Cyclophane

[n]-Cyclophanes

Structures

Paracyclophanes adopt the boat conformation normally observed in cyclohexanes. Smaller value of n lead to greater distortions. X-ray crystallography on '[6]paracyclophane' 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. Despite their distorted structures, cyclophanes retain their aromaticity, as determined by UV-vis spectroscopy.

Reactivity

With regards to their reactivity, cyclophanes often exhibit diene-like behavior, despite evidence for aromaticity in even the most distorted [6]-cyclophane. This highly distorted cyclophane photochemically converts to the Dewar benzene derivative. Heat reverses the reaction. With dimethyl acetylenedicarboxylate, [6]metacyclophane rapidly undergoes the Diels-Alder reaction.

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).

NMR properties

The proton NMR spectra of cyclophanes have been intensively examined to gain insights into the aromaticity of the benzene ring. Also of great interest is the shielding effects of the aromatic ring on the hydrocarbon strap. Generally the aromatic protons appear near their usual positions around 7.2 ppm, indicating that even with severe distortions, the ring retains aromaticity. The central methylene protons in the aliphatic bridge are shielded to a position of around - 0.5 ppm.

Synthesis

[6]paracyclophane can be synthesized beginning with the Bamford–Stevens reaction to form the spiro ketone 1 in scheme 3, rearranging in a pyrolysis reaction through the carbene intermediate 4. A separate route to the Dewar form involves a Ag⁺-induced rearrangement reaction of the bicyclopropenyl compound 7.

Metacyclophanes are generally less strained and thus more easily prepared than paracyclophanes. Shown below is the route to a [14][14]metaparacyclophane in scheme 4 featuring a in-situ Ramberg-Bäcklund Reaction converting the sulfone 3 to the alkene 4.

Naturally occurring [n]-cyclophanes

A few cyclophanes 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.

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.