Helicene

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Hexahelicene
General structure formula of [n]helicene

In

chiral molecules.[1] The chemistry of helicenes has attracted continuing attention because of their unique structural, spectral, and optical features.[2][3][4][5][6][7][8]

Structure and properties

The

steric collisions. For helicenes with six benzene units, a 360° turn is completed. In the helicene series the dihedral angles
between the extremities increases going from [4]helicene (26°) to [6]helicene (58°) and then decreases again for example in [7]helicene (30°).

Helicenes are notable for having chirality despite lacking both

dextrorotatory
.

The stability of the two complementary helical enantiomers with respect to interconversion and the mechanism by which they interconvert depend on n.[9]

Synthesis

The first helicene structure was reported by

2-nitronaphthalene.[10] [5]helicene was synthesized in 1918 by Weitzenböck & Klingler.[11] The first [6]helicene (also called hexahelicene) was synthesized by M. S. Newman and D. Lednicer in 1955 via a scheme that closed the two central rings by Friedel–Crafts cyclization of carboxylic acid compounds.[12][13] Since then, several methods for synthesizing helicenes with different lengths and substituents are used. The oxidative photocyclization of a stilbene-type precursor is used most often as the key step. The longest helicene prepared by this method is [16]helicene in 2015.[14]

In one study,

Grubbs' second generation catalyst
:

Helicene synthesis by olefin metathesis
Helicene synthesis by olefin metathesis

Other approach is also non-photochemical and is based on assembly of biphenylyl-naphthalenes and their platinum-catalyzed double cycloisomerization leading to various [6]helicenes:[16]

  • [4]Helicene
    [4]Helicene
  • [5]Helicene
    [5]Helicene
  • [6]Helicene
    [6]Helicene
  • [6]Helicene, other chirality
    [6]Helicene, other chirality
  • [7]Helicene
    [7]Helicene
  • [7]Helicene, other chirality
    [7]Helicene, other chirality
  • [8]Helicene
    [8]Helicene
  • [9]Helicene
    [9]Helicene
  • [10]Helicene
    [10]Helicene
  • [11]Helicene
    [11]Helicene
  • [12]Helicene
    [12]Helicene
  • [13]Helicene
    [13]Helicene
  • [14]Helicene
    [14]Helicene
  • [15]Helicene
    [15]Helicene
  • [16]Helicene
    [16]Helicene
  • [18]Helicene
    [18]Helicene

Applications

Helicenes have been studied with respect to

chemical sensors[23] and hetero-atom substitution.[24][25][26][27]

See also

References

  1. ^
    doi:10.1351/goldbook.H02762
  2. doi:10.1002/anie.197406491
  3. doi:10.1021/cr200087r
  4. doi:10.1021/jo048707h
  5. doi:10.1039/C2CS35154D
  6. doi:10.1039/C2CS35111K
  7. doi:10.1039/C2CS35134J
  8. doi:10.5059/yukigoseikyokaishi.72.58
  9. doi:10.1021/jacs.3c07032
    .
  10. doi:10.1002/cber.19030360481
  11. doi:10.1007/BF01524529
  12. doi:10.1021/ja01617a097
  13. doi:10.1021/ja01599a060
  14. doi:10.1002/anie.201502436
    .
  15. doi:10.1002/anie.200504150
  16. doi:10.1021/jo900077j
  17. ^ Helquat Dyes: Helicene-like Push–Pull Systems with Large Second-Order Nonlinear Optical Responses Benjamin J. Coe, Daniela Rusanova, Vishwas D. Joshi, Sergio Sánchez, Jan Vávra, Dushant Khobragade, Lukáš Severa, Ivana Císařová, David Šaman, Radek Pohl, Koen Clays, Griet Depotter, Bruce S. Brunschwig, and Filip Teplý The Journal of Organic Chemistry 2016 81 (5), 1912-1920
    doi:10.1021/acs.joc.5b02692
  18. doi:10.1002/chem.201504048
  19. doi:10.1021/acs.jpcc.6b01123
  20. doi:10.1002/ajoc.201600025
  21. ^ Synthesis and Structural Features of Quadruple Helicenes: Highly Distorted π Systems Enabled by Accumulation of Helical Repulsions Takao Fujikawa, Yasutomo Segawa, and Kenichiro Itami Journal of the American Chemical Society 2016 138 (10), 3587-3595
    doi:10.1021/jacs.6b01303
  22. doi:10.1021/acs.joc.6b00130
  23. doi:10.1002/elan.201600104
  24. doi:10.1021/jacs.6b01498
  25. doi:10.1016/j.tetasy.2016.06.020
  26. doi:10.1021/acs.joc.6b01395
  27. doi:10.1021/acsomega.6b00335
General references
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