Cyclophane

Source: Wikipedia, the free encyclopedia.
Structures of some fundamental cyclophanes: [n]-paracyclophanes (left), [n]-metacyclophanes, and [n.n]paracyclophanes (right).

In

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 examples of strained organic compounds.[1][2]

[n]-Cyclophanes

Structures

Structural details of [6]paracyclophanes, illustrating the distortion of the aromatic ring imposed by the (CH2)6 strap.

Paracyclophanes adopt the

UV-vis spectroscopy.[1]

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.[5] With

Diels-Alder reaction.[6]

A non-bonding nitrogen to arene distance of 244 pm is recorded for a pyridinophane and in the unusual

in-methylcyclophanes,[7] in-ketocyclophanes[8] and in,in-Bis(hydrosilane).[9]

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.[6]

Synthesis

Original synthetic route to [6]-paracyclopane.[10]

[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.[10]

Metacyclophanes are generally less strained and thus more easily prepared than paracyclophanes. Shown below is the route to a [14][14]metaparacyclophane

Ramberg-Bäcklund Reaction converting the sulfone 3 to the alkene
4.

Scheme 4. [14][14]metaparacyclophane
Scheme 4. [14][14]metaparacyclophane

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

Diels-Alder reaction in the crucial step with expulsion of carbon dioxide (scheme 5).[13]

Scheme 5. Haouamine A
Scheme 5. Haouamine A

In this compound the deviation from planarity is 13° for the benzene ring and 17° for the bridgehead carbons.[14] An alternative cyclophane formation strategy in scheme 6[15] was developed based on aromatization of the ring well after the formation of the bridge.

Scheme 6. Haouamine cyclophane substructure synthesis
Scheme 6. Haouamine cyclophane substructure synthesis

Two additional types of cyclophanes were discovered in nature when they were isolated from two species of cyanobacteria from the family Nostocacae.[16] 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.n]Paracyclophanes

Superphane.

A well studies member of the [n.n]paracyclophane family is [2.2]paracyclophane.[17][18] One method for its preparation is by the 1,6-Hofmann elimination of 4-methylbenzyltrimethylammonium hydroxide:[19]

Synthesis of 2.2-paracyclophane.
Synthesis of 2.2-paracyclophane.

The [2.2]paracyclophane-1,9-diene has been applied in

Grubbs' second generation catalyst:[20]

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:[21]

Formation of Octahedrane by Photochemical Dimerization of Benzene
Formation of Octahedrane by Photochemical Dimerization of Benzene

The product formed has an

LUMO
.

Phanes

Generalization of cyclophanes led to the concept of phanes in the IUPAC nomenclature. Some example systematic phane names are:

  • [14]metacyclophane is 1(1,3)-benzenacyclopentadecaphane
  • [2.2']paracyclophane (or [2.2]paracyclophane) is 1,4(1,4)-dibenzenacyclohexaphane

In "1(1,3)-benzenacyclopentadecaphane", the "1" refers to the first position of the ring as a "superatom", the "(1,3)" describes the "meta" location, "benzena" refers to the ring, and the "pentadeca" (15) describes the chain length counting the ring as one atom.

See also

General sources

  • B. H. Smith, Bridged Aromatic Compounds, Academic Press, New York, 1964.
    ISBN 978-0-323-16321-7
  • P. M. Keehn, S. M. Rosenfeld (eds.), Cyclophanes, Vols. 1 and 2, Academic Press,New York, 1983.
    ISBN 978-0-12-403001-5
  • F. Vögtle, F., G. Hohner, Top. Curr. Chem. 1978, 74, 1.
    doi:10.1007/BFb0050144
  • F. Vögtle, P. Neumann, Top. Curr. Chem. 1983, 113, 1; 1985, 115, 1.

References

  1. ^
    ISBN 978-0-471-72091-1
  2. ISBN 3527603964
    .
  3. doi:10.1021/ja00238a024
    .
  4. doi:10.1002/cber.19861190904
    .
  5. doi:10.1021/ja00459a055
    .
  6. ^
    S2CID 55279751
    .
  7. PMID 16089445
    .
  8. PMID 20112943
    .
  9. PMID 23971948
    .
  10. ^
    doi:10.1021/ja00815a070
    .
  11. PMID 12662074
    .
  12. Ramberg-Bäcklund Reaction with halide donor dibromodifluoromethane and base potassium hydroxide. Final step hydrogenation pf alkene by hydrogen and palladium on carbon
  13. PMID 16551088
    . The authors mark the biosynthetic origin as mysterious
  14. PMID 16623580
    .
  15. Nosylate
  16. doi:10.1021/ja00166a066
    .
  17. PMID 30065985
    .
  18. PMID 31283092
    .
  19. doi:10.15227/orgsyn.042.0083
    .
  20. PMID 17061303
    .
  21. PMID 17177393
    .
  22. PMID 23126565
    .
Retrieved from "https://en.wikipedia.org/w/index.php?title=Cyclophane&oldid=1207364361"