Cyclophanes importance

Cyclophanes or 7r-spherands have played a central role in the development of supramolecular chemistry forming an important class of organic host molecules for the inclusion of metal ions or organic molecules via n-n interactions. Particular examples are provided by their applications in synthesis [80], in the development of molecular sensors [81], and the development of cavities adequate for molecular reactions with possible applications in catalysis [82]. The classical organic synthesis of cyclophanes can be quite complex [83], so that the preparation of structurally related molecules via coordination or organometallic chemistry might be an interesting alternative. 

Template effects have been used in rotaxane synthesis to direct threading of the axle through the wheel. Since macrocycHc compounds such as cyclodextrins, crown ethers, cyclophanes, and cucurbiturils form stable complexes with specific guest molecules, they have been widely used in the templated synthesis of rotax-anes as ring (wheel) components. Here, we briefly discuss macrocycles used in the synthesis of rotaxane dendrimers and their important features. 

As seen, cyclophane structures shown in Schemes 1.4b through 1.4e have the following unique feature The through-bond distance within the paracyclophane fragment is held constant, whereas the spatial distance between the ion-radicalized and neutral moieties is changed. Therefore, the relative importance of through-bond and through-space mechanisms for intramolecular electron transfer can be learned directly from experimental data on these molecules. 

Molecules with a single helical unit, such as in hydrogen peroxide, are rare. More commonly found are bundles of two to nine units with coincident central bonds (see Table 1). The combination of two, e.g., in cyclophanes, and four, e.g., in biaryls and allenes, helical units are particularly important. Such compounds have a stereogenic axis, as stereoisomers may be... 

Because of totally synthetic nature, cyclophanes are one of the most important class of hosts which will contribute to understanding of the detailed aspects of... 

From this important subgroup of cyclophanes [2.2]metacyclophane (5) and its derivatives (including [3.3]precursors for synthetic purposes) as well as some related carbo- and heterophanes have attracted much attention. Syntheses 30), the stereochemistry as well as several reactions and spectra 12) have been reviewed earlier. 

The scope of the tether-directed remote functionalization has been expanded from Cgo to the higher fullerene C70, and the described reactions are completely regioselective, featuring, in the case of C70, the kinetically disfavored addition pattern. The crown ether is a real template, since it can be readily removed by transesterification, giving a much-improved access to certain bis-adducts that are not accessible by the direct route. Cation-binding studies by CV reveal that cyclophane-type crown ethers derived from C60 and C70 form stable complexes with metal cations, and a perturbation of the fullerene reduction potentials occurs because the cation is tightly held close to the fullerene surface. This conclusion is of great importance for future developments of fullerene-based electrochemical ion sensors. 

An important reason for the historical reliance on local geometry was eliminated by the postulation of p bonds, especially for the class of cyclophanes. 

The experimental CD spectra of these donor-acceptor cyclophanes were very well reproduced by the theoretical calculations at the TD-DFT-BH-LYP/TZV2P level [32]. The amount of the exact exchange in the functional to be used turned out to be important for better agreement with experiment. The analysis of the configuration contributions showed that the observed CD spectra essentially resulted from simple overlap of the Cotton effects of pure n-n transitions in the donor and acceptor parts, in addition to the CT transitions. Thus, the exciton chirality method cannot be applied to the absolute configuration determination for these cyclophanes. The experimental CD spectrum of more congested donor-acceptor cyclophane 22 was reproduced in a less satisfactory manner compared to the cases of 20 and 21. The absolute configuration was safely determined by a comparison of the experimental spectrum with the theoretical data at the TD-DFT level however, the satisfactory reproduction of the whole CD spectrum of 22 was only possible by... 

Macrocycles are a highly topical subject. They constitute a large spectrum of compounds involving both artifical substances and natural products such as crowns, cryptands, cyclophanes, porphyrins, or macrolides. The former initiated the exiting area of host-guest supramolecular chemistry, which was highlighted by the award of the Nobel Prize for Chemistry to D. J. Cram, J.-M. Lehn, and C. J. Pedersen in 1987 but is still developing enormously. Porphyrins and macrolides are important active substances. No wonder that macrocycles are of immediate interest and everyone wants to know how they can be synthesized efficiently. 

The pyrolytic elimination of sulfur dioxide from cyclic sulfones to give (strained) macrocycles (sulfone pyrolysis) counts among the most important reactions in cyclophane chemistry. There is hardly any research group dealing with cyclophanes which did not make use of the sulfone pyrolysis for the synthesis of such molecules. Since its first application in cyclophane synthesis over twenty years ago [1], the sulfone pyrolysis enabled researchers to synthesize a wealth of strained cyclophanes and developed into a reaction of general importance. 

Direct pyrolysis of the cyclic sulfides without prior chemical modification [11] special method without general importance in cyclophane chemistry. 

A special issue devoted to molecular machines appeared in Accounts of Chemical Research in 2001. It reflects the current interest for this field in which ruthenium complexes act as important tools. Molecular machines are characterized by a mobile part and a stationary part. Photochemical and electrochemical inputs can make a machine work, offering the advantage of being switched on and off easily and rapidly. Mechanically interlocked molecules, such as rotaxanes and catenanes, are suitable candidates. Crown ethers, cyclophanes, and calixarenes are representative families of the cyclic... 

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