Catenane
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A catenane is a mechanically-interlocked molecular architecture consisting of two or more interlocked macrocycles. The interlocked rings cannot be separated without breaking the covalent bonds of the macrocycles. Catenane is derived from the Latin catena meaning "chain". They are conceptually related to other mechanically-interlocked molecular architectures, such as rotaxanes, molecular knots or molecular Borromean rings. Recently the terminology "mechanical bond" has been coined that describes the connection between the macrocycles of a catenane.
[edit] Synthesis
There are two primary approaches to the organic synthesis of catenanes. The first is to simply perform a ring closing reaction with the hope that some of the rings will form around other rings giving the desired catenane product. This so-called "statistical approach" lead to the first successful synthesis of a catenane, however the method is highly inefficient and is not usually used.
The second approach relies on supramolecular preorganization of the macrocyclic precursors utilizing hydrogen bonding, metal coordination, hydrophobic forces, or coulombic interactions. These non-covalent interactions offset some of the entropic cost of association and help position the components to form the desired catenane upon the final ring-closing. This "template-directed" method has dramatically increased the yields that can be obtained for catenanes and thus has increased their potential for application. An example of this approach used bis-bipyridinium salts which form strong complexes threaded through crown ether bis(para-phenylene)-34-crown-10.
[edit] Families of catenanes
There are a number of distinct methods of holding the precursors together prior to the ultimate ring-closing reaction in a template-directed catenane synthesis. Each noncovalent approach to catenane formation results in what can be considered different families of catenanes.
Another family of catenanes are called pretzelanes or bridged [2]catenanes after their likeness to pretzels with a spacer linking the two macrocycles. In one such system one macrocycle is an electron deficient oligo Bis-bipyridinium ring and the other cycle is crown ether cyclophane based on paraphenylene or naphthalene. X-ray diffraction shows that due to pi-pi interactions the aromatic group of the cyclophane is held firmly inside the pyridinium ring. A limited number of (rapidly-interchanging) conformers exist for this type of compound.
In handcuff-shaped catenanes , two connected rings are threaded through the same ring. The bis-macrocycle (red) contains two phenanthroline units in a crown ether chain. The interlocking ring is self-assembled when two more phenanthroline units with alkene arms coordinate through a copper(I) complex followed by a metathesis ring closing step.
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Handcuff-shaped catenane
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[edit] References
- ↑ Dynamic Chirality in Donor-Acceptor Pretzelanes Y. Liu, S. A. Vignon, X. Zhang, P. A. Bonvallet, S. I. Khan, K. N. Houk, and J. F. Stoddart J. Org. Chem.; 2005; 70(23) pp 9334 - 9344; (Article) DOI: 10.1021/jo051430g Graphical Abstract detailed molecular structure
- ↑ A catenane consisting of a large ring threaded through both cyclic units of a handcuff-like compound Julien Frey, Tomá Kraus, Valérie Heitz and Jean-Pierre Sauvage Chemical Communications, 2005, (42), 5310 - 5312 Abstract detailed molecular structure
