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 carbon 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).
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.
Two additional types of cyclophanes were discovered in nature when they were isolated from two species of cyanobacteria from the family Nostocacae. These two classes of cyclophanes are both [7,7] paracyclophanes and were named after the species from which they were extracted: cylindrocyclophanes from Cylindrospermum lichenforme and nostocyclophanes from Nostoc linckia.
[n]Cycloparaphenylenes ([n]CPPs) consist of cyclic all-para-linked phenyl groups. This compound class is of some interest as potential building block for nanotubes. Members have been reported with 18, 12, 10, 9, 8, 7, 6 and 5 phenylenes. These molecules are unique in that they contain no aliphatic linker group that places strain on the aromatic unit. Instead the entire molecule is a strained aromatic unit.
Generalization of cyclophanes led to the concept of phanes in the IUPAC nomenclature.
The systematic phane nomenclature name for e.g. metacyclophane is 1(1,3)-benzenacyclopentadecaphane; and [2.2']paracyclophane (or [2.2]paracyclophane) is 1,4(1,4)-dibenzenacyclohexaphane.
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