Helicene

Structure and properties

The systematic naming for this class of compounds is based on the number of rings: [n]helicene is the structure consisting of n rings. According to IUPAC, only structures where n is at least 5 are considered helicenes. Some specific compounds also have alternate or trivial names. As the number of rings increases, starting at four, the structure becomes non-planar, but instead the planes of consecutive rings tilt to prevent 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 asymmetric carbons and chiral centers. Instead, there is axial chirality, which results from the handedness of the helicity itself. The clockwise and counterclockwise helices are non-superposable. By convention a left-handed helix is minus and labeled (M), a right-handed helix is plus and labeled (P). Evidence from CD spectroscopy suggests left-handed helices are levorotatory and right-handed helices are dextrorotatory.

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

Synthesis

The first helicene structure was reported by Jakob Meisenheimer in 1903 as the reduction product of 2-nitronaphthalene.

In one study, [5]helicene was synthesized in an olefin metathesis reaction of a divinyl compound (prepared from 1,1′-bi-2-naphthol (BINOL) in several steps), with Grubbs' second generation catalyst:

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:

:[[File:Helicene Cycloisomerization.png|center|500px]]

File:Tetrahelicene.jpg|[4]Helicene File:Pentahelicene.jpg|[5]Helicene File:Hexahelicene2.jpg|[6]Helicene File:Hexahelicene.jpg|[6]Helicene, minus chirality File:Heptahelicene2.jpg|[7]Helicene File:Heptahelicene.jpg|[7]Helicene, minus chirality File:Octahelicene.jpg|[8]Helicene File:Nonahelicene.jpg|[9]Helicene File:Decahelicene.jpg|[10]Helicene File:Undecahelicene.jpg|[11]Helicene File:Dodecahelicene.jpg|[12]Helicene File:Tridecahelicene.jpg|[13]Helicene File:Tetradecahelicene.jpg|[14]Helicene File:Pentadecahelicene.jpg|[15]Helicene File:Hexadecahelicene.jpg|[16]Helicene File:Octadecahelicene.jpg|[18]Helicene

Applications

Helicenes have been studied with respect to nonlinear optics, CPL, organocatalysis, conformational analysis, chirality sensing, chemical sensors and hetero-atom substitution.

References

; General references

References

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