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A cyclophane is a hydrocarbon consisting of an aromatic unit (typically a benzene ring) and an aliphatic chain that forms a bridge between two non-adjacent positions of the aromatic ring. More complex derivatives with multiple aromatic units and bridges forming cagelike structures are also known. Cyclophanes are well studied in organic chemistry because they adopt unusual chemical conformations due to build-up of strain. Despite this, cyclophane structures are not unknown to biomolecules.
Basic cyclophane types are [n]metacyclophanes (I) in scheme 1, [n]paracyclophanes (II) and [n,n']cyclophanes (III). the prefixes meta and para correspond to the usual arene substitution patterns and n refers to the number of atoms making up the bridge.
Additional recommended knowledge
Paracyclophanes adopt the boat conformation normally observed in cyclohexanes but are still able to retain aromaticity. The smaller the value of n the larger the deviation from aromatic planarity. In paracyclophane which one of the stable cyclophanes X-ray crystallography shows that the aromatic bridgehead carbon atom makes an angle of 20.5° with the plane. The benzyl carbons deviate by another 20.2°. The carbon to carbon bond length alternation has increased from 0 for benzene to 39 pm  .
Yet the proton NMR spectrum displays the aromatic protons and their usual deshielded positions around 7.2 ppm and the central methylene protons in the aliphatic bridge are even severely deshielded to a position of around - 0.5 ppm, that is, even deshielded compared to the internal reference tetramethylsilane. With respect to the diamagnetic ring current criterion for aromaticity this cyclophane is still aromatic.
One particular research field in cyclophanes involves probing just how close atoms can get above the center of an aromatic nucleus . In so-called in-cyclophanes with part of the molecule forced to point inwards one of the closest hydrogen to arene distances experimentally determined is just 168 picometer.
paracyclophane can be synthesized   in the laboratory by a Bamford-Stevens reaction with spiro ketone 1 in scheme 3 rearranging in a pyrolysis reaction through the carbene intermediate 4. The cyclophane can be photochemically converted to the Dewar benzene 6 and back again by application of heat. A separate route to the Dewar form is by a cationic silver perchlorate induced rearrangement reaction of the bicyclopropenyl copound 7.
Naturally occurring cyclophanes
Despite carrying strain, the cyclophane motif does exist in nature. One example of a metacyclophane is cavicularin.
Haouamine A is a paracyclophane found in a certain species of tunicate. Because of its potential application as an anticancer drug it is also available from total synthesis via an alkyne - pyrone Diels-Alder reaction in the crucial step with expulsion of carbon dioxide (scheme 5) .
In this compound the deviation from planarity is 13° for the benzene ring and 17° for the bridgehead carbons . An alternative cyclophane formation strategy in scheme 6  was developed based on aromatization of the ring well after the formation of the bridge.
The driving force for ring-opening and polymerization is strain relief. The reaction is believed to be a living polymerization due to the lack of competing reactions.
Because the two benzene rings are in close proximity this cyclophane type also serves as guinea pig for photochemical dimerization reactions as illustrated by this example :
The product formed has an octahedrane skeleton. Interestingly when the amine group is replaced by a methylene group no reaction takes place: the dimerization requires through-bond overlap between the aromatic pi electrons and the sigma electrons in the C-N bond in the reactants LUMO.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Cyclophane". A list of authors is available in Wikipedia.|