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catenane
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.
== 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" led to the first successful synthesis of a catenane; however, the method is highly inefficient, requiring high dilution of the "closing" ring and a large excess of the pre-formed ring, and is rarely used.
The second approach relies on supramolecular preorganization of the macrocyclic precursors utilizing hydrogen bonding, metal coordination, hydrophobic effect, 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" approach, together with the use of high-pressure conditions, can provide yields of over 90%, thus improving the potential of catenanes for applications. An example of this approach used bis-bipyridinium salts which form strong complexes threaded through crown ether bis(''para''-phenylene)-34-crown-10.
Template directed syntheses are mostly performed under kinetic control, when the macrocyclization (catenation) reaction is irreversible. More recently, the groups of Sanders and Otto have shown that dynamic combinatorial approaches using reversible chemistry can be particularly successful in preparing new catenanes of unpredictable structure. The thermodynamically controlled synthesis provides an error correction mechanism; even if a macrocycle closes without forming a catenane it can re-open and yield the desired interlocked structure later. The approach also provides information on the affinity constants between different macrocycles thanks to the equilibrium between the individual components and the catenanes, allowing a titration-like experiment.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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