Cyclophane

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. 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. Basic cyclophane types are metacyclophanes (I) in scheme 1, paracyclophanes (II) and 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. 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 is one of the smallest, yet 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. In organic reactions cyclophane tends to react as a diene derivative and not as an arene. With bromine it gives 1,4-addition and with chlorine the 1,2-addition product forms. 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 shielded to a position of around - 0.5 ppm, that is, even shielded 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 picometers (pm). A non-bonding nitrogen to arene distance of 244 pm is recorded for a pyridinophane and in the unusual superphane the two benzene rings are separated by a mere 262 pm. Other representative of this group are in-methylcyclophanes, in-ketocyclophanes and in,in-bis(hydrosilane). 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 compound 7. Metaparacyclophanes constitute another class of cyclophans like the metaparacyclophane in scheme 4 featuring a in-situ Ramberg-Bäcklund Reaction converting the sulfone 3 to the alkene 4.