans-bis (pyridinium)ethylene unit. The comparison with the tetracationic cyclophane also evidences that all these reductions are shifted toward more negative potential values (Fig. 13.33b). 

The acyloin condensation was used in an ingenious manner to prepare the first reported catenane (see p. 91).727 The catenane (39) was prepared by a statistical synthesis (p. 91) in the following manner An acyloin condensation was performed on the diethyl ester of the C34 dicarboxylic acid (tetratriacontandioic acid) to give the cyclic acyloin 37. This was reduced by a Clemmensen reduction with DCI in D20 instead of HC1 in H20, thus producing a C34 cycloalkane containing deuterium (38) 728... 

I was intrigued. Mulling over the problem later that evening, and considering the need both to form and to detect catenanes, a statistical approach seemed attractive. Synthesis of a C30 ring from a linear precursor in the presence of an inert C20 cyclic compound should enable threading of some of the smaller rings before cyclization. A C50 product would indicate the presence of an interlocked system. 

Two-dimensional NMR studies and mass spectrometry identified the isomers as being the cyclic tetramer 6 and the [2]catenane 7 consisting of two interlocked cyclic dimers. Later X-ray crystal structures confirmed these results [16, 17],... 

It is clear from comparison of the reactivity towards polycondensation of the difunctionalized [2]catenand 50b and 53 with their corresponding difunctionalized [2]catenates 50a and 54, respectively, that the mobility of the interlocked macrocycles of catenanes plays a fundamental role for the nature of the resulting polycondensates - either cyclic oligo[2]catenanes 52,54 or linear high molecular-weight poly[2]catenanes 51b,56 (Schemes 18-20). 

Scheme 26. Proposed synthesis of polymeric catenane 11, with anion-exchange separation of cyclic species from linear polymers [33].

With the exception of DNA catenanes and protein catenanes, and despite various synthetic attempts, only one polymeric catenane structure, i.e. the catenated block copolymer 72, is known [31]. Evidently, the fact that the two constitutive cyclic polymers have two different chemical structures greatly facilitates the syn-... 

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