Crown ethers mechanically interlocked

However, produced a neutral [2]catenane (Scheme 11.5) [lib] bearing a mechanically interlocked bis-NDI macrocycle in 53% yield. This yield is far more attractive for a bis-NDI macrocycle than the 10% yield originally obtained in macrocycles [30a]. This approach involved an oxidative coupling of two acetylenic naphthalene diimides in the presence of crown ether. The efficiency of the catenation can be attributed to the crown ether component acting as a permanent template for the forming cyclophane. 

This topic was partially covered in CHEC-II(1996) <1996CHEC-II(9)809> under the subentry Catenanes and Rotaxanes . In this section, emphasis is given to the design and construction (and to some extent, the properties) of supramolecular architectures derived from or incorporating crown ethers rather than to the synthesis of the crown ether component present in them. The crown ether rings described herein are either covalently linked (dendrimers), mechanically interlocked (rotaxanes, catenanes), or just bound by noncovalent interactions (pseudorotaxanes) to the rest of the supermolecule to which they belong. 

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... 

A main chain polyrotaxane, a mechanically interlocked structure that can be considered as a string of pearls, in which the strand is the polymer backbone and the pearls are the cychc species threaded onto the strand. The backbone polymer can be a polyester, polyamide, poly(ethylene oxide), or virtually any linear polymer. The cyclic species are typically crown ethers, cyclodextrins (CDs), cucurbimrils, and calix-arenes. Polyrotaxanes can be made by two different methods statistical threading or via the template approach (enthalpi-cally driven). The statistical approach utilizes le Chatelier s principle with an excess macrocycle during the polymerization. The template approach is driven by attractive interactions of the macrocyclic species with either the monomer... 

One of the most widely used supramolecular interactions in the field of mechanically interlocked molecules, so-called mechanostereochemistry, is that of the strong hydrogen bonds between ammonium ions and crown ethers, which were first reported by Stoddart and coworkers in the mid-1990s. This versatile interaction has fonnd nse in the construction of a myriad of interlocked systems and bistable molecules. 

FIGURE 2.29 Electron donors covalently bonded to crown ethers and then mechanically interlocked with C o-ammonium salt derivatives. 

Polyrotaxanes, in which many cyclic molecules are mechanically interlocked onto a polymeric chain, have been synthesized using macrocyclic compounds such as cyclodextrins, crown ethers and blue boxes. Synthesis of pillar[5]arene-based polyrotaxanes consisting of cationic polymers, such as viologen polymer, is relatively easy because pillar[5]arene forms stable host-guest complexes with cationic molecules (see details in Chapter 9) however, it is difficult to synthesize nonionic polymer-based polyrotaxanes. To synthesize nonionic polymer-based polyrotaxanes, we used a liquid... 

Interlocking of the tetracationic cyclophane with a crown ether through the superimposition of a chemical to a supramolecular bond according to the scheme in Fig. 9B produces a 2-concatenane [27], The latter is another example of the exciting concept of a mechanical bond between two closed structures not directly linked by covalent bonds. The structures must remain constrained but are otherwise endowed with the possibility of movement, as in a macroscopic chain. Continuous catenane sequences with DP > 5 appear difficult to prepare. However, newer polymers based on 2-concatenane units connected by segments of a covalent chain are described in this volume in Chapter 8. 

---